強震即時警報系統之相關研究

強震即時警報系統之相關研究
鄧大量1 李汯鑑2
1.美國南加州大學南加州地震中心
2.美國地質調查所
吳逸民1 辛在勤2 蕭乃祺2
1.國立台灣大學
2.中央氣象局
1
FINAL REPORT
TO THE CENTRAL WEATHER BUREAU
ON
Strong-Motion Real-time Warning Systems and
Related Research
Submitted by
Ta-liang Teng
Southern California Earthquake Center
University of Southern California
Los Angeles, California 90089-0740
William H. K. Lee
U.S. Geological Survey (Retired)
Menlo Part, California 94025
Also at 862 Richardson Court
Palo Alto, California 94303
In collaboration with
Yih-Min Wu1, Tzay-Chyn Shin2, Nai-Chi Hsiao2
1. National Taiwan University, Taipei, Taiwan
2. Central Weather Bureau, Taipei, Taiwan
November 15, 2004
2
Executive Summary
This contract performs work that assists the Central Weather Bureau (CWB) in
improving and executing the on-going large seismological observation and research
programs of its Seismological Center. The work further help expand the capability of
the Earthquake Rapid Reporting System (RRS) and the Earthquake Early Warning
System (EWS) to include more realistic moment magnitude information, near real-time
seismic intensity, damage, and casualty assessment, so that governmental emergency
response agencies can more effectively dispatch the rescue resources. A good part of
this contract work also helps in defining the instrumentation specifications and
evaluating bidder’s technical proposals during the CWB acquisition activities,
instrument calibrations, data quality control and database construction – all directly
related to the successful Taiwan Strong-Motion instrument Program (TSMIP). A
number of scientific papers have also published that, together with earlier published
works, has made Taiwan Central Weather Bureau a well-known scientific governmental
agency in the world.
Task A.
Research work in Strong-motion studies and earthquake early warning:
(1) A published scientific paper: Near Real-Time Magnitude Determination for Large
Crustal Earthquakes by Wu, Y. M. and T. L. Teng has been published in
Tectonophysics, 390, 205-216, (2004).
Abstract: We introduce and empirical method of near real-time, near-field
magnitude determination for large ( M > 6.5 ) crustal earthquakes. Time integration
over the strong shaking during on the absolute values of the acceleration records is
carried out for nearby stations surrounding many large earthquake sources in
Taiwan. The integrated quantity, here denoted as total effective shaking, is used in
3
a regression process to derive and empirical relationship for a quick Mw
determination useful for a reliable real-time operation in earthquake rapid reporting
and earthquake early warning systems. [Note: This manuscript was reported in our
final report of 2003.
Formal publication with some revisions of the original
manuscript is reported here for completeness. The body of the entire text is omitted
as it can be found from the published journal].
(2) A submitted manuscript of a scientific paper: A Study on Near-Fault Mortality from
the 1999 Chi-Chi, Taiwan Earthquake by Chih-Hung Pai, Yong-Ming Tien, and TaLiang Teng, submitted to Bull. Seism. Soc. Am. (2004).
Abstract: A new approach to estimate the relations between mortality and the
closest distance to the Chelungpu fault surface trace, causal to the 1999 Chi-Chi,
Taiwan earthquake is introduced. We have constructed the database giving the
attributes of victims through a compilation of various documents of field survey
made immediately after the big damaging earthquake. These survey documents
were resulted from comprehensive filed visits recording actual locations of victims
and types of buildings in which victims were found. Among the total 2492 victims
of the Chi-Chi earthquake, 2039 victims (more than 80% of the total) were located
by GPS. Through the combined use of the attributive database of victims, digital
maps and Geographic Information Systems (GIS), we map the spatial distribution
and the attributive nature of victims with resolution of the smallest administrative
districts in Taiwan. A regression analysis gives equations for the mortality as
functions of the closest distance to the surface trace of the Chelungpu fault. We
find that the percentage of the mortality M can be expressed as
M ＝ 0 . 08 exp( 2 . 97 − 0 . 0097 d )
4
Here d is the closest distance to the fault surface trace in meter. As expected, the
shorter distance d causes higher mortality. We device three disastrous levels and
then suggest orders and scopes of an effective earthquake disaster rescue strategy
according to the regression curve of the mortality and the closest distance d to the
fault surface trace. The difference in mortality between the hanging-wall and the
footwall areas is remarkable and is described in separate regression curves. In nearfault regions, the death tolls and mortality for the residents lived in the hangingwall block (1348, and 0.23%) is significantly higher than those in the footwall
block (557, and 0.01%). The deaths ratio of the hanging-wall vs. the footwall block
is approximately 2.4:1. Finally, find that the mortality is nearly zero in areas
experiencing a PGA below 220 gals; and increases dramatically from 0.2% up to
2% of the local population when the PGA exceeds 400 gals. This rapid increase at
about 400 gals also shows up in the building damage. This close correlation clearly
indicates that earthquake death by-and-large are caused by the building collapse.
(3) An accepted manuscript by Yunfeng Liu, Ta-Liang Teng, and Yehuda Ben-Zion to
appear in Geophysical Journal International: Near-surface seismic anisotropy,
attenuation and dispersion in the aftershock region of the 1999 Chi-Chi, earthquake
Abstract: Seismograms from local aftershocks of the 1999 Chi-Chi, Taiwan,
earthquake recorded at a 200 m deep downhole station CHY of the Taiwan Central
Weather Bureau Seismic Network (CWBSN) have clear direct up-going shear
waves and their surface-reflected down-going phases. Measurements of time
difference between the direct and reflected phases of the fast and slow components
of split shear waves show approximately 8% velocity anisotropy in the top 200 m
of the crust. The phase velocities extracted from the direct and reflected waveforms
display clear evidence of attenuation-related dispersion. Taking the dispersion and
geometrical spreading factor into account, we estimate the Q value of the shear
waves by fitting calculated results to the observed reflected waveforms. The
5
amplitude spectral density ratios between the direct and reflected phases are
approximately linear within the frequency range 2 - 15 Hz. This allows us also to
estimate the Q value from the slope of the amplitude spectral ratio (in dB/Hz) in
this range. The estimated Q values with both methods, based on a set of similar
waveforms and additional 156 high-quality records, are 61 - 68 for the fast
components and 43 - 52 for the slow components. The observed attenuation
anisotropy may be, similarly to velocity anisotropy, a manifestation of microcracks
alignment and their response to in-situ stress. Strong attenuation anisotropy (23 30% in this study) along with attenuation-related dispersion in the shallow crust can
affect significantly the properties of shear waves and should be taken into account
in studies employing surface and shallow borehole records of shear waveforms.
(4) A conference paper by Ta-liang Teng, Yih-Min Wu, Tzay-Chyn Shin, Yi-Ben Tsai,
William H. K. Lee and Chung Chi Liu, Nai-Chi Hsiao for 2004 兩岸強地動觀測暨
地震測報研討會: Development of Modern Seismic Monitoring in Taiwan and
Progress on Earthquake Rapid Reporting and Early Warning Systems
Abstract: A recollection of the recent history on the developmental events that lead
to the current CWBSN and TSMIP, both of these two seismic networks have
contributed significantly not only in Taiwan’s earthquake science, but also have a
great impact on the holding of the seismological data of the entire world – in quality
and in quantity. Taiwan also leads the world in the development, operation, and
accomplishments of earthquake rapid reporting (RRS) and earthquake early
warning (EWS) systems.
Task B.
Specifications and Evaluations of Strong-Motion Instruments
6
Instrument specifications and evaluations were performed in 2004 in support of the
CWB 2004 procurements of free-field digital accelerographs. Instrument
specifications were written for 24-bit digital accelerographs and are given in
Appendix B1 of Section B. We also evaluated a technical proposal submitted by the
Refraction Technology for bidding the CWB 2004 digital accelerographs. A
preliminary analysis of the Reftek’s technical proposal was sent to CWB on April 6,
2003 (see Appendix B2 of Section B).
Task C.
Strong-Motion Data Processing and Software Development
During 2004, we made slow but steady progress in systematic processing of the
strong-motion data recorded by the Central Weather Bureau (CWB). The quality
assurance tasks are performed with the aid of a computer program called
SmBrowser, which has been described in the 2003 Annual Report. Enhancement has
been made to SmBrowser to improve data processing efficiency, and considerable
efforts have been made to verify station coordinates, which is still now underway.
A field test of multiple co-located accelerographs was conducted in Hualien from
end of March to early June, 2004. To analyze the recorded data, we developed some
pre-processing software and application code for coherence analysis. In particular,
computing the coherence function between two time-series signals was implemented,
and over 1,000 correlation pairs had been computed for up to 16 earthquakes that were
recorded by the six deployed accelerographs. A detailed report analyzing the
performance of the 24-bit acclerographs is given in Appendix C2 of Section C of this
Report. The results indicate that these accelerographs performed not as 99% perfect
with respect to each other as we would like, but not as bad as we might have feared.
7
Section A :
Research work in Strong-motion studies and earthquake early warning
Paper (1) Text omitted.
Paper (2)
A Study on Near-Fault Mortality from the 1999 Chi-Chi,
Taiwan Earthquake
Chih-Hung Pai, Yong-Ming Tien, and Ta-Liang Teng
Abstract
A new approach to estimate the relations between mortality and the closest
distance to the Chelungpu fault surface trace, causal to the 1999 Chi-Chi, Taiwan
earthquake is introduced. We have constructed the database giving the attributes of
victims through a compilation of various documents of field survey made immediately
after the big damaging earthquake. These survey documents were resulted from
comprehensive filed visits recording actual locations of victims and types of buildings
in which victims were found. Among the total 2492 victims of the Chi-Chi earthquake,
2039 victims (more than 80% of the total) were located by GPS. Through the combined
use of the attributive database of victims, digital maps and Geographic Information
Systems (GIS), we map the spatial distribution and the attributive nature of victims with
resolution of the smallest administrative districts in Taiwan. A regression analysis gives
equations for the mortality as functions of the closest distance to the surface trace of the
Chelungpu fault. We find that the percentage of the mortality M can be expressed as
M ＝ 0 . 08 exp( 2 . 97 − 0 . 0097 d )
8
Here d is the closest distance to the fault surface trace in meter. As expected, the
shorter distance d causes higher mortality. We device three disastrous levels and then
suggest orders and scopes of an effective earthquake disaster rescue strategy according
to the regression curve of the mortality and the closest distance d to the fault surface
trace. The difference in mortality between the hanging-wall and the footwall areas is
remarkable and is described in separate regression curves. In near-fault regions, the
death tolls and mortality for the residents lived in the hanging-wall block (1348, and
0.23%) is significantly higher than those in the footwall block (557, and 0.01%). The
deaths ratio of the hanging-wall vs. the footwall block is approximately 2.4:1. Finally,
find that the mortality is nearly zero in areas experiencing a PGA below 220 gals; and
increases dramatically from 0.2% up to 2% of the local population when the PGA
exceeds 400 gals. This rapid increase at about 400 gals also shows up in the building
damage. This close correlation clearly indicates that earthquake death by-and-large are
caused by the building collapse.
Introduction
Human fatality due to destructive earthquakes is a matter of the most important
concern. It implicitly describes the degree of resiliency of a country, due to its
socioeconomic structures and physical assets, to the impact of earthquakes. As human
safety is a primary goal of most modern earthquake hazard mitigation programs, reliable
analyses and estimations on modes of human fatalities during earthquakes are necessary.
Nevertheless, data and documents of the occurrences of fatalities during earthquakes are
relatively rare (Coburn and Spence, 1992) and mostly qualitative, which prevent a
quantitative statistical analysis of human casualties caused by earthquakes. The
extensive field data from the 1999 Chi-Chi earthquake provides an opportunity to
proceed with a necessary quantitative research.
According to reports published by the Architecture and Building Research Institute,
Ministry of Interior of Taiwan, the Chi-Chi earthquake has caused 2492 deaths, 739
severely injured (Hsiao et al., 2001b), and 51,778 and 53,852 buildings, respectively,
9
totally and partially collapsed (Hsiao et al., 2001a). Because the earthquake struck the
central Taiwan in the early morning, almost all residents were sleeping at home, 94% of
deaths resulted from lost their dwellings collapsed (Tien et al., 2002). It is fortunate,
because of the availability of excellent field data on (1) fault rupture, (2) ground motion,
(3) building damage, (4) well-documented fatality statistics and (5) demographic data
for the destructive earthquake that a reasonably quantitative analysis can be carried out.
The great (100 km × 40 km) rupture of the causal Chelungpu fault and the strong
shaking produced by the Chi-Chi earthquake were responsible for this great loss. New
guidelines introduced after this disastrous event require building sites to set back at least
15 m away from an well-defined trace of the Chelungpu fault, or 30 m away from west
and 50 m away from east side from a not well-defined trace of that fault. Based on these
guidelines, the active Chelungpu fault zone with which construction of structures for
human occupancy is prohibited, affects 15 towns and the total area equals 363 hectares.
Surveys to identify and define active fault zone are a common practice in
earthquake countries like the U.S.. Taiwan has Building Codes and Regulations
prohibited construction on a slope site which is within 100 m from an active fault zone
when the maximum historic earthquake magnitude (Mmax) is over or equal to 7.0; 50 m
for 7.0＞Mmax ≧6.0; and 30 m for Mmax ＜6.0 for cases of uncertain earthquake records.
Here Mmax is considered as the largest earthquake magnitude that is believed to occur on
an active fault or fault segment in the future. In this Regulation, the term “slope sites”
must conform to either of the following two conditions:
(1) The land must have an elevation of more than 100 meters.
(2) The land’s elevation is less than 100 meters but with a slope greater than 5%.
The Regulations are as shown in Table 1 (Construction and Planning Agency,
1997).
In U.S., the Alquist-Priolo Special Studies Zones Act (renamed the Alquist-Priolo
Earthquake Fault Zoning Act in 1994) was signed into law in 1972. The primary
purpose of the Act is to mitigate the hazard of fault rupture on structure by prohibiting
the construction of structures for human occupancy across the trace of an active fault.
10
Earthquake Fault Zones are delineated on U.S. Geological Survey topographic base
maps at a scale of 1:24,000 (1 inch equals 2,000 feet). The boundary of an “Earthquake
Fault Zone” is generally about 500 feet (150 meters) away from major active faults, and
200 to 300 feet (60 to 90 meters) away from well-defined, minor faults. The Act
requires that cities and counties withhold development permits for sites within an
Earthquake Fault Zone within their jurisdiction until geologic investigation demonstrate
that the site is not threatened by surface displacements from future faulting (California
Department of Conservation, Division of Mines and Geology, 1997).
Immediately after the occurrence of the Chi-Chi earthquake, more than 1,200
scientists and engineers were mobilized to conduct investigations and to collect data in
order to document as much as possible from this catastrophe. These investigations
recorded building types, the number of floors, building ages, building usages and
construction type etc. of 8,773 totally or partially collapsed structures (Architecture and
Building Research Institute, 1999). Nevertheless, the data associated with human
fatality were not included in the original report. Tien et al. (2002) had performed
another field work and compiled the complete statistical data of human fatality
according to the death certificates, the relief fund distribution lists, and construction
types of buildings. In this study, we constructed “the attributive database of victims”
which was further completed through a joint analysis of various after-earthquake survey
documents. The database from these various surveys is the result of a comprehensive
field visit confirming exact locations of victims and the type of buildings in which
victims were found. With these detailed descriptions, we can easily map and present the
spatial distribution and the attributive data of victims with the accuracy in terms of the
smallest administrative districts in Taiwan. Moreover, we have proposed a new
estimation approach for determining the mortality as function of the closest distance to
the ruptured fault surface trace. This differs from earlier report where the mortality
determined by the calculation of death tolls with regards to population after the
earthquake in an administrative district. A regression analysis gives correlated equations
between the mortality and the closest distance to the ruptured fault surface trace. We
11
device three different disastrous levels and then suggested orders and scopes of an
effective earthquake disaster rescue strategy according to the regression curve of the
mortality and the closest distance d to the fault surface trace. The difference in mortality
between hanging-wall and footwall areas and effect of building construction types are
particularly discussed with emphasis. Finally, we have established relations between
PGA and mortality as function of the closest distance on both sides to the ruptured fault
surface trace. From these relations have provided a quantitative method of the mortality
estimation and confirmed the adequacy of the research results against the real data. This
leads to a new procedure for the estimation of mortality. The results give a clearer
assessment of hazard levels induced to the destructive earthquake in near-fault regions.
Attributive Database of Victims
In this study, we try to quantify the spatial relationship between near-fault
mortality and the Chelungpu fault, causal to the Chi-Chi earthquake. Since the
documentation and factual data of deaths generally are in descriptive form, we have first
built an attributive database of victims by gathering quantitative data with GPS
positions of victims. In this attributive database of victims we have constructed for
every positioned victim not only has (1) a GPS coordinates but also (2) ID number, (3)
name, (4) address at which the victims was found, (5) buildings type and age, (6) floor
numbers, as well as (7) situation descriptions for surrounding areas of destructed
buildings. This database is also complemented by on-site photos, records of
investigation staffs and dates and so on (Pai et al., 2004). Specifically, to confirm the
types and characteristics of buildings in which victims were found, the original data
bank of damage buildings that was established by the Architecture and Building
Research Institute was also reexamined and verified by field visits.
Among the total 2492 victims of the Chi-Chi earthquake, there were 2039
positioned victims (more than 80%) by using GPS in this study. There were 1082 (43%)
12
and 752 (30%) victims mainly lived in Taichung and Nantou counties, respectively,
where the loss was heaviest. There were 205 (8%) positioned victims in Taichung city,
Taipei city, Changhua county, and Yunlin county of Taiwan that were hit less hard.
Among the 2039 positioned victims, there were 1921 victims who died from the
collapse of buildings, 97 victims died from landslides and 21 victims with unidentifiable
cause of death.
It took our research team two-years, with many graduate students and substantial
research funding to finish the GPS location of victims and the attributive database of
victims. Due to the remoteness of the areas and damaged roads, it was sometimes quite
difficult to get the GPS locations of victims of the Chi-Chi earthquake. We usually took
a whole working day just to complete the required data of 2-3 victims during field
investigation. The establishment of a complete database of human fatalities is thus not
feasible and some minor errors are not avoidable. Through the attributive database of
victims we have constructed, we desire to find information in the assessment of
earthquake fatality.
Digital Maps and Application of GIS
Data in this study are presented by a series of digital maps, including 1:25,000
scale digital Taiwan regional geographical maps and 1:1,000 scale digital Chelungpu
fault trace map, supplied by the National Center for Research on Earthquake
Engineering and the Central Geological Survey. High-resolution digital regional
geographical map are used that enables us to conduct analysis at the level of villages,
the smallest administrative unit in Taiwan. It will help to enhance the accuracy of our
research results when we process a series of spatial analyses. In the major disastrous
region on both sides of the Chelungpu fault, the area and population of villages are
uneven, ranging from (0.006 km2, 88) to (425.5 km2, 29487) before the Chi-Chi
earthquake.
The power of a GIS comes from the ability to relate different information in a
spatial context, so as to facilitate a relationship to be derived. Therefore, the application
13
of the GIS software in supporting analyses of near-fault mortality arises directly from
the benefit of integrating attributive database of victims and the digital maps to process
numerous operations that address any desirable spatial relations.
Methods for Mortality Calculation
The nearly NS–striking Chelungpu fault, causal to the Chi-Chi earthquake,
produced a sudden surface rupture for a total length of about 100 km (Central
Geological Survey, 1999; Ma et al., 1999; Chang et al., 2000; Lee et al., 2000). Most of
the collapsed and heavily damaged buildings were concentrated along the Chelungpu
fault zone, especially in the areas east of the fault (hanging-wall) that has experienced
most intense shaking (Tsai et al., 2001). As far as the characteristics of near-fault
ground motion is concerned, the PGA contour lines of all three components are
elongated along the fault line, and the regression of ground motion is strongly
dependent on distance to the fault rather than the distance to the epicenter (Wang et al.,
2002). Therefore, the distance to the causative fault is specifically defined as the closest
distance between the location of victims and the surface rupture trace as used in this
study. The values of the closest distance to the fault surface trace used in this study were
calculated by GIS. The definition of the closest distance to the fault surface trace is
illustrated in Figure 1.
The spatial analysis approach to near-fault mortality can be thought of as a process
that combines and transforms data of victims into a spatial resultant multi-layered
database and then process a series of the GIS analyses to finish the relationship between
mortality and the distance to the Chelungpu fault. The proposed process is an iterative
one that combines several operations from the technique of the GIS spatial analysis.
Published human fatalities research has not yet been developed in a quantitative form
because of the lack of a thorough database established for human fatalities. Therefore,
we will introduce calculation methods to which can best be adopted to generate
mortalities estimates. The mortalities calculation methods are available to serve this
purpose and many of them use the GIS software, together with algorithms to calculate,
14
map, and display damage and mortalities estimates according to particular methods that
meet our research needs.
Figure 2 schematically shows the essential concept and procedure for the
calculation method of mortality. The proposed procedure involves several main stages
presented as follows:
Data Integration
The first stage of the proposed methodology is to integrate a set of digital maps
and the attributive database of victims through the GIS software-Arcview. It provides
the spatial field which includes Taiwan regional geographical boundaries, traces of the
Chelungpu fault and spatial coordinates of victims for examining, presenting and
calculating the spatial relations of victims and the Chelungpu fault, as shown in Figure 3.
In fact, this is the essential part in the overall approach because we need to make sure
that the spatial field we have constructed is sufficient to process the research objectives.
In choosing these data, we attempted to cover all aspects of possible human fatalities
data in the earthquake and avoid redundancy. Most of the mortalities estimates can be
calculated as a function of population density and the closest distance to the fault
surface trace.
Selection of Analysis Zone
To process a series of spatial analyses and calculation of mortalities, we firstly
capture different analysis zones for equivalent closest distance to both sides of the fault
and then produced the spatial buffer space by the GIS software. In Figure 4a and b, we
present the two spatial buffer spaces in different closest distance to the fault surface
trace (d= 500m, 1000m) to explain that part of procedures. Once analysis zone of the
spatial buffer space is chosen, we can get a region with respect to the closest distance to
the fault surface trace. Therefore, many buffer spaces will be produced in this stage to
handle analysis of mortalities in individual closest distance to the fault surface trace.
15
Determination of Death Tolls
Once we choose one of the closest distances to the fault surface trace, the
corresponding analysis zone of spatial buffer space which includes areas far away from
the both side of the fault is produced. In every analysis zone of the spatial buffer space,
the victims involved are scattered over in the individual spatial buffer space according
to their own GPS locations. We can conduct and obtain building types and other
attributive data related to victims by operation of the GIS within analysis zone of the
spatial buffer space. Besides, we can handle and differentiate the information of victims
separately in the hanging-wall and footwall areas based on the construction of the
spatial buffer space and related information of victims.
Estimation of Population
In this stage, it is important how to accurately determine the population of each
analysis zone. Firstly, we gathered the document of “current” population and areas of
each village in Taiwan at the end of June 1999, which was about three months before
the Chi-Chi earthquake. These data are routinely documented by the Population Affairs
Administration, Ministry of Interior. The population density of each village was
calculated by gathered data accordingly.
Secondly, any analysis zone is composed of pieces or whole areas of many
villages. We separately calculated the occupied areas of each village in the
corresponding analysis zone by the GIS. Estimated population of each village was
calculated by the product of the occupied areas and population density. Finally, total
population of the analysis zone was obtained by the summation of estimated population
of all villages.
Mortalities Calculation
From the death tolls and population of each analysis zone, the mortalities rate,
which is given by the death tolls divided by population in an individual closest distance
to the fault surface trace, are calculated. Results are shown in Table 2.
16
Spatial Distribution of Victims
Among the 2039 GPS-located victims, 1082 victims are located at Taichung
County; 752 at Nantou County; 21 at Taichung City; 23 at Changhua County; 74 at
Yunlin County (See Fig. 3). Additional victims (87) are located in the city of Taipei,
some 145 km away from the epicenter. We have left these out because victims in Taipei
are caused by complex causes including strong basin amplification effect and fraudulent
construction practice.
In order to explain features of the attributive database and GPS locations of victims,
we present the data from the Taichung County, where most of the victims are located.
The geographical location of Taichung County almost equally seat over both sides of
the Chelungpu fault. Before the Chi-Chi earthquake, the total population of the county
was 1,475,254 in 21 towns. In the attributive database of victims, the distribution of
1082 positioned victims there was mainly concentrated in 9 townships. The population
before the earthquake, the number of positioned victims, and the statistics of types of
buildings in which victims were found are presented in Table 3 and Figure 5. The
spatial distribution of victims in two of the nine towns is presented in Figure 6a and b,
which is based on the scale of villages. From Figure 6a and b, we can easily map out the
spatial relations between the locations of each victim and the Chelungpu fault.
Moreover, we can conduct a series of spatial analyses to resolve many research
problems through application of the GIS such as the analysis of mortality, difference in
mortality between the hanging-wall and footwall areas, effect of building construction
types and ground motions.
Regression Analysis of Mortality
In the following we present results from analyses to show the relation between
mortality and the Chelungpu fault. A relationship between the mortality and the closest
17
distance to the fault surface trace can be obtained by a regression analysis. Results are
given in Figure 7 and an equation for the mortality can be expressed by:
M = 0.08 exp(2.97 − 0.0097d )
(1)
Here M is the percentage of the mortality, and d is the closest distance to the fault
surface trace (in meter). The corresponding curve matches the data closely, with the
multiple correlation coefficient (R2) =0.98. The regression curve clearly points out that
the closer to the Chelungpu fault the higher mortality is observed, and vice versa. In
Figure 7, we have marked off the three regions: the first region (Ⅰ)--d＜30 m. The
mortalities are the highest. In the d=30 m situation, where the mortality and the numbers
of cumulative victims, respectively, 1.43% and 128. In the second region (Ⅱ)--d=30
m~1000 m, the mortalities are sharply reduced with increasing distance to the fault as
shown in Figure 7. From the results of analyses tabulated in Table 2, the mortality and
the numbers of cumulative victims reduces from a high (1.43%, 128) to (0.2%, 529).
Finally, in the third region (Ⅲ)--d＞1000 m, the mortality is approaching zero. There
were the 1562 cumulative victims and about 77% of the 2039 located victims. After the
Chi-Chi earthquake, a new guideline issued by the Executive Yuan (the office of the
Prime Minister) requiring building sites that to be set back at least 15 m from an
identified Chelungpu fault. It is interesting to note that as a whole, there were 65 victims
(3.2% of the population lived within the region of 15 m from the identified Chelungpu
fault) in that 15-meter zone. A relatively high rate among the total 2039 located victims.
Higher mortalities in the near-fault regions are a logical expectation and described
qualitatively from several documents on disastrous earthquakes. With excellent database,
we have preceded an analysis to obtain quantitative results in terms of equations more
clearly define the relationship between mortalities and closest distance to the fault. Of
course, other parameters enter into this relation, too. These include the faulting
geometry, site effects, etc. that will be discussed later.
Difference between Hanging-wall and Footwall Areas
18
Due to the unique nature of thrust faulting, distribution of ground motions
intensity was highly asymmetrical about the fault trace, with the hanging-wall
displaying much higher ground acceleration. The rupture of the fault and the strong
ground motion definitely influenced the distribution of mortalities on both east and west
blocks of the fault. Therefore, it is helpful to understand and verify the difference on
mortalities between the hanging-wall (eastern block of the fault) and the footwall areas
(western block of the fault).
There are three major building types which victims lived, mud-brick, masonry
(including reinforced masonry) and Reinforced Concrete (RC) shorter than 6 stories,
were discussed according to statistic data (See Table 4). Based on the same method for
mortality estimation, we separately process the mortalities estimation of the hangingwall and the footwall areas. The cumulative death tolls, population and mortalities of
the hanging-wall and the footwall areas are tabulated against the closest distance to the
fault surface trace; these are presented in Table 5. The relations between the mortality of
the hanging-wall and the footwall areas and the individual closest distance to the fault
surface trace are presented in Figure 8. These relations between the closest distance to
the fault surface trace and mortality of the hanging-wall and the footwall areas can be
separately represented by the regression curves given by:
M HW = 0.33 + 2.22 exp(−15.45d )
(2)
M FW = 0.095 + 0.78 exp(−4.11d )
(3)
in which MHW and MFW are the mortality of the hanging-wall and the footwall areas (in
percentage), and d is the closest distance to the fault surface trace (in meter). These
corresponding curves matches the data closely, with R2 =0.98 and 0.84 in the hangingwall and the footwall areas. It shows that the mortality for residents lived in hangingwall block is significantly higher than those in footwall block especially within areas of
100 m on both sides of the Chelungpu fault. Specifically, the closer to the Chelungpu
fault the differences in mortality between the hanging-wall and the footwall areas are
more pronounced. On both sides of the fault, the death tolls and mortality for the
19
residents lived in the hanging-wall block (1348, and 0.23%) are significantly higher
than those in the footwall block (557, and 0.01%). The deaths ratio of the hanging-wall
vs. the footwall block is approximately 2.4:1.
Effect of Building Construction Types
Damage to or collapse of buildings resulting from an earthquake is very complex.
In addition to the characteristics of earthquake strong shaking, other factors, such as the
building design codes, the construction quality, architectural designs, the usage behavior
of householders, and so on, can lead to different degrees of damage to or collapse of
buildings. Because the Chi-Chi earthquake struck central Taiwan at 1:47 AM local time,
almost all victims were crushed to death by the collapsed buildings in the dormant state.
Before the Chi-Chi earthquake, the number of mud-brick (adobe) residences was only
about 5% of total buildings in Taiwan. However, there were 962 victims (43%) staying
in mud-brick residences. Mud-brick residences are not strengthened by any materials
and they are very old. In addition, the seismic resistance capacity of mud-brick
residences is clearly much lower than that of masonry (particularly the reinforced
masonry) and RC buildings; and they are very likely to collapse in a strong earthquake.
The seismic resistant capacity of mud-brick residences is the most important factor that
caused great loss of lives in the Chi-Chi earthquake.
Masonry (including reinforced masonry) and RC buildings under 6 stories in height
are currently the two most common types of building in Taiwan. In general, the more
urbanized the district the higher the percentage of RC buildings which can be found. In
contrast, the percentage of masonry (included reinforced masonry) buildings is higher in
rural areas. In the disaster area, the average number of masonry buildings in each town
is approximately one third to two thirds of the total buildings. In addition, the total
number of masonry buildings in Nantou and Taichung counties, areas most affected by
the earthquake in Taiwan, masonry structures make up to about 44% of the total
buildings. The total number of RC buildings in the two counties is about 38% of the
total buildings. There were 384 and 363 victims, respectively, who stayed in masonry
20
buildings and RC buildings. We infer that the seismic risk of masonry buildings and RC
buildings are almost equivalent, and the seismic risk of these two types of buildings is
much lower than that of mud-brick strictures (Tien et al., 2002).
Statistically, we found that these three major building construction types account
for 841 deaths (43.7%) in mud-brick (adobe); 368 (19.2%) in masonry and reinforced
masonry buildings and 459 (23.9%) in RC buildings under 6 stories (Fig. 9a~9c).
Based on the procedures of preceding estimation, we are able to process the
mortality of the hanging-wall and the footwall areas in different building construction
types with respect to the individual closest distance to the fault. The mortalities and preChi-Chi population in the hanging-wall and footwall areas are given in Table 6. A
relationship between the mortality of the hanging-wall and footwall areas and the
closest distance to the fault surface trace in different building construction types are
given in Figure 10a and b. From Figure 10, we find the mortality of the hanging-wall
areas is clearly higher than that of the footwall areas in the same building construction
types. Mud-brick buildings play a dominant role. Mud-brick buildings are rapidly
collapsed during the fault thrust motion. On the other hand, for both of masonry and RC
buildings no clear difference on mortality in either the hanging-wall or the footwall
areas. The lower mortality of masonry and RC buildings is attributed to their better
resistant to strong shakings.
Effect of Ground Motions
After the Chi-Chi earthquake, recordings of more than 400 out of 650 free-field
ground-motion stations deployed throughout Taiwan Central Weather Bureau have
obtained excellent records. These data represent an unprecedented wealth of waveform
information invaluable for ground-motion studies, as well as for the correlative study of
human fatalities engaged in this paper. To understand the relations between
characteristics of ground motion and the mortality, as well as the distances to the
Chelungpu fault, PGA data of 66 ground-motion stations in the immediate
21
neighborhood on both sides of the Chelungpu fault were gathered. Among the 66
stations, there were 11 stations in the hanging-wall block (also foothill side) and 55
stations in the footwall block (sedimentary plain side). Figure 11 shows the spatial
distribution of these ground-motion stations. We have defined the Mean PGA Index
(MPI) of a station as the mathematic mean calculated from PGA values of the three
directions for every ground-motion station. In the meantime, we have calculated the
closest distance (L) of the location of every ground-motion station with respect to the
Chelungpu fault surface trace by the GIS operations. Figure 12 shows the relation that is
given by a regression line having the following form:
MPI = 451.34 − 95.34 ln( L + 1.13)
(4)
where MPI has a unit of gals and L is in kilometer. Through the combination of Eqn. (1)
and Eqn. (4), Figure 13 shows a regression curve of the MPI value and mortality
resulted from the Chi-Chi earthquake as functions of the closest distance to the
Chelungpu fault surface trace of every ground-motion station. We found that the
mortality is nearly zero in the areas experienced the MPI below 220 gals. On the other
hand, the mortality increases dramatically from 0.2% up to 2% when the MPI exceeds
400 gals.
Discussions and Conclusions
Human safety has been a primary concern in most modern earthquake damage
mitigation programs. However, there was not sufficient knowledge related to earthquake
fatalities, damage and distance to potential active faults. In this research, we have
carried out a study employing a large amount of quantitative data obtained through
extensive after-earthquake field investigations. Positioning of the victims is done by the
GPS. We have built-up a comprehensive attributive database of victim to explore the
factors leading to human fatalities caused by the Chi-Chi earthquake. Our research has
finished the relationship between the mortality and the closest distance to the fault
surface trace, as obtained by a regression analysis. The regression curve clearly points
22
out, though not surprising, that the closer to the Chelungpu fault surface trace the higher
mortality is observed, and vice versa. Therefore, we have devised three disastrous levels
and then suggest the orders and scopes of the earthquake disaster rescue according to
the regression curve on the mortality and the closest distance d to the fault surface trace.
Level 1: People live within 100 meters from the rupture surface trace, especially
in the hanging-wall areas.
Level 2: Large fraction of mud-brick buildings is in presence in those areas.
Level 3: PGA in those areas exceeds 400 gals. As we have found that the
mortality is nearly zero in the areas having shaking below 220 gals and increases
dramatically up to 2% when the PGA value exceeds 400 gals.
Thus, for emergency response agencies, conditions meet the above three “levels”
should receive the highest priority in the dispatching of rescue resources, which
unfortunately are never enough during a major disaster such as a major disaster like the
1999 Chi-Chi, Taiwan earthquake. However, rapid and logical dispatching however
limited resources will still maximize the rescue effectiveness.
Acknowledgements
This research was supported by the National Science Council under Contract No.
NSC 90-2211-E-008-068 and NSC 93-2625-Z-253-001. TLT is also support by the
National Science Foundation (grant EAR-0124926) and the Taiwan Central Weather
Bureau (Contract MOTC-CWB-93-E-06). We sincerely thank the Central Weather
Bureau and the Architecture and Building Research Institute for providing the groundmotion data and 1:25,000 scale Taiwan digital geographical maps. We especially
appreciate Prof. Yi-Ben Tsai and Dr. Li Zhao for their valuable comments. Finally, we
dedicate this study in memory of the victims in the Chi-Chi earthquake.
References
23
Architecture and Building Research Institute of Taiwan (1999). Preliminary report on
surveys of damaged buildings from the great 921 Chi-Chi earthquake (in Chinese),
178pp.
California Department of Conservation, Division of Mines and Geology (1997). Report
on Fault-rupture Hazard Zones in California, Special Publication 42, 19pp.
Central Geological Survey of Taiwan (1999). Report on geological investigations of the
921 earthquake (in Chinese). 315pp.
Central Weather Bureau of Taiwan (2003). PGA data of Chi-Chi earthquake,
http://gov.tw/V4/ind-ex.htm (last accessed 30 September 2003).
Chang, C. H., Wu, Y.M., Shin, T. C., and Wang, C. Y. (2000). Relocation of the 1999
Chi-Chi earthquake in Taiwan, TAO, 11, no. 3, 581-590.
Coburn, A., and R. Spence (1992). Earthquake Protection, John Wiley & Sons,
Chichester, U.K., 355 pp.
Construction and Planning Agency of Taiwan (1997). The Building Construction
Regulations for Slope Sites (in Chinese), Taiwan Building Codes and Regulations,
Chapter 13, Section 261-3, http://www.cpami.gov.tw/law/law/lawe-2/b-rule.htm.
Hsiao, C. P., Lee, B. J., and Chou, T. Y. (2001a). Analyses and statistics of the
characteristics of buildings damage in the 1999 Chi-Chi earthquake (in Chinese),
Report for the Architecture & Building Research Institute, Ministry of the Interior,
159pp.
Hsiao, C. P., Tien, Y. M., Chen, J. C., Juang, D. S., and Pai, C. H. (2001b).
Investigation and statistical analyses of the characteristics of buildings in which
victims stayed in the 1999 Chi-Chi earthquake (Ⅰ) & (Ⅱ) (in Chinese), Report for
the Architecture & Building Research Institute, Ministry of the Interior, 394pp.
Lee, C. T., Kang, K. H., Cheng, C. T., and Liao, C.W (2000). Surface rupture and
ground deformation associated with the Chi-Chi, Taiwan earthquake (in Chinese),
SINO-GEOTECHNICS, 81, 5-16.
Ma, K. F., Lee, C. T., Tsai, Y. B., Shin, T. C., and Mori, J (1999). The Chi-Chi, Taiwan
earthquake: large surface displacements on inland thrust fault, EOS, 80, 605-611.
24
Pai, C. H., Tien, Y. M., Juang, D. S., and Wang, Y. L. (2004). A Study on near-fault
mortality from the Taiwan Chi-Chi earthquake, 13th World Conference on
Earthquake Engineering, paper no. 2542, 15pp
The Executive Yuan of the Republic of China (1999). The limitations of Fault zoning
for prohibiting buildings construction (in Chinese), Special News Publication.
Tien, Y. M., Juang, D. S., Pai, C. H., Hisao, C. P., and Chen, C. J. (2002). Statistical
Analyses of Relation between Mortality and Building Type in the 1999 Chi-Chi
Earthquake, Journal of the Chinese Institute of Engineers, 25 no. 5, 577-590.
Tsai, Y. B., Yu, T. M., Chao, H. L., and Lee, C. P. (2001). Spatial distribution and age
dependence of
human-fatality rates from the Ch-Chi, Taiwan, earthquake of
21human-fatality rates from the Chi-Chi, Taiwan, earthquake of 21 September
1999, Bulletin of the Seismological Society of America, 91, no. 5, 1298-1309.
Wang, G. Q., Zhou, X. Y., Zhang, P. Z., Igel, H. (2002). Characteristics of amplitude
and duration for near fault strong ground motion from the 1999 Chi-Chi, Taiwan
earthquake, Soil Dynamics and Earthquake Engineering, 22, no. 1, 73-96.
25
Department of Civil Engineering
National Central University at Chung-Li
Taoyuan County, Taiwan 320, R.O.C.
(C.-H. P., Y.-M. T.)
Department of Civil Engineering
Nanya Institute of Technology at Chung-Li
Taoyuan County, Taiwan 320, R.O.C.
(C.-H. P.)
Southern California Earthquake Center
University of Southern California
Los Angeles, California
(T.-L.T.)
26
Table 1. Building Construction Regulations for slope sites in Taiwan
Max. historic earthquake magnitude
(Mmax)
Distance away from the fault (m)
Mmax ≧7.0
100
7.0＞Mmax ≧6.0
50
Mmax ＜6.0 or
30
no earthquake records
27
Table 2. Cumulative death tolls, population and mortality in regions of increasing
distance from the surface fault trace.
Analysis
Zone*
(m)
Closest
distance
Cumulative
to the fault, d
death toll
Population
Mortality
(%)
(m)
0~10
10
42
3,197
1.31
0~20
20
86
6,375
1.35
0~30
30
128
9,541
1.43
0~40
40
145
12,699
1.02
0~50
50
162
15,856
1.02
0~100
100
232
31,872
0.73
0~200
200
265
62,128
0.43
0~300
300
300
90,387
0.33
0~400
400
333
116,402
0.29
0~500
500
363
141,351
0.26
0~600
600
376
166,347
0.23
0~700
700
402
190,749
0.21
0~800
800
480
214,936
0.22
0~900
900
519
239,232
0.22
0~1,000
1,000
529
263,283
0.20
0~2,000
2,000
712
519,599
0.14
0~5,000
5,000
1,322
1,305,427
0.10
1,562
2,327,196
0.07
0~10,000 10,000
*Analysis Zone as defined in Figure 4a.
28
Table 3. Statistics of death tolls and types of buildings in which positioned victims were
located (in Taichung County).
Under 6 stories
Town
District
Population Victims*
Victim
Mud-
Location brick
Masonry
and
reinforced
High-rise
Residential
RC
Building
Others
(RC)
masonry
Hoping
11,041
36
26
0
0
0
0
26
Shihgang
15,563
173
168
113
38
15
0
2
Hsinshe
27,024
120
105
82
15
7
0
1
Dongshi
59,413
357
329
207
29
55
31
7
Wufeng
68,126
86
74
22
22
30
0
0
Tanz
86,037
7
5
3
2
0
0
0
Fengyuan 160,863
157
155
64
10
27
45
9
Taiping
164,246
89
70
24
11
18
12
5
Dali
171,215
165
150
1
4
2
136
7
Others
711,726
2
0
0
0
0
0
0
Total
1,475,254
1192
1082
516
131
154 224
57
*the data of victims were supplied by Reconstruction Committee of Taiwan Chi-Chi
earthquake.
29
Table 4. Statistics on death tolls and building types in which victims stayed of
positioned victims
Under 6 stories
Districts Victims
Victim
Mud-
Location
brick
Masonry and
High-rise
Residential
reinforced
RC Building (RC)
Other
types
Landslide
(Non-
Unidentified
building)
masonry
Taichung
1192
1082
516
131
308
70
28
26
3
927
752
319
222
151
0
10
32
18
108
21
6
15
0
0
0
0
0
87
87
0
0
0
87
0
0
0
33
23
0
0
0
23
0
0
0
82
74
0
0
0
35
0
39
0
Others
63
0
0
0
0
0
0
0
0
841
368
459
215
38
Total
2492
2039
97
21
County
Nantou
County
Taichung
City
Taipei
City
Changhua
County
Yunlin
County
1921
30
Table 5. Results of cumulative death tolls, population and mortality in regions of
increasing distance from the fault. Different data from hanging-wall and footwall areas
are separately tabulated.
Footwall Areas
Closest distance to the
fault, d (m)
Cumulative
death tolls
Population
Hanging-wall Areas
Mortality
Cumulative
(%)
death tolls
Population
Mortality
(%)
10
8
1,735
0.46
34
1,462
2.33
20
25
3,405
0.73
61
2,970
2.05
30
53
4,834
1.1
75
4,707
1.59
40
58
6,444
0.9
87
6,255
1.39
50
67
8,062
0.83
95
7,794
1.22
60
76
9,681
0.79
109
9,606
1.13
70
78
11,297
0.69
122
11,179
1.09
80
84
12,895
0.65
133
12,737
1.04
90
87
14,475
0.6
140
14,285
0.98
100
90
16,052
0.56
142
15,820
0.9
200
103
31,746
0.32
162
30,382
0.53
300
127
47,650
0.27
173
42,737
0.4
400
138
63,730
0.22
195
60,452
0.33
500
156
80,055
0.19
207
61,296
0.33
600
163
97,323
0.17
213
69,024
0.31
700
189
114,772
0.16
213
75,977
0.28
800
260
133,124
0.2
220
81,812
0.27
900
269
151,767
0.18
250
87,465
0.29
1,000
273
170,619
0.16
256
92,664
0.28
2,000
333
399,664
0.08
378
119,935
0.32
5,000
494
1,083,865
0.05
824
221,562
0.37
10,000
494
2,028,872
0.02
1,056
298,324
0.35
31
Table 6. Death tolls, estimated population and mortality of the hanging-wall and
footwall areas in different analysis zones.
Analysis
Zone (m)
Building Type
*
MB
0~10
0~20
0~30
0~60
0~100
0~500
0~1,000
0~5,000
Footwall Areas
E.P.‡
Deaths
Hanging-wall Areas
M (%) ♀
Deaths
E.P.
M (%)
1
106
0.94
26
131
19.89
MRM
7
524
1.33
1
570
0.18
RC
0
645
0
7
664
1.05
MB
11
211
5.22
34
262
12.98
MRM
11
1,051
1.05
19
1,137
1.67
RC
0
1,331
0
7
1,288
0.54
MB
21
309
6.79
36
397
9.07
MRM
22
1,548
1.42
27
1,730
1.56
RC
7
1,915
0.37
11
2,010
0.55
MB
29
613
4.73
52
795
6.54
MRM
28
3,087
0.91
50
3,459
1.45
RC
12
3,821
0.31
11
4,022
0.27
MB
44
1,009
4.36
58
1,330
4.36
MRM
28
5,088
0.55
67
5,787
1.16
RC
15
6,298
0.24
13
6,724
0.19
MB
69
4,818
1.43
95
5,906
1.61
MRM
41
24,019
0.17
85
26,269
0.32
RC
43
29,642
0.15
20
30,900
0.06
MB
83
9,501
0.87
124
10,533
1.18
MRM
67
46,714
0.14
102
46,785
0.22
RC
76
57,505
0.13
21
55,263
0.04
MB
105
46,181
0.23
464
34,540
1.34
MRM
98
250,603
0.04
160
152,789
0.10
RC
236
350,998
0.07
133
193,691
0.07
†
MB*: Mud-brick (adobe)
MRM†: Masonry and reinforced masonry
E.P. ‡: Estimated Population
M ♀: Mortality=Deaths/Estimated Population
32
The Fault Trace
(Xi,Yi)
B(X b,Yb )
db
da
A(X a,Ya )
Hanging-wall
Footwall
Figure 1 : Definition of the closest distance from a victim to the Chelungpu fault
trace. For examples, d a and d b give the closest distance from GPS locations of
victim A and B to the fault trace.
33
Data Collections
Attributive Database
of Victims
Digital Maps
Population & Areas
of Each District
Data Integration
Selection of Analysis
Zone
Calculation of
Population Density
Determination of
Death Tolls
Estimation of
Population
Mortalities
Calculation
Finish
Figure 2 : Procedures of near-fault mortality Estimation.
34
Figure 3 : Spatial distribution of victims from Taiwan Chi-Chi Earthquake (near-
fault region).
35
(a)
(b)
Figure 4 : (a) Shows the analysis zone; (b) Shows the patterns of the spatial
buffer space for an individual located closest distance to the fault surface trace
(d=500 m & 1000 m).
36
350
Death Tolls
others
300
high-rise residences
250
reinforced concrete
masonry & reinforced masonry
200
mud-brick
150
100
50
0
Shihgang
Hsinshe
Dongsi
Taiping
Fengyuan
Dali
Wufeng
Hoping
Tanz
Town
Figure 5: Death tolls of each town in Taichung County classified with building
types in which victims were founded.
37
(a).
(b).
Figure 6: Spatial distribution of victims based on each village (the smallest
administrative district in Taiwan). (a) Shihgun town of Taichung county; (b)
Hsinshe town of Taichung County.
38
1.6
1.4
Mortality (%)
1.2
1.0
(Ⅰ)
(Ⅱ)
(Ⅲ)
0.8
0.6
0.4
0.2
0.0
1
10
100
1000
10000
Closest distance to Chelungpu Fault (m)
Figure 7: Mortality as plotted against the closest distance to the fault surface
trace.
39
Mortality (%)
2.5
2.0
Footwall Areas
Hanging-wall Areas
1.5
1.0
0.5
10
1
0.1
0.01
0.1
1
Closest Distance to the Fault (km)
Figure 8: Mortalities of the hang-wall and footwall areas as plotted against the
closest distance to the fault surface trace.
40
10
(a)
(b)
41
(c)
Figure 9:(a) A partially collapsed mud-brick (adobe) residences in Dongshi town
during the Chi-Chi earthquake. Its distance to the Chelungpu fault trace is 1.643
km. (b) Most of the ground floor, especially at the corner buildings, collapsed and
practically all un-reinforced masonry and poorly constructed reinforced masonry
structures were destroyed in the town of Puli during the Chi-Chi Earthquake. Its
distance to the Chelungpu fault trace is farther 27.858 km. Puli is a small yet soft
basin. Basin amplification of strong-ground motion is the primary reason. (c)
Tilting of the RC building in Chungshan Gobow Community of Douliu city
during the Chi-Chi Earthquake. Its distance to the Chelungpu fault trace is 13.292
km. It is apparently a problem of poor foundation preparation.
42
Mortality (%)
25
Mud-brick (Adobe)
Masonry and reinforced masonry
Reinforced Concrete
20
15
Footwall Areas
Hanging-wall Areas
10
5
0
10000
1000
100
10
100
1000
10000
Closest Distance to the Fault (m)
(a)
Mortality (%)
2.0
Masonry and reinforced masonry
Reinforced Concrete
1.5
Footwall Areas
Hanging-wall Areas
1.0
0.5
0.0
10000
1000
100
10
100
1000
10000
Closest Distance to the Fault (m)
(b)
Figure 10: Mortalities of the hang-wall and footwall areas as plotted against the
closest distance to the fault surface trace according to the classification of
buildings types in which victims stayed.
43
Figure 11:Spatial distribution of ground-motion stations on both sides of the
Chelungpu fault.
44
Mean PGA Index, MPI (gals)
1000
100
Hanging-wall Stations (11)
Footwall Stations (55)
10
0.1
1
10
100
Closest distance to the fault, L (km)
Figure 12: The Mean PGA Index (MPI) as plotted against the closest distance to
the fault surface trace of every ground-motion station.
45
2.0
1.8
Mortality, M (%)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150 200 250 300 350
Mean PGA Index, MPI (gals)
400
Figure 13: Relations between mortality and Mean PGA Index (MPI).
46
450
500
Paper (3)
Near-surface seismic anisotropy, attenuation and dispersion in the
aftershock region of the 1999 Chi-Chi, earthquake
Yunfeng Liu, Ta-Liang Teng, and Yehuda Ben-Zion
Department of Earth Sciences, University of Southern California, Los Angeles, CA,
90089-0740, USA
Email: [email protected], [email protected], [email protected]
Submitted March, 2004
Abstract
Seismograms from local aftershocks of the 1999 Chi-Chi, Taiwan, earthquake
recorded at a 200 m deep downhole station CHY of the Taiwan Central Weather Bureau
Seismic Network (CWBSN) have clear direct up-going shear waves and their surfacereflected down-going phases. Measurements of time difference between the direct and
reflected phases of the fast and slow components of split shear waves show
approximately 8% velocity anisotropy in the top 200 m of the crust. The phase
velocities extracted from the direct and reflected waveforms display clear evidence of
attenuation-related dispersion. Taking the dispersion and geometrical spreading factor
into account, we estimate the Q value of the shear waves by fitting calculated results to
the observed reflected waveforms. The amplitude spectral density ratios between the
direct and reflected phases are approximately linear within the frequency range 2 - 15
Hz. This allows us also to estimate the Q value from the slope of the amplitude spectral
ratio (in dB/Hz) in this range. The estimated Q values with both methods, based on a set
of similar waveforms and additional 156 high-quality records, are 61 - 68 for the fast
components and 43 - 52 for the slow components. The observed attenuation anisotropy
may be, similarly to velocity anisotropy, a manifestation of microcracks alignment and
their response to in-situ stress. Strong attenuation anisotropy (23 - 30% in this study)
along with attenuation-related dispersion in the shallow crust can affect significantly the
47
properties of shear waves and should be taken into account in studies employing surface
and shallow borehole records of shear waveforms.
1.INTRODUCTION
The heavily damaged material in the top few hundred meters of the crust, with a
low overburden pressure, contains a high density of cracks, pores and other defects. The
near-surface material can thus have a large influence, in terms of attenuation, dispersion
and anisotropy, on seismic recordings made at the surface. Many studies (e.g. Aster and
Shearer 1991; Fletcher et al. 1990; Hauksson et al. 1987) employed borehole
observations to investigate the seismic attenuation and anisotropy in the shallow crust
and their effects on surface recordings. In this paper we perform such a study using high
quality seismograms from local aftershocks of the 1999 Chi-Chi earthquake recorded at
a 200 m deep downhole station CHY of the Taiwan Central Weather Bureau Seismic
Network (CWBSN).
The borehole seismograms we use have clear direct up-going shear waves Sup and
surface-reflected down-going phases Sdown. In a previous study based on this data set
(Liu et al. 2004), we found a strong anisotropy of shear-wave velocity in the top 200 m,
which contributes about 20% of the total shear-wave splitting (SWS) time delay in the
upper crust. In the present work we employ two methods to calculate the quality factors
(Q) of both the fast and slow shear-wave components determined from the previous
SWS analysis of Liu et al. (2004). In the first method, the quality factor is estimated
from the slope of the amplitude spectral ratio of the direct and reflected waves versus
frequency. In the second method, we estimate the quality factor by comparing the
observed reflected waveform with a calculated one, generated by applying an
attenuation equation to the observed direct waveform. The quantity and quality of the
recordings enable us to measure the quality factor reliably. The results show clear
evidence of attenuation anisotropy in the near-surface structure. The phase velocities
extracted from the direct and reflected waveforms indicate the existence of attenuationrelated dispersion. The inferred dispersion curves fit the theoretical logarithm dispersion
48
equation well. Because of the difficulty of isolating the reflected P wave phases, we
only analyze the attenuation and dispersion properties of shear waves.
2.DATA SET AND GEOLOGICAL BACKGROUND
Modern digital seismic monitoring in Taiwan began in the early 1970s and at
present the Taiwan Central Weather Bureau Seismic Network (CWBSN) has 75
telemetered stations (Shin and Teng 2001). One of these short-period stations, CHY, is
installed in a 0.2 km deep borehole. The 1999 Chi-Chi earthquake sequence was highly
energetic, with many M ≥ 6.0 aftershocks, two of which and many other smaller
aftershocks occurred in the area close to CHY. The data used in this study extends from
January 1997 to March 2002 and the sampling rate of the employed seismograms is 50
sps.
As shown in Figure 1, the borehole station CHY is located in the eastern boundary
area of the west coast Holocene alluvium plain, southwest of the southern end of the
Chelungpu Fault (CLF), which ruptured during the Chi-Chi main shock. The Meishan
fault (MSF), a strike-slip fault associated with the 1906 M 7 earthquake (Wu and Rau
1998), is located at the northern boundary of the study region. The Chukuo fault (CKF),
another well-known active structure, is about 10 km to the east of the region.
Observations from hydro-geological drilling reveal that the top 200 - 300 m of the
crust in the study area consists of inter-fingered fine-, medium- and coarse- grain
sandstones and gravel beds. Figure 2 shows drill core samples from a nearby hydrogeological well #200201G1 that illustrate visually some characteristics of the nearsurface material. The core samples from 88 m to 90 m are coarse- grain (0.50 mm –
1.00 mm) sandstone, those from 90 m to 92 m are medium- to fine- grain (0.125 mm –
0.5 mm) sandstone, and those from136 m to 140 m are finer siltstones or mudstones.
3.METHODS FOR ATTENUATION ANALYSIS
49
The amplitude spectrum Ai ( f ) recorded at the ith station for a given earthquake can
be expressed as (Bath, 1974)
Ai ( f ) = Gi K ( f ) S i ( f ) I i ( f ) exp(− πfRi Qv) ,
(1)
where Gi is the geometrical spreading factor, K ( f ) is the source spectrum, S i ( f )
is the site response, I i ( f ) is the instrumental response, Ri is the travel distance of the
seismic wave, v is the average wave velocity and Q is the assumed frequencyindependent quality factor averaged along the path. Because the travel times can be
measured directly from the recordings, they can be used to determine interstation Q
values without making assumptions about the velocity structure. For two stations along
essentially the same ray path, we have approximately
πf (t 2 − t1 )
A1 ( f ) G1 S1 ( f ) I 1 ( f )
exp(
)
=
A2 ( f ) G2 S 2 ( f ) I 2 ( f )
Q
(2)
where t1 and t 2 are the travel times from the source to the first and second stations,
respectively, and Q is the average quality factor along the path between these two
stations. Here the source spectrum k ( f ) is eliminated, since both recordings are from
the same source.
In this study, we adopt this method for downhole recordings in which reflected
waves from the free surface are viewed as waveforms that are recorded by another
virtual station. Figure 3 illustrates schematically the geometry of the direct and surfacereflected waves in the borehole configuration. Since the up- and down-doing phases are
both recorded by the same physical station, we have S1 ( f ) = S 2 ( f ) and I 1 ( f ) = I 2 ( f ) .
Thus, we can eliminate the site and instrumental response terms as well and get
Aup ( f )
Adown ( f )
=
Gup
Gdown
exp(
πf (t down − t up )
Q
),
(3)
where the subscripts ‘up’ and ‘down’ replace ‘1’ and ‘2’, respectively. We can
develop the following two procedures for estimating the quality factor Q based on
equation (3):
50
(a) Amplitude spectral ratio method
Taking denary (base 10) logarithms on both sides of equation (3), we get
log
Aup ( f )
Adown ( f )
= log
Gup
Gdown
+ log e
π (t down − t up )
Q
(4)
f
The first item in the right side of equation (4) is independent of the frequency f .
Thus a plot of log Aup ( f ) Adown ( f ) versus f gives a line with a slope m , from which
the Q value can be estimated as
Q = π ⋅ (t down − t up ) /(log e 10 ⋅ m)
(5)
A similar method is adopted by Hauksson et al. (1987) and Aster and Shearer
(1991) for attenuation analysis in borehole experiments.
(b) Waveform fitting
For a layered velocity model, the geometrical spreading factor G for a shallow
depth range is, from ray theory, approximately proportional to 1 / R , where R is the
distance from source to receiver (Hauksson et al., 1987). Therefore, equation (3) can be
rewritten as
Adown ( f ) =
Rup
Rdown
exp(
πf (t down − t up )
Q
) Aup ( f )
(6)
As will be discussed later, for a frequency-independent Q there must be a
frequency-dependent phase velocity c( f ) , which can be represented (Aki and Richards,
2002) as
⎛f ⎞
1
1
1
=
+
log⎜⎜ 0 ⎟⎟ ,
c( f ) c( f 0 ) πQc( f 0 ) ⎝ f ⎠
(7)
where f 0 is a reference frequency. In this case we can combine the amplitude and
phase spectra and get
X down ( f ) =
Rup
Rdown
X up ( f ) exp[−
πf (t down − t up )
Q
51
+i
2πf
( Rdown − Rup )]
c( f )
=
=
Rup
Rdown
Rup
Rdown
X up ( f ) exp[−
X up ( f ) exp[−
πf (t down − t up )
Q
πf (t down − t up )
Q
+i
+i
2 f ( Rdown − Rup )
Qc( f 0 )
2 f (t down − t up )
Q
Rdown − Rup
⎛f ⎞
log⎜⎜ 0 ⎟⎟ + i 2πf (
)]
c( f 0 )
⎝ f ⎠
⎛f ⎞
log⎜⎜ 0 ⎟⎟ + i 2πf (t down − t up )] , (8)
⎝ f ⎠
where X up ( f ) and X down ( f ) are the Fourier spectra of the direct waveform
xup (t ) and the reflected waveform x down (t ) , respectively.
In this study we use equation (8) as follows. We first calculate X up ( f ) from
k
( f ) using equation (8) with an assumed Q k ,
waveform xup (t ) . Then we calculate X down
where k is the index of different trial values of Q.
In a third step we calculate
k
k
( f ) by inversely transforming X down
x down
( f ) and compare the result with the observed
waveform x down (t ) . The fitting errors between the calculated and observed waveforms
are defined as
E (k ) =
N
∑ (x
l =1
k
down
(t ) − x down (t )) 2 / N ,
(9)
where N is the number of data points in the waveforms. The minimum value of
E (k ) indicates the best fitting result and the corresponding Q k gives an estimate of Q .
4 RESULTS
4.1 Estimated Q values from stacked waveforms of multiplets
Multiplets are a set of earthquakes with similar waveforms, and by implication
they have similar locations, focal mechanisms, and ray paths to the station. Several sets
of multiplets are identified by cross-correlating the observed horizontal waveforms with
each other for all 360 events. Figure 4 shows the stacked horizontal seismograms of a
set of 7 multiplets. These seismograms are projected into the resolved fast and slow
directions of shear-wave splitting, as determined by the detailed SWS analysis of Liu et
al. (2004). As expected, the average waveforms for all these events display a high
signal-to-noise ratio. The nearly ideal impulse-like waveforms of the shear-wave phases
52
allow us to window the direct and reflected phases properly. A cosine taper is used to
reduce the effect of data truncation (Kanasewich 1981).
We calculate the amplitude spectra of the direct and reflected phases for both the
fast and slow components and show them in Figure 5a. Since the data sample rate is 50
sps, the spectra are cut off at the Nyquist frequency of 25 Hz. The energy of shear
waves is seen to be mainly distributed within the frequency range 2 – 15 Hz. We also
present corresponding results based on the data prior to and following the first S wave
(“boxes” on the seismograms of Figure 4). It appears that the amplitude spectra of the
direct and reflected S phases are sufficiently larger than the background signals within
the frequency range 2 – 15 Hz. We note that the above amplitude spectra are obtained
from stacked waveforms, which usually have a higher signal-to-noise ratio. The
amplitude spectral ratios between the reflected and direct phases for the fast and slow
components are shown in Figure 5b. As discussed in section 3, we can estimate the Q
values of the fast and slow shear waves by fitting the observed amplitude spectral ratios
versus frequency (in db/Hz) to equation (4). We fit the curves to the equation within the
2 - 15 Hz range and calculate Q values from the estimated slope m through equation
(5). The measured values are Q f = 68 ± 8 for the fast shear wave and Qs = 52 ± 3 for
the slow wave.
The main advantage of estimating Q values from amplitude spectral ratios is that
we can avoid making assumptions on geometrical spreading factor and considering the
effect of dispersion. However, the strong dependency of the estimated results on the
employed frequency range reduces from the robustness of the measurements.
Using the same waveforms, we also estimate Q f and Qs by fitting calculated
reflected waveforms to the observed ones as discussed in method (b) of section 3. The
fitting errors, defined by equation (9), with different trial Q values are shown in Figure
6a. As indicated by arrows in the figure, the minimum values of the fitting errors are
associated with Q f = 61 for the fast shear wave and Qs = 43 for the slow shear wave,
respectively. Figure 6b shows the corresponding results for best fitting waveform results.
53
This method is not strongly affected by the background noise since multiple reflections
and scattering signals have much smaller amplitudes than the direct or free-surface
reflected phases, and therefore contribute less to the fitting errors than the main phases.
As a consequence, the estimated results are insensitive to factors such as the fitting
frequency range, and are therefore relatively robust.
Figure 6a shows that the analysis of individual measurements is not very sensitive
to small changes in the estimated Q value, in our cases with relatively high Q and
corresponding relatively small attenuation. Nevertheless, we can use the outlined
procedure to derive automatically “best” estimated Q values from a large data set, and
then estimate the error of the obtained values from the standard deviation of the results.
4.2 Estimated Q values from a set of 156 recordings
There is typically a large scatter in Q measurements made from an individual
recording. This raises doubts on the reliability of Q values estimated from few
observations. In this section we employ 156 events that produce clear direct and
reflected phases in our borehole data set for additional attenuation analysis. Since these
events are located within the shear-wave window (Liu et al. 2004, and references
therein), the waves generated by them have nearly vertical ray paths when they
approach the free surface. Therefore, the reflected waves propagate to the borehole
receiver through essentially identical paths. This implies that the amplitude spectral
ratios between the reflected and direct phases for these waveforms should follow the
same relationship.
The direct and reflected phases of the employed seismograms for the fast and slow
components are windowed with a cosine taper and shown in Figure 7a. We then
calculate the amplitude spectral ratios from these phases and give the stacked results in
Figure 7b. We find that the amplitude spectral ratios for frequencies larger than 15 Hz
display a large scatter. We estimate the Q values by fitting the average amplitude
54
spectral ratios versus frequency curves to equation (4) in the range 2 – 15 Hz. The
results are Q f = 62 ± 5 and Qs = 45 ± 5 for the fast and slow shear waves, respectively.
We also estimate the Q values with the waveform fitting method for each of these
recordings. The distributions of the estimated Q values are shown in Figure 8. The
average value of Q f is 62 with a standard deviation of 11 for the fast shear wave, and
the average value of Qs is 48 with a standard deviation of 11 for the fast shear wave.
5.DISCUSSION
5.1 Attenuation in the crust
Several mechanisms have been identified to contribute to seismic attenuation and
velocity dispersion (Winkler and Murphy III, 1995). In homogeneous rocks attenuation
and dispersion appear to be dominated by viscous fluid/solid interactions. In
heterogeneous rocks, scattering can lead to dispersion and energy diffusion. A frictional
mechanism is only important at large strain amplitudes in the near field of seismic
sources. The shapes of seismograms are strongly affected by seismic attenuation. This
obscure source properties that are very important for earthquake physics studies. To
characterize the source properties of earthquakes, it is important to separate the source
from the path and site effects. Moreover, attenuation analysis also provides a tool to
probe rock properties along the ray path. Downhole experiments provide the most
reliable information on attenuation properties of the shallow crust. It is usually difficult
to estimate near-surface attenuation from surface observations because of the free
surface amplification and other complexities. To avoid the interference of near-surface
amplification, it is desirable to use clear surface-reflected waves in downhole recordings
(Hauksson et al., 1987). The results of this study, based on such surface-reflected
waves, provide robust estimates of near-surface attenuation of fast and slow shear
waves over the frequency range 2 - 15 Hz for the study area.
5.2 Body-wave dispersion
55
Dispersion of body waves is a consequence of any causal theory of absorption. Aki
and Richards (2002) show that the assumptions of constant Q and linearity of seismic
waves lead to non-causality if waves are non-dispersive. Since observed data indicate
that those assumptions typically characterize the solid earth materials, a dispersion must
exist to preserve causality of a propagating wave. From Figures 5b and 6b we see that
the curves of amplitude spectral ratio versus frequency are well represented by a linear
line in the approximate frequency range 2 - 15 Hz. This indicates that the assumption of
frequency-independent Q also characterizes well that frequency range in our data.
Various theories of dispersion (Lomnitz,1957; Futterman 1962 ; Kolsky, 1957; Liu et al.
1976) have the logarithmic dispersion form
⎛ω
c(ω1 )
1
log⎜⎜ 1
= 1+
c(ω 2 )
πQ ⎝ ω 2
⎞
⎟⎟ .
⎠
(10)
A waveform distortion due to dispersion of local earthquake records can affect
studies based on waveform shape, such as shear-wave splitting analysis. Since seismic
waves attenuate greatly when propagating through the near-surface crust, they are
expected also to be distorted by the corresponding dispersion. Figure 4 shows that the
shape of the surface-reflected phase differs significantly from that of the direct one.
Similar results can been found in the borehole observations by Hauksson et al. (1987).
To estimate the dispersion of shear waves in the top 200 m of the crust, we first
calculate the phase difference ∆Φ 'down ,up between the direct and reflected waves for both
the fast and slow components. In the calculation, the direct and reflected waves are
aligned using cross-correlation. This portion of phase change is apparently due to bodywave dispersion. The total phase change between the direct and reflected waves during
wave propagation can be written as
From equation (8), we have
56
∆Φ down ,up =
2πf
( Rdown − Rup ) .
c( f )
∆Φ down ,up = ∆Φ 'down ,up + 2πf (t down − t up ).
∆Φ down ,up = ∆Φ 'down ,up + 2πf (t down − t up ).
(11)
(12)
Thus, we can calculate the phase velocity as
c( f ) =
2πf ( Rdown − Rup )
∆Φ down ,up
.
(13)
Similarly to equation (8), we can get from equation (10)
c( f ) = c( f 0 ) +
c( f 0 )
f
log( ) .
πQ
f0
(14)
The dispersion curves extracted from the stacked waveforms (Figure 4) using
equation (13) are shown in Figure 9a. The measured phase velocities for the fast and
slow components are represented by small circles and triangles, respectively. The solid
and dashed lines in the figures give the theoretical dispersion curves based on equation
(14) with Q = Q f
and Q = Qs , respectively. The theoretical curves with
Q = Q f appears to fit well the measurements in the frequency rang 4-15 Hz for both the
fast and slow components. Beyond 15 Hz, the noise-to-signal ratio is too high to extract
stable phase information. The measured phase velocities drop rapidly below 4 Hz and
the theoretical results can not fit this portion of the data well. Similar results can be
found in Wuenschel (1965).
Figure 9b shows the impact of the dispersion on the waveform shape. The
calculated waveform without dispersion is significantly different from the observed one,
while the calculated results with dispersion improve the fit. We note that although the
fast and slow shear waves attenuate differently, they disperse almost the same and the
theoretical curves with Q = Q f (solid lines) fit the measured data for the slow wave
better than with Q = Qs . It seems that the mechanisms that result in additional
57
attenuation for the slow shear wave have very small contribution to the body-wave
dispersion.
5.3 Attenuation anisotropy
It has been observed from VSP data that transmitted amplitudes display a
systematic variation with azimuth (Liu et al., 1993; Horne and MacBeth, 1997). The
amplitude variation is commonly interpreted as attenuation that is related to the
fractures.
Anisotropic attenuation has been also observed in laboratory measurements on
rock samples containing aligned cracks (Thomsen, 1995). Attenuation anisotropy is one
of the main seismic signatures of cracks that could be used for fracture detection. Aster
and Shearer (1991) found evidence for preferential attenuation of the slow horizontal
component relative to the fast horizontal component in borehole data near the San
Jacinto Fault Zone in southern California. They suggested the existence of anisotropic
shear-wave attenuation between 150 and 300 m, which is below the top weathered layer.
They also argued that such phenomena may be partially responsible for the clear fast
shear waves and the general lack of distinct slow shear waves.
Our high quality data set provides excellent opportunity to study systematically the
attenuation anisotropy in the near-surface crust. Based on our pervious SWS analysis
(Liu et al. 2004), the horizontal shear wave can be separated clearly into fast and slow
shear wave components. We can therefore estimate separately Q values for those two
components of the split shear waves. The estimated results from the two employed data
subsets using the two methods of section 3 are listed in Table 1. The obtained values of
Q f for the fast shear wave range from 61 to 68, while the values of Qs for the slow
shear component range from 43 to 52. The attenuation anisotropy ranges from 23% to
30%. The analysis assumes that the polarization directions are the same for the sections
above and below the borehole station. As discussed in Liu et al. (2004), there is
probably a 6 degree difference between the polarization directions for these two sections.
58
However, we found that such small vertical variation of polarization direction can
change the estimated Q value only by 1 - 3, which is much smaller than the
uncertainties of the estimated results.
Because of attenuation anisotropy, the amplitude decays differently for different
polarization directions. We calculate the amplitude ratios, defined as the ratio of peak to
trough amplitude, between the direct and reflected waves in the time domain. The
results for both the fast and slow shear waves obtained from 156 events are shown in
Figure 10. The distribution of the amplitude ratios for the fast shear waves peaks at
around 0.65, while the distribution of the amplitude ratios for the slow waves peaks at
around 0.55.
6 CONCLUSIONS
Estimating the Q values from the direct and reflected seismic phases in downhole
recordings has the advantage of using signals that have the same instrument and site
responses. In addition, the analysis avoids the distortions and amplification due to the
near-surface structure and the free surface, which can complicate the measurements of
attenuation from surface seismic data.
We estimate the Q values from a set of earthquake multiplets and 156 events with
high-quality recordings using both the amplitude spectral ratio and a waveform fitting
method. The estimated value for the fast shear wave component is Qf = 61 - 68, and the
estimated value for the slow shear wave component is Qs = 43 - 52. The results reveal a
substantial difference of attenuation between the fast and slow shear wave components
and show a clear evidence of attenuation anisotropy in the near-surface structure. The
observed attenuation anisotropy may be a manifestation of microcracks alignment and
their response to in-situ stress, as is commonly assumed for the velocity anisotropy.
An attenuation-related dispersion is clearly observed and it has a significant effect
on the shapes of waveforms. The observed dispersion curves fit the theoretical
logarithm dispersion equation well in the frequency range 4 - 15 Hz. The mechanisms
59
that result in additional attenuation for the slow shear wave appear not to contribute
measurably to the body-wave dispersion.
The observed strong attenuation anisotropy (23% - 30% in this study) along with
attenuation-related dispersion is likely to characterize the near-surface structure in other
locations. These effects can modify significantly the properties of observed shear-wave
seismograms and should be taken into account in studies employing such data.
ACKNOWLEDGEMENTS
We thank Rick Aster, anonymous referee and Associated Editor Raul Madariaga
for useful comments. The research was supported by the National Science Foundation
(grant EAR-0124926) and the Taiwan Central Weather Bureau (MOTC-CWB-93-E-06).
60
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Aster, R.C. and Shearer, P.M., 1991. High-frequency borehole seismograms recorded in
the San Jacinto fault zone, Southern California, Part 2. Attenuation and site effects,
Bull. Seism. Soc. Am., 88, 1081-1100.
Bath M., 1974. Spectarl analysis in geophysics, Elsevier scientific publish company,
Amsterdam, The Netherlands, 563 pp.
Fletcher, J.B., Fumal, T., Liu, H.-P., and Carroll, L.C., 1990. Near-surface velocities
and attenuation at two boreholes near Anza, California, from logging data, Bull.
Seism. Soc. Am., 80 , 807-731.
Futterman, W.I., 1962. Dispersive body waves, J. Geophys. Res. ,67, 5279-5291.
Hauksson, E., Teng, T.-L., Henyey, T.L., 1987. Results from a 1500 M deep, three-level
downhole seismometer array: site response, low Q values, and fmax, Bull. Seism. Soc.
Am., 77, 1883-1904.
Horne, S. and MacBeth, C., 1997. AVA observations in walkaround VSPs, 67th Ann.
Internat. Mtg: Soc. Of Expl. Geophys. , 290-293.
Kanasewich, E.R., 1981. Time sequence analysis, The University of Alberta Press,
Third Edition.
Kolsky, H., 1956. The propagation of the stress pulses in viscoelastic solids, Phil. Mag.,
8, 1,673-710.
Liu, E. Crampin, S. Queen, J.,H., and Rizer, W.D., 1993. Velocity and attenuation
anisotropy caused by microcracks and macro fractures in a multiazimuth reverse
VSP, Can. J. Expl. Geophys. 19, 162-176.
Liu, Y., Teng, T.-L., Ben-Zion, Y., 2004. Systematic analysis of shear-wave splitting in
the aftershock zone of the 1999 Chi-Chi, Taiwan, Earthquake: Shallow crustal
anisotropy and lack of precursory variations, Bull. Seism. Soc. Am., in press.
Liu, H.P., Anderson, D.L. and Kanamori, H., 1967. Velocity dispersion due to
anelasticity; implications for seismology and mantle composition, Geophys. J.R.
Astron. Soc., 47, 41-56, 1976
61
Lomnitz, C., 1957. Linear Dissipation in solids, J. Appl. Phys., 28, 201-205.
Shin, T.-C. and Teng, T.-L. 2001.. An overview of the 1999 Chi-Chi, Taiwan,
earthquake, Bull. Seism. Soc. Am. 91, 5, 895-913.
Winkler K.W. and Murphy III W.F., 1995. Acoustic velocity and attenuation in porous
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Thomsen, L., 1995. Elastic anisotropy due to aligned cracks in porous rock, Geophys.
Prosp., 43, 805-829
Wu, F. T. and R-J Rau, 1998 Seismotectonics and Identification of Potential Seismic
Source Zones in Taiwan, TAO, 9, 4, 739-754
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62
Table 1. The estimated Q from two data sets using two methods
Data
7 multiplets
156 events
Method
a
b
a
b
Qf
68
61
62
62
Qs
52
43
45
48
Attenuation
24%
30%
27%
23%
anisotropy
63
Figure 1 : A location map for the study region with the Meishan fault (MSF), the
Chelungpu fault (CLF) and the Chukou fault (CKF). Solid triangles indicate
short-period seismic stations including a 200 m deep downhole station CHY.
Solid stars represent the September 20, 1999 Mw 7.6 Chi-Chi earthquake and its
two large aftershocks. Solid circles represent other small aftershocks recorded by
the borehole station CHY.
64
Figure 2 : Photos of drill core samples from a hydro-geological well #200201G1
in the study area. The total depth of the well is 250 m and the depths of core
samples are indicated in the figure. (From the Hydrogeology Data Bank, the
Central Geological Survey of Taiwan.)
65
Figure 3 : Schematic geometry for the direct and surface-reflected waves in the
borehole configuration.
66
Figure 4 : Stacked fast and slow components of horizontal shear waveforms for a
set of 7 earthquake multiplets. The vertical shaded areas indicate portions of the
seismograms used in the following attenuation and dispersion analyses, while the
horizontal boxes mark portions used for background spectral analysis in Figure 5.
67
Figure 5 : (a) Amplitude spectra of the direct and reflected shear-wave phases
and the background noise prior to and following the direct shear waves. (b)
Amplitude spectral ratios versus frequency and linear fitting to equation (4) in the
frequency range 2 – 15 Hz.
68
Figure 6: (a) Errors of fitting between the observed waveforms and the calculated
waveforms by equation (9) with different trial values of Q . The minimum error
values indicated by arrows correspond to the estimated
comparison between
Qf
and Qs . (b) A
observed waveforms and calculated ones using the
estimated Q values.
69
Figure 7 : (a) Direct and reflected windowed phases of the fast and slow shear
wave components for 156 recordings.
(b) Amplitude spectral ratios versus
frequency. The heavy solid lines represent the average values and their linear
fitting to equation (4) in the frequency range 2 – 15 Hz give estimates of
Qs .
70
Qf
and
Figure 8 : Distributions of
Qf
and Qs calculated with the waveform fitting
method for 156 recordings.
71
Figure 9 : Distributions of the amplitude ratios between the reflected and direct waves
in time domain for the fast and slow shear waves. The mean and standard deviation of
the amplitude ratios are 0.65 and 0.10 for the fast component, while the corresponding
values for the slow component are 0.55 and 0.08.
72
Paper 4
Development of Modern Seismic Monitoring in Taiwan and Progress
on Earthquake Rapid Reporting and Early Warning Systems
Ta-liang Teng1, Yih-Min Wu2, Tzay-Chyn Shin3, Yi-Ben Tsai4,William H. K. Lee1,5
and Chung Chi Liu6, Nai-Chi Hsiao2
1.University of Southern California, Los Angeles, California ([email protected])
2.National Taiwan University, Taipei, Taiwan
3. Central Weather Bureau, Taipei, Taiwan
4. National Central University, Chungli, Taiwan
5. U.S. Geological Survey, Menlo Park, California
6. Institute of Earth Science, Academic Sinica, Taipei, Taiwan
Historical Remarks
Seismic monitoring in Taiwan began in the 1897 with the installation of the first
seismic station in Taipei, it was during the period of Japanese occupation. A small
group of Milne-Ewing type 3-component instruments were later installed that formed
the basis of the early 1900s version of a seismic network. After the WWII, this network,
with some augmentation, became the Central Weather Bureau Seismic Network
(CWBSN). With 17 stations by 1968, this seismic network was operated as part of the
meteorological monitoring activities within the Central Weather Bureau (CWB). This
early version of CWBSN has provided Taiwan seismological data mainly from 1900 to
early 1970s and has sketched out the basic seismicity framework of Taiwan. In the
interim, several damaging earthquakes occurred in the Chia-nan area and in Hsinchu.
This early version of CWBSN has published regular seismic catalogs, as well as number
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of valuable special volumes after damaging earthquakes (such as the 1935 Hsinchu
Earthquake Report). Because of sparse population before the recent great economical
development, earthquake damage in Taiwan was not too expensive, but sometimes the
loss of lives was considerable because of substandard constructions.
First Stage Taiwan Seismic Monitoring Development in 1968
In the late 1960s, Taiwan organized a Long-Term Scientific Development
Commission, headed by the famed physicist Dr. Ta-You Wu. He was soon contacted
by a group of newly graduated seismologists from the U.S., and with his strong
encouragement, this group of scientists* was organized and drew up “A Long-Range
Taiwan Earthquake Research Development and Implementation Plan”, which was
approved swiftly and fund allocated by the National Science Council.
Work progressed efficiently, a Special Task Force on Earthquakes (STFE or 地震專
案小組 ) was organized, TL Teng made several trips to Taiwan in 1969 to conduct a
field site survey for possible quiet seismic stations. In Taiwan, Dr. SK Yiu assembled a
group of young geophysics graduates (YT Yeh and YH Yeh, for examples) from the
National Central University and established a local office to carry out day-to-day
administrative work, in excellent coordination with the group in the U.S. (mainly FT
Wu, TL Teng and WHK. Lee) where actual design and acquisition of modern seismic
instrumentation system were being carried out. Dr. Yiu also recruited an excellent
electrical engineering graduate CM Lo from the then Taipei Technical College, who
was sent to the U.S. to work with the group there on the seismic network system design,
data communication and recording technique, as well as all acquisition and testing of
delivered equipment. A man of total dedication, he also assembled the complete seismic
network system on the University of Southern California campus to make sure that all
components performed according to specifications before packaging and shipping the
entire system back to Taiwan. This part of preparatory work was very well done,
making the system immediately operational as soon as it arrived and installed in Taiwan
74
in late 1972. By that time, this STFE group was temporally housed in borrowed space
from the Institute of Oceanography of National Taiwan University in Taipei. A very
able seismologist, Dr. YB Tsai, a fresh Ph.D. from MIT, was recruited to head the STFE
group, who has since for more than a dozen years made major contributions in all
aspects not only during the formative stage of the STFE, but also during the later
development and expansion stage when the Institute of Earth Sciences was established
in the Academia Sinica.
Background of TTSN
While in the late 1960s, the Caltech Seismological Laboratory as well as UC
Berkeley and U.S. Geological Survey at Menlo Park, California were experimenting
with analog seismic data telemetry, the STFE has examined their results and adopted the
method of frequency division multiplexing (FDM) as its basic telemetry scheme, using
300 Hz – 3600 Hz commercial telephone circuit to transmit 7 signal channels with 250
Hz bandwidth each channel and 90 Hz guard band. Remember at that stage that, almost
all seismic stations in the world were isolated single stations each with their own clocks
and recordings, which used either smoke-paper recorders or photographic recorders.
However, after WWII, radio time synchronization became available, making the
correlation of travel times between stations much more accurate.
Since the expected total quantity of seismic equipment need for Taiwan was small,
STFE decided to out-source all equipment from the U.S. This job fell on the able hands
of an engineer CM Lo mentioned before, with the expertise help from WHK Lee who
was very rigorous in setting the instrument specifications and in examining the
performance of the delivered items. At the same time, the STFE team in Taiwan, led by
SK Yiu and later by YB Tsai, prepared the local infrastructure of field station sites and
instrument housings as well as the application and installation of telephone data
transmission links. In six months after the equipment arrival in Taiwan in late 1972, the
TTSN was up and going for test runs. Formal operation, however, started in 1973.
Early Taiwan Telemetered Seismic Network
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The newly set up modern telemetered system was named Taiwan Telemetered
Seismic Network (TTSN). It became fully operational in 1973 with published seismic
catalogs. This represents a modern mode with telemetered seismic monitoring in
Taiwan. The timing error was reduced to milliseconds level. Regularly published
seismicity catalogs were part of its products.
In late 1972, as the TTSN begun, it was at first composed of 6 stations. Later, it was
expanded into16 stations in a year. It has substantially higher gain capable of detecting
M > 2.5 events and gave location with adequate accuracy. Besides drum recording,
develocorders, multiplexed large analog tape unit, and later digital computer were used
as recording media, pretty much kept pace with the advance of technology in the world.
An advanced SRO station was installed in 1976, with 3-component long-period
instruments. By 1982, TTSN was further expanded into a 22-station seismic network.
A refined seismicity pattern of Taiwan was soon emerged, regular seismic bulletins
published, also published was an academic journal bimonthly containing scientific
contributions of TTSN, then a unit in the newly established IES (see next section).
Second Stage Seismic Monitoring Development
The success of STFE, under the able directorship of YB Tsai, was making great
progress. TTSN increased again to 25 stations by 1988. The STFE was approved to be
organized into the future Institute of Earth Sciences (IES) of Academia Sinica by 1976,
with substantially broaden scientific scope.
In the mean time, Dr. TC Shin joined the Central Weather Bureau in mid 1988 and
continued to upgrade CWBSN into a network employing all digital operations based on
a Teledyne/Geotech system, utlizing matched 3-comp S-13 short-period seismometers
for all its stations. Digital telemetry (using 9600 baud rate telephone lines) and digital
computer recording were employed.
In 1989, CWBSN has added another 10 stations. By 1990, because of the primary
function of IES was fundamental research, the daily operation of TTSN was thus
integrated into CWBSN, forming a single and much more complete digital seismic
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network. The augmented CWBSN also established a real-time data line transmitting all
signals to IES as a backup recording site. The new CWBSN has a total 65-station - an
all-digita telemetered, 3-component, high-gain short-period seismic network. This
network is the backbone of Taiwan seismicity database from then on (Figures 4 and 5).
Auxiliary Developments
During the period of 1970 -1980, several related developments deserve mention,
all having contributed to the seismic monitoring effort in Taiwan, some for special
purposes:
1. The SMART-1 strong-motion array in the Lanyang Plain.
2. The SMART-2 strong-motion array in the Hualien area.
3. The WWSSN station ANPU at a saddle point in the Tatun volcano.
4. The first downhole Seismological Research Observatory (SRO) station TATO in
the southern hillside of the Taipei basin.
5. Several large-scale portable network deployments, such as Panda Arrays, a
collaborative work with Dr. JM Chiu of Memphis University in the U.S..
A Major Undertaking of TSMIP
In 1989, Taiwan was undertaking a number of large-scale infrastructure construction
projects (Six-Year Large Construction Program). YB Tsai proposed jointly with TL
Teng to Dr. CY Tsai, then Director of CWB, an ambitious Taiwan Strong-Motion
Instrumentation Program, later becomes the world well-known TSMIP.
TSMIP consists of modern digital strong-motion instruments with large digital
memory and absolute timing, some also with telemetry capability. By-and-large, the
stringent specifications of TSMIP instrument followed closely with WHK Lee’s
untiring effort, who was mainly in charge of the TSMIP basic instrument specifications
and software development. Much of the TSMIP specifications developed by WHK Lee
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were leading the world seismic equipment industry, and become the standard
specifications of present-day strong-motion instruments.
TSMIP consists of two principal parts:
1. 700 free-field strong-motion instruments, deployed with more concentration in
metropolitan areas of Taiwan (Figure 3). A portion of the instruments have
digital telemetry capability. This dense set of stations allows snap shots of
strong-motion wave propagation to be directly observed (Figures 6 -9). This has
never been done before in the history of observational seismology. Figure 10
gives the distribution of TSMIP stations with site geology, and Figure 11 gives
the corresponding site response spectrums.
2. More than 50 structure arrays, each with 30 data channels at each structure and
recorded by two digital computers (one online and one offline) – another first in
the world when most instrumented structures in the U.S., particularly in
California, have only 9 data channels, and most of them are analog instruments
then.
Real-Time Seismology
A very important decision made in early stage of TSMIP was to make all
telemetered CWBSN stations also equipped (or co-sited) with strong-motion
instruments. Each CWBSN will then consist of (Figures 1 and 2):
1. 3-component, high-gain, short-period seismometers (Teledyne Geotech S-13)
for routine seismic monitoring.
2. 3-component, low-gain, broadband, strong-motion instruments (with 1-2 g
clipping level) for near-field recording with the foresight of future development
of earthquake rapid reporting (RRS) and early warning (EWS) systems.
EWS and RRS Developments in CWB
78
We report in the following the most recent progress on real-time seismic monitoring
in Taiwan. Particularly on:
1. Earthquake rapid reporting (RRS) system, and
2. Earthquake early warning (EWS) system.
These are developed at the Central Weather Bureau (CWB). This is done using the
telemetered signals from strong-motion instruments in the free-field (clearly, all nearfield high-gain seismic stations would only give saturated square waves useless for
waveform analysis). Taiwan today, leads the world in the development and operation of
RRS and EWS for about five years. A good number publications in scientific journals
have established this claim. Since a lot of papers have been published in these two
subjects by scientists related to CWB and IES, we only give a brief list of these papers
in the references section where interested readers can check the details. Only a brief
account of the important and practical contributions are given below.
For RRS
The CWB can offer information about one minute after an earthquake occurrence
and prediction of PGA and PGV distributions can be reported within two minutes. The
empirical relationships between PGA, PGV and earthquake loss can also be determined
about the same time. Thus, the system can achieve near real-time damage assessment in
Taiwan for earthquake emergency response operations. A summary of its results shows:
2000/01-2003/11 reported 289 events
Successful rate > 99% for ML>4.5 events
Location difference < 10 km
Magnitude difference < 0.2
Reporting time 58 +- 16 seconds
For EWS
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The CWB can achieve earthquake early reporting time about 23 sec. At the same
time, research is devoted into more accurate determination of Mw, PGA, PGV, intensity,
and damage potential of Taiwan in cases of future earthquakes. This will offer
earthquake early warning for metropolitan areas located more than 75 km from the
epicenter. A simple summary of its results shows:
23 seconds reporting time
It possible gives early warning for metropolitan areas located more than one 75
km from the epicenter.
Near-real time prediction of PGA, PGV, and strong-motion and damage
potential in Taiwan.
Additional Remarks:
The CWBSN and the TSMIP networks have both stricken a gold mine in
seismological data recovery as the consequence of the energetic earthquake sequence
leading off by the 1999 Chi-Chi, Taiwan earthquake. As of this writing, Taiwan holds
the most important ( M > 7) strong-motion, near-field data, both in quality and in
quantity, anywhere in the history of the earth. From these data, hundreds scientific
papers, including an entire issue of the Bulletin of Seismological Society of America
(2001, Volume 91, No. 5), have already been published that have substantially
advanced the science of seismology.
So, from a meager start in1897 to today, especially during the past 30 years, the
seismological monitoring and network development in Taiwan have indeed come a long
way and have made milestone contributions in observational seismology.
Some Short Notes
1. We realized at the outset that the total instrument need in Taiwan is limited (say
1000 units). It was impractical to develop our own seismic instrument
manufacturing facility.
80
2. We have developed very detailed and stringent instrumental specifications, and
have enforced that any qualifying manufacturer must meet these specifications
by subjecting its product to extensive shaking-table tests performed by WHK
Lee. Through Lee’s untiring effort, he has very important contribution in the
quality control of the TSMIP instrument program.
3. With CC Liu’s technical guidance and untiring effort of the CWB staff members
under the leadership of TC Shin (now Deputy Director of CWB), installation of
free-field instruments and structure arrays has successfully accomplished.
Acknowledgements:
Authors gratefully appreciate generous research supports from the Central Weather
Bureau and National Science Council, without which the CWBSN, TSMIP and all
related scientific work would not be possible. In the implementation of TSMIP, Dr. CY
Tsai (then Director of CWB) had the foresight and wisdom to push the program to
fruition.
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Selected References in the past ten years:
Wu, C.F., T. L.Teng, and T.C. Shin (1994) Taiwan Strong-Motion Instrumentation
Program -- Building array system and data analysis for Min-Li Elementary School,
Meteorological Bulletin, Vol. 39, No. 3,151-164.
Lee, W.H.K., T.C. Shin, and T.L. Teng (1996) Design and implementation of
earthquake early warning system in Taiwan, Proc. 11th World Conf. Earthq. Eng.,
Paper N0. 2133.
Lee, W. H. K., T. C. Shin, and T. L. Teng, (1996). Design and implementation of
earthquake early warning systems in Taiwan, Paper No. 2133, Eleventh World
Conference on Earthquake Engineering, Elsevier Science Publishers, Amsterdam.
Teng, T.L., Y.M. Wu, T.C. Shin, Y.B. Tsai, and W.H.K. Lee (1997) Development on
Earthqyake Rapid Reporting: One Minute after: Intensity Map, Epicenter, and
Magnitude, 1997 Proceedings of Meteorology and Seismology, Central Weather
Bureau of Taiwan, 781-792.
Lee, W. H. K. and T. C. Shin (1997). Realtime seismic monitoring of buildings and
bridges in Taiwan, in "Structural Health Monitoring" edited by F. K. Chang, p.
777-787, Technomic Pub. Co, Lancaster, PA.
Wu, Y. M., Y. B. Tsai, and T. C. Shin (1997). Discrimination between earthquake and
noise signals on strong motion records by artificial neural networks (in Chinese).
Meteorol. Bull., 41, 235-245.
Wu, Y. M., C.C. Chen, T.C. Shin, Y.B. Tsai, W.H.K. Lee, and T. L. Teng (1997).
Taiwan Rapid Earthquake Information Release System, Seism. Res. Lett., 68, 931943
Teng, T.L., L. Wu, T.C. Shin, Y.B. Tsai, and W.H.K. Lee (1997) One Minute after:
strong-motion map, effective epicenter, and effective magnitude, Bull. Seismo.
Soc. Am., Vol. 87, No. 5,1209-1219.
Wu, Y. M., T. C. Shin, and Y. B. Tsai (1998). Quick and reliable determination of
magnitude for seismic early warning, Bull. Seism. Soc. Am., 88, 1254-1259.
82
Lee, W. H. K., T. C. Shin, K. W. Kuo, and K. C. Chen (1999). CWB Free-Field StrongMotion Data from the 921 Chi-Chi Earthquake: Volume 1. Digital Acceleration
Files on CD-ROM, Pre-Publication Version (December 6, 1999), Seismology
Center, Central Weather Bureau, Taipei, Taiwan.
Wu, Y. M., J. K. Chung, T. C. Shin, N. C. Hsiao, Y. B. Tsai, W. H. K. Lee, and T. L.
Teng (1999). Development of an integrated seismic early warning system in
Taiwan, Terrestrial, Atmospheric and Oceanic Sciences, 10, 719-736.
Chang, C. H., Y. M. Wu, T. C. Shin, and C. Y. Wang (2000). Relocating the 1999 ChiChi Earthquake, Taiwan. Terrestrial, Atmospheric and Oceanic Sciences, 11, 581590.
Shin, T. C., K. W. Kuo, W. H. K. Lee, T. L. Teng, and Y. B. Tsai (2000) A Preliminary
Report on the 1999 Chi-Chi (Taiwan) Earthquake, Seismological Research Letters,
71, N0.1, 24 – 30.
Wu, Y. M., W. H. K. Lee, C. C. Chen, T. C. Shin, T. L. Teng, and Y. B. Tsai (2000).
Performance of the Taiwan Rapid Earthquake Information Release System (RTD)
during the 1999 Chi-Chi (Taiwan) earthquake. Seismo. Res. Let., 71, 338-343.
Shin, T. C. and T.L.Teng, (2001) An overview of 1999 Chi-Chi (Taiwan) Earthquake,
Bull. Seismo. Soc. Am., 91, No. 5, 895-913.
Wu, Y. M., T. C. Shin, and C. H. Chang (2001). Near Realtime Mapping of Peak
Ground Acceleration and Peak Ground Velocity following a Strong Earthquake.
Bull. Seism. Soc. Am, 91, 1218-1228.
Shin, T. C., F. T. Wu, J. K. Chung, R. Y. Chen, Y. M. Wu, C. S. Chang, T. L. Teng,
(2001). Ground displacement around the fault of the September 20th, 1999, Chi-chi
Taiwan earthquake. Geophysical Research Letters, 28, 1851-1654.
Wu, Y. M. and T. L. Teng (2002). A virtual sub-network approach to earthquake early
warning. Bull. Seism. Soc. Am, 92, 2008-2018
Wu, Y. M., N. C. Hsiao, T. L. Teng, and T. C. Shin (2002). Near real-time seismic
damage assessment of the rapid reporting system. Terrestrial, Atmospheric and
Oceanic Sciences, 13, 313-324.
83
Wu, Y. M., T. L. Teng, T. C. Shin, and N. C. Hsiao (2003). Relationship between peak
ground acceleration, peak ground velocity, and intensity in Taiwan. Bull. Seism.
Soc. Am., 93, 386-396.
Wu, Y. M., J. K. Chung, C. C. Chen, N. C. Hsiao, T. C. Shin, Y. B. Tsai, and K. W.
Kuo (2003). On the establishment of an automatic earthquake information
broadcast system in Taiwan, in “Early Warning Systems for Natural Disaster
Reduction” edited by J. Zschau and A. N. Kuppers, p. 461-464, Springer, Berlin
Shin, T. C., Y. B. Tsai, Y. T. Yeh, C. C. Liu, and Y. M. Wu (2003). Strong-Motion
Instrumentation Programs in Taiwan, in "Handbook of Earthquake and
Engineering Seismology" edited by W. H. K. Lee, H. Kanamori, and P. C.
Jennings, Academic Press, p. 1057-1602.
Wu, Y. M., T. L. Teng, N. C. Hsiao, T. C. Shin, W. H. K. Lee and Y. B. Tsai, 2004:
Progress on earthquake rapid reporting and early warning systems in Taiwan, in
“IUGG Special Volume on Earthquake Hazard, Risk, and Strong Ground Motion”
edited by Y. T. Chen, G. F. Panza, Z. L. Wu, Seismological Press, Beijing, p. 457480.
Wu, Y. M., N. C. Hsiao, and T. L. Teng (2004). Relationships between strong ground
motion peak values and seismic loss during the 1999 Chi-Chi, Taiwan earthquake.
Natural Hazards 32, 357-373.
Wu, Y. M. and T. L. Teng (2004). Near Real-Time Magnitude Determination for Large
Earthquakes. Accepted by Tectonophysics.
Wu, Y. M. and H. Kanamori (2004). Experiment on an onsite early warning method for
the Taiwan early warning system. Accepted by Bull. Seism. Soc. Am.
84
Station Instrument A900 & S13
Figure 1 : Instruments of typical CWBSN station, a GPS receiver is shown on
the top left.
85
Figure 2 : CWBSN Real-time Seismic Station Distribution: Weak motion
(squares) 73 stations; Strong motion (triangles) 86 stations, most co-sited on the
same pier with S-13 short-period instruments.
86
Figure 3 : Distribution of the TSMIP stations. Also shown are the mainshock and
larger aftershocks of the 1999 Chi-Chi earthquake.
87
Figure 4 : Seismicity of ML > 3 event in Taiwan between 1990- 1999. Stars show
damage earthquakes.
88
Figure 5 : Distribution of the 1999 Chi-Chi earthquake sequence.
89
Figure 6 : TSMIP instrumental intensity map of the 1999 Chi-Chi mainshock.
90
Figure 7 : Four snap shots at 14 s, 20 s, 23 s and 24 s of observed strong-motion
wave propagation of the 1999 Chi-Chi mainshock derived from the TSMIP
instrument data.
91
Figure 8 : Four snap shots at 27 s, 32 s, 43 s and 44 s of observed strong-motion wave
propagation of the 1999 Chi-Chi mainshock from the TSMIP instrument data
92
Figure 9 : A snap shot at 88 s, at a time the rupture process has long completed,
of observed strong-motion wave propagation of the 1999 Chi-Chi mainshock
derived from the TSMIP instrument data. It shows the long duration of basin
(Taipei, Lanyang and Chia-Nan) reverberations.
93
Figure 10 : Distribution of TSMIP stations and their corresponding site
classification.
94
Figure 11 : Response spectrums from TSMIP 1999 Chi-Chi data for four
different site classifications.
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Section B :
Specifications and Evaluations of Strong-Motion instruments
W. H. K. Lee
November 11, 2004
Contents
II. Instrument Specifications........................................................................................... 97
III. Instrument Evaluation .............................................................................................. 97
Appendix B1. 2004 CWB Specifications for Digital Earthquake Strong-motion
Accelerographs ............................................................................................................... 98
Appendix B2. A Preliminary Evaluation of Technical Compliance Test of a Reftek
Model 130-SMA/01 Accelerograph (S/N 9080; S/N 9164)......................................... 125
I. Introduction ............................................................................................................... 148
II. Software Development ............................................................................................ 148
III. Coherence Analysis of Multiple Co-Located, “24-bit” Strong-Motion Instruments
...................................................................................................................................... 149
References .................................................................................................................... 150
Appendix C1. Script Code for Coherence Analysis ..................................................... 151
Appendix C2. A Coherence Analysis of Data Recorded by Multiple Co-Located “24bit” Strong-Motion Instruments at the Hualien Seismic Station, Taiwan .................... 170
Abstract......................................................................................................................... 170
1. Introduction .............................................................................................................. 171
2. Recorded Earthquakes .............................................................................................. 176
3. Coherence Analysis .................................................................................................. 179
4. Comparisons of “24-bit” Strong-Motion Instruments .............................................. 180
4.1. Earthquake at 09:03 on April 3, 2004 (Event #1)...............................................181
4.2. Earthquake at 05:33 on April 9, 2004 (Event #2)...............................................182
4.3. Earthquake at 02:26 on April 23, 2004 (Event #3).............................................185
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4.4. Earthquake at 02:27 on April 23, 2004 (Event #4).............................................190
4.5. Earthquake at 15:20 on April 24, 2004 (Event #5).............................................195
4.6. Earthquake at 19:26 on April 24, 2004 (Event #6).............................................199
4.7. Earthquake at 22:29 on April 24, 2004 (Event #7).............................................204
4.8. Earthquake at 14:28 on April 25, 2004 (Event #8).............................................209
4.9. Earthquake at 07:56 on May 1, 2004 (Event #9)................................................214
4.11. Earthquake at 20:06 on May 9, 2004 (Event #11)............................................219
4.12. Earthquake at 15:28 on May 13, 2004 (Event #12)..........................................224
4.13. Earthquake at 06:04 on May 16, 2004 (Event #13)..........................................229
4.14. Earthquake at 07:04 on May 19, 2004 (Event #14)..........................................234
4.15. Earthquake at 20:25 on May 22, 2004 (Event #15)..........................................239
4.16. Earthquake at 16:56 on June 2, 2004 (Event #16)............................................244
Discussions ................................................................................................................... 249
Acknowledgements ...................................................................................................... 250
References .................................................................................................................... 250
I. Introduction
Instrumentation specifications and evaluation were performed in 2004 in support of
the CWB 2004 procurements of free-field digital accelerographs.
II. Instrument Specifications
In support of the CWB procurements in 2004, instrument specifications were written
for 24-bit digital accelerographs. These specifications are given in Appendix B1.
III. Instrument Evaluation
I received the technical proposal submitted by the Refraction Technology for
bidding the CWB 2004 digital accelerographs. A preliminary analysis of the Reftek’s
technical proposal was sent to CWB on April 6, 2003 (see Appendix B2).
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Appendix B1.
2004
CWB
Specifications
for
Digital
Earthquake
Strong-motion
Accelerographs
January 1, 2004
I. Introduction
In this fiscal year, CWB would like to purchase ___ 24-bit digital
accelerographs. By 24-bit, we mean that a 24-bit A/D chip is used in digitizing the
accelerometer signals and the accelerograph achieves 20 bits (120 dB dynamic range) or
better in the overall system performance for seismic signals in the earthquake frequency
band.
II. Required Items
For 2004, the following items are required:
(1) ___ units of 24-bit digital earthquake strong-motion accelerographs. Each unit must
be able to maintain absolute time to +/- 0.005 sec of UTC when a GPS timing
device is connected to it, and is ready for Internet access from anywhere in the
world when the unit is deployed in the field and is connected to the Internet. [See
Section IV below for specifications).
(2) ___ GPS timing devices (each with a 50-feet receiver cable) that can be used to
connect to the accelerograph for maintaining absolute time to within +/- 0.005 sec of
UTC at all times and to provide geographic location of the accelerograph. [See Item
11 of Sub-section 4 of Section IV].
(3) Recommended spare parts for Item (1) and (2) for three years operation, and a
listing of their prices. [See Section V below].
(4) A training program for installation, operation, and maintenance of Item (1) and (2).
[See Section VI].
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(5) The required accelerographs and GPS timing devices must carry 3 years' full
warranty and maintenance service (see Note 5 below).
NOTE 1: All bidders must arrange with Mr. Chien-Fu Wu (phone: 02-2-709-5603; fax:
02-3-707-3220) for the Internet access test (see Section IV.10) during the following
time period: from _______________ to _______________.
NOTE 2: A bidder must submit a report of the test results (including computer readable
data files and the required software [see Section IV.6] on floppy disks or CD-ROM) in
their proposal in support of their claims that the proposed model meets the CWB 2004
specifications (see Appendix 1). [See Note 6 for exemption].
NOTE 3: A bidder must submit the proposed model for test at the CWB Headquarters
and at the CWB Hualien Station for a field test during the following time period: from
_______________ to _______________. Details are specified in Appendix 2. [See Note
7 for exemption].
NOTE 4: All delivered units from the awarded bidder will be subjected to performance
acceptance tests as specified in Appendix 3.
NOTE 5: Full warranty for three years after the final acceptance by CWB or its
designated agent is required. This warranty must include parts and labor for fixing any
breakdown of accelerographs and GPS timing devices under normal operating
conditions in the field (i.e., anywhere in Taiwan) free of charge. Repair or replacement
must be completed within 5 working days after notification by CWB, except if any
replacement parts require importing from outside of Taiwan, an additional 10 working
days will be granted by CWB if requested.
NOTE 6: Accelerographs that were qualified in the CWB 2002 bidding of the 24-bit
digital accelerographs [Model K2 by Kinmetrics, and Model CV575C by Tokyo
Sokushin] are exempted from requirements specified in Note 2 and Note 3 above.
NOTE 7: If the bidder wins the bid with a new accelerograph, then the same tests as
specified in Appendix 1 must be repeated and witnessed by a CWB appointed observer.
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In this case, the bidder is required to give CWB a two-week advance notice for the time
and place for the repeated testing.
III. Technical Evaluation
Each bidder is required to bid an accelerograph model that are in production and
meet all the specifications listed below. The bidder should prepare in their bid proposal
a clause by clause commentary indicating compliance with each specification. The bid
proposal must contain a report of the technical tests as specified in Appendix 1. This
technical test report must contain a written account of the technical tests (including the
specs of the shaking table system used), and the recorded data files and the required
software (see Section IV.6) on floppy disks or CD-ROM. The technical tests must be
conducted in an appropriate test laboratory by the bidder at their own expenses. In
addition, the bidder must submit their recorded data at the CWB Headquarters test and
at the Hualien field test to CWB immediately after the tests, as specified in Appendix 2.
As indicated in Note 6 in Section II, accelerographs that were qualified in the CWB
2002 bidding of the 24-bit digital accelerographs are exempted from the above test
requirements. However, all bidders must arrange with Mr. Chien-Fu Wu for the Internet
access test [see Section IV.10].
The CWB's Instrumentation Advisory Subcommittee will analyze all the recorded
data files from the proposed accelerograph (and the reference unit if applicable) to
determine if the new accelerograph meets the specifications. A bid of an accelerograph
will be automatically rejected if its technical test report (with data files and required
software [see Section IV.6] on floppy disks or CD-ROM for personal computers) is not
included in the bid proposal, or if the bidder failed to provide the test data recorded at
the CWB Headquarters and at the Hualien field test. In addition, the bidder of a new
accelerograph must provide the specifications of the shaking table system used in the
technical test (see Appendix 1). If the specifications do not meet the CWB required
specs for the shaking table system, then the bid will be automatically disqualified.
However, accelerographs that were qualified in the CWB 2002 bidding of the 24-bit
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digital accelerographs are exempted from these requirements because these
accelerographs had already been subjected to and passed the CWB 2002 tests before.
Technical evaluation will be carried out in the following steps.
(1) Technical evaluation will be based on the bidders' proposals, their technical test
report (including using a shaking table system that meets the CWB specs), test data
recorded at the CWB Headquarters and at the Hualien field test, the Internet access
test, and their reputation with respect to customers' satisfaction of their
accelerograph products. Any bidder whose accelerograph failed the Internet access
test will be automatically disqualified, and any bidder who used a shaking table
system that does not meet the CWB shaking table system specs will also be
disqualified.
(2) Based on results of the technical evaluation in (1), the CWB's Instrumentation
Advisory Subcommittee will decide whether or not a given bid proposal is
technically acceptable.
NOTE 1: The exact bidding and instrument evaluation procedures are given in the
Chinese version of the “CWB (2004) 24-bit Free-Field Accelerograph Specifications”.
NOTE 2: Bidders whose accelerographs were qualified in the CWB 2002 bidding of
the 24-bit digital accelerographs [i.e., Model K2 by Kinmetrics, and Model CV575C by
Tokyo Sokushin] are exempted from the above technical evaluation if they submitted
the same 24-bit digital accelerographs.
However, these accelerographs must be
modified to include the Internet access capability, and must pass the Internet access test
to be qualified in the CWB 2004 bidding.
IV. Specifications for Earthquake Strong-Motion Accelerographs
1. General Features
The accelerograph must be rugged, compact, weighing less than 25 kilograms,
transportable over rough terrain by vehicle, and then capable of being installed and field
calibrated with a minimum amount of adjustments. The accelerograph will be installed
101
in all types of environments and should be designed to withstand extremes of humidity,
dust, and temperature, and to be waterproof [see 2.1(5) below].
After installation, the accelerograph shall remain in a standby condition until
actuated manually for test purposes or triggered by ground motions satisfying the trigger
criteria. After actuation, it shall record data for a prescribed time period, and return to
the standby condition ready to record the next event without servicing or attention.
The accelerograph must be designed for quick trouble-shooting by performing
functional tests so that a technician can locate faulty component(s) or circuit board(s)
under field conditions. A field installation site may be a simple instrument shelter in a
remote region with extreme environment conditions.
2.System Operation
The accelerograph is normally packaged in a single unit and consists of four
components: the transducers (triaxial accelerometer), a solid-state digital recorder, a
GPS receiver, and battery power supply. It must be capable of connecting by means of
a user-supplied modem to telephone lines for remote interrogation and data
downloading, and for Internet access of its recorded data files when it is connected to
the Internet [see subsection 10 below]. The case enclosing the accelerograph shall be
rugged enough to permit the accelerograph to operate after having typical non-structural,
earthquake-caused debris, such as plaster, ceiling panels, light fixtures, falling on the
unit from a height of 2.5 meters. The accelerograph must have handle(s) for ease of
carrying and facility for leveling adjustment. If necessary, the triaxial accelerometer can
be packaged separately from the recording unit.
System operation shall be such that it will automatically start recording when the
ground acceleration exceeds a preset triggering criterion. The trigger may actuate from
any selected combination of the three transducer signals.
A scheme for protected and externally visible indicator(s) must be provided to
show the event status. The memory status must be displayed upon user's interrogation
via a PC, and optionally by visible indicator(s).
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2.1 System Characteristics
(1) System Accuracy: A "static" system accuracy of +/- 0.03 g for any sensitive axis
aligned with gravity from a tilt test is required, and a "dynamic" system accuracy of
+/- 3% on a RMS basis at room temperature from a shaking table test is required.
(2) System Response: nominally flat (+/- 3 dB) from DC to 50 Hz.
(3) System Noise: The overall system noise must be less than the equivalent of 1 digital
count of a 20-bit system on a RMS basis in the seismic frequency range of 0.01 to
50 Hz.
(4) Temperature Stability: Sensitivity change due to temperature effect must be less than
+/- 0.06% per degree C for the operating temperature range (-10 degree C to 60
degree C). Similarly, zero-level change due to temperature effect must be less than
+/- 0.06% per degree C.
(5) Humidity and Waterproof: Must be able to handle high humidity (up to 100%), and
must be waterproof according to the NEMA (US National Electrical Manufacturers
Association) Standards Publication 250 for NEMA Type 6P enclosures (i.e.,
protection against the entry of water during prolonged submersion at a limited
depth), or the IEC standard IP67.
(6) Auto-zeroing of DC level: If the accelerograph has the software feature of autozeroing of DC level, the user must be able to turn it off if necessary.
(7) System DC-Level Drift in Field Operation: After removing the temperature effects
(see Item 4 above), a daily drift of less than +/- 240 digital counts (of a 20-bit
system) and a cumulative drift of less than +/- 720 digital counts (of a 20-bit system)
over a period of 5 days are required in a typical field environment (for a 2g fullscale accelerograph when auto-zeroing of DC level is turned off).
2.2. Trigger Operation
(1) Trigger Level: Selectable from 0.0001g to 0.1g of any one or more of the 3
accelerometer channels.
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(2) Trigger Frequency Response: Triggering criterion is applied only in the frequency
range from 0.1 to 12 Hz. The trigger filter's parameters must be given by the
manufacturer.
(3) Trigger Accuracy: Must be within +/-10% at 1% full-scale trigger level in the
frequency range from 0.1 to 12 Hz.
3. Transducer Sub-Unit
Orthogonally oriented, triaxial (two horizontal and one vertical) accelerometers
must be mounted internally to the recording unit.
(1) Type: Force-balance or force-feedback accelerometers.
(2) Full scale: +/-2g standard.
(3) Dynamic Range: at least 120 dB.
(4) Frequency Response: nominally flat (+/- 3 dB) from DC to 50 Hz.
(5) Damping between 0.6 and 0.7 critical damping.
(6) Accuracy: The relationship between output signal and input acceleration is to be
within +/- 1% of full scale for all frequencies from DC to 50 Hz at room temperature.
(7) Cross-axis Sensitivity: 0.03 g/g maximum; 0.02 g/g desirable.
(8) Output: Nominally +/- 2.5 volts full scale, or must match the input requirement of
the recording unit.
(9) Noise: less than 3 dB (on a RMS basis) with respect to a 120 dB system.
(10) The unit itself or its transducer unit must have the facility for tilt testing. There
must also be an adjustment so that each axis's zero-level may be reset to compensate
for non-level mounting surface (< 2 degree ) by either one of the following methods:
(i) by individual axis, or (ii) simultaneously on all three axes. A reference line
indicating each sensor's orientation and polarity shall also be provided.
(11) The unit itself or its transducer unit must have an indicator for leveling the
transducer.
(12) Calibration data (voltage per g and accurate to better than +/- 1%) for the three
internal transducers must be provided with the accelerograph.
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4.Digital Recording Sub-Unit
The recording sub-unit shall record three channels with appropriate signal
conditioning, A-D conversion, and solid-state memory. The retrieved digital data must
contain sufficient coded information to enable proper and complete decoding of the data
by the retrieval system using supplied program(s). The format of this recorded digital
data shall be in a form suitable for rapid data reduction by modern computer methods
and existing standard computer systems. Absolute timing to within +/- 5 msec of UTC
must be maintained at all times by the accelerograph if the GPS timing device is used.
In the event of losing the external GPS timing signal, the accelerograph must be capable
of maintaining absolute timing with a drift of less than +/- 26 milliseconds per day.
(1) Filtering: Anti-aliasing filter must be provided suitable for the maximum sampling
rate (see item 3).
(2) Analog Channel-to-Channel Sampling Skew: The channel-to-channel sampling must
be completed within 10% of the sample rate in a known fixed manner so that
corrections can be applied.
(3) Sample Rate: 200 samples/sec/channel.
(4) Pre-event Data Storage: 0-30 seconds, selectable in steps of 1 second by software.
(5) Recording Type: Digital, solid-state memory and/or IC memory card.
(6) Resolution: 20 bits or better.
(7) Noise: less than 3 dB with respect to a 120 dB system (on a RMS basis) when the
signal input is shorted.
(8) Full Scale: Matching that of the output of the accelerometer.
(9) Total Recording Capacity: At least 180 minutes of recording time at 200 samples per
second for 3 channels.
(10) Removable Recording Device: A removable recording device (e.g., a PC-standard
removable memory card) of at least 20 megabytes must be provided for ease of
data transfer to a PC for data processing.
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(11) Absolute Time and Location: A GPS device is required to provide geographical
location and absolute time to within +/- 0.005 sec of UTC at all the time by the
accelerograph. Data acquisition must not be interrupted by GPS timing adjustments.
In the event of losing the external GPS timing signal, the accelerograph must be
capable of maintaining absolute timing with a drift of less than +/- 26 milliseconds
per day.
(12) Coded Information: In addition to the recorded acceleration data, all relevant
instrument parameters are to be recorded in a header for each event. These items
include (but are not limited to): (a) the instrument's serial number, (b) the day and
time as synchronized by a servicing technician or as received from an external time
code, and (c) coded indicators for any options (gain, etc.) that are preset at the
factory, and would be required for processing the data.
(13) IASPEI Software Compatibility: Recorded data must be either written directly in
the PC-SUDS format, or a format conversion routine must be provided for
conversion to the PC-SUDS format. The PC-SUDS format is required so that the
recorded data are compatible with the IASPEI Software Library (jointly published
by the International Association of Seismology and Physics of the Earth's Interior
and the Seismological Society of America; see Sub-Section 6. “Required
Software” below).
(14) Post Event Shut Off Delay: The system shall continue to record for 10 to 60
seconds (selectable in steps), after the signal drops below the trigger level.
(15) Facility for field calibration must be provided and described.
(16) At least 2 serial ports must be provided: Port #1 provides direct or external modem
(supplied by the user) communications for setup and/or download data; Port #2 is
dedicated to realtime digitized data stream output as specified in Section VII.
(17) Realtime digitized data stream in 16-bit data format: The system must be able to
provide (on a dedicated serial port) a serial stream of digitized 3-component
ground acceleration data at 50, 100, or 200 (user selectable) samples per second
per channel for transmission by hardwire or a suitable modem (supplied by the user)
106
to a receiving station of the USGS Digital Telemetry System for realtime operation
at all time. The digitized data at 50 or 100 samples per second per channel may be
derived from decimation of the 200 sampling rate data. Suitable anti-aliasing
filtering to 50 or 100 samples per second is required. A mating connector to the
realtime digitized data stream must be provided (see Section VII below). Please
note that the 16-bit realtime data stream format is required in order to be
compatible with the existing CWB telemetry system.
5. Power Supply
The accelerograph shall operate from an internal battery that can be charged either
from solar cells or from an 110V +/- 20% AC power source. The accelerograph must
meet the following requirements:
(1) Internal Battery: 12 volt rechargeable, sufficient to operate the system on standby
for a minimum of 36 hours with the GPS timing device (or for a minimum of 48
hours without the GPS timing device) and then record for 90 minutes without
external power source for charging.
(2) If the external power source for the accelerograph were cut off by more than 36
hours, then the accelerograph must be able to restart automatically and function
properly after the external power source is restored.
(3) Supplemental Power: The accelerograph shall be configured so that an auxiliary
external 12 Vdc power source may be connected in such a way as to add to the
Amp-hour capacity of the internal battery.
(4) Because a rechargeable battery can create a safety hazard in a waterproof
accelerograph as hydrogen gas can accumulate and cause an explosion, the
accelerograph must have a safety device (e.g., breather valves) to guard against this
safety hazard.
6. Required Software
There are two main categories of required software.
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(1) Instrument Firmware: The instrument's firmware program consists of the code
(normally embedded in EPROMs) to perform the basic functions of recording and
retrieval of earthquake records. Internal data recording format must be able to
store 24-bit data samples and should be clearly described. Other important
functions are event triggering and pre-event memory control. Also, the programs
normally allow the user to examine and set the instrument's operating parameters,
and perform important diagnostic functions. They should be upgradeable. In
addition, a user must be able to select either the required 16-bit data stream output,
or the manufacturer’s 24-bit data stream output of its internal recorded data.
(2) External Support and Communications Programs: These programs must run on a
typical personal computer (running under either Microsoft Windows or DOS), and
provide the user interface to the instrument. They must support remote
communications via telephone, including Internet access of the recorded data either
via anonymous FTP or by the TCP/IP based software provided by the manufacture.
They are also used to retrieve the data and display it. The display of earthquake
records should be able to be accomplished with a minimum of processing. A
stand-alone utility program to convert the 24-bit recorded data (if it is not written
directly in the PC-SUDS format) to the standard PC-SUDS format for IASPEI
software compatibility must be provided. IASPEI Software (executable code and
source code) packages are published jointly by the International Association of
Seismology and Physics of the Earth's Interior and the Seismological Society of
America. They are available for sale from the Seismological Society of America,
201 Plaza Professional Building, El Cerrito, CA 94530, USA (Phone: 1-510-5255474; Fax: 1-510-525-7204).
7. Interconnection with Other Identical Accelerographs
The accelerograph shall be capable of being interconnected for common timing
and common triggering with identical accelerographs. When interconnected, a trigger
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signal from any one accelerograph shall cause simultaneous triggering in all
interconnected accelerographs.
8. Ancillary Requirements
A convenient means for system calibration and checkout shall be provided. The
calibration of the total system for sensitivity shall be possible by a physical tilt test.
Operability of the total system shall be possible by application of functional test
voltages under software control which stimulate the accelerometer mass, permitting the
determination of the damping and frequency response of the system. In addition, testing
and data retrieval shall be performed with a typical personal computer (running under
either Microsoft Windows or DOS).
Remote interrogation shall be possible so that parameters of the data, including
event count, battery voltages, amount of memory used, and accelerogram parameters
(such as peak value and trigger-time) shall be available via telephone.
A manual shall be provided with complete description in full detail of all
operational characteristics and of all adjustments or options capable of being made in
the factory, in the shop, and in the field. The manual must be sufficiently clearly
written that a trained electronic technician in a shop along with the manufacturer's
recommended test equipment could thoroughly test out every operating feature of the
system and therefore be in a position to judge whether (1) repairs or adjustments are
necessary to bring the system up to the required specifications or (2) a return to the
factory is necessary. The manual must contain a complete and detailed description of
the format of the recorded data. The factory calibration data for individual components,
including those for the transducers, filters, and clocks, shall be provided.
9. Training and Support
The seller must provide a training course at CWB, Taipei, Taiwan. The training
program must provide sufficient instruction on the installation, operation, maintenance
and repair of the accelerograph. The course must also include sufficient instruction on
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the installation and operation of all provided software and timing systems. The maker
must supply a copy of their course outline within one month after signing of the contract.
10. Internet Access Capability
The proposed accelerograph must have the Internet access capability; i.e., when the
unit is deployed in the field and is connected to the Internet, data recorded by the
accelerograph must be accessible from anywhere on the Internet for downloading the
recorded data files in near real time either via anonymous FTP or by the TCP/IP based
software provided by the bidder. The test for the Internet access capability must be
performed by all bidders with an arrangement with Mr. Chien-Fu Wu (phone: 02-2-7095603; fax: 02-3-707-3220) within the specified time period given above [see Note 1 of
Section II]. A bidder must first set up the proposed accelerograph and connect it to the
Internet at a site with telephone communication. He then arranges with Mr. Chien-Fu
Wu to set up the necessary software (if necessary) in a PC at CWB that is connected to
the Internet. When the bidder is happy with the connection (both Internet and telephone
communication), he requests a formal test. Mr. Wu will then instruct the bidder to tell
the person at the accelerograph site to start recording and to tap the accelerograph at
certain time intervals to generate sudden “pulses”. The recorded file (typically 1 minute
in length) should appear for download either via anonymous FTP or by the bidder’s
TCP/IP based software in near real time (i.e., within 2 minutes after the recording
ended). The downloaded file should be plotted by the bidder using his software to show
that “pulses” did occur at the specific times given over the telephone. A bidder will be
automatically disqualified if the trigger recorded data files can not be downloaded and
shown to have the specified “pulses” after 3 formal requested trials. We realize that
there can be Internet problems beyond the bidder’s control. Therefore, a bidder should
check out everything at CWB first before requesting a formal test.
V. Recommended Spare Parts for Three Years Operation
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The bidder must quote the recommended spare parts with an itemized price list,
valid and firm for one year after the contract is signed, needed for the 3-year operation
of the delivered accelerographs.
VI. Specifications for Training and Support
CWB specifications for training at CWB have been given in Subsection 9 of
Section IV above. The seller must provide the training free of charge as follows:
On-site training of CWB staff (20 maximum) and demonstration of installation,
operation, and maintenance for the accelerographs and related items in Taiwan are
required during the period in which the Post Award Performance Acceptance Tests are
conducted.
VII. Specifications for Realtime Digital Data Stream Output in 16-bit
Format
The proposed accelerograph must have two user selectable realtime digital data
stream output formats: (1) a 24-bit format with time tag as designed by the manufacturer,
and (2) the 16-bit data format as specified below. Bidder must provide the 24-bit format
in detail in the proposal.
In order to be compatible with existing accelerographs in CWB, digital data are to
be streamed out in packets immediately upon completion of a sample scan of all three
channels
by
the
accelerograph.
The
output
rate
is
50,
100,
or
200
samples/channel/second (user selectable by either hardware jumpers or software
commands) at 4800, 9600, or 19200 baud, respectively, and each sample packet consists
of eight bytes with the following format:
Byte No.
1
Description
Sync character (user programmable)
111
2
Most significant byte (MSB) of first channel (16-bit) data
3
Least significant byte (LSB) of first channel (16-bit) data
4
Most significant byte (MSB) of second channel (16-bit) data
5
Least significant byte (LSB) of second channel (16-bit) data
6
Most significant byte (MSB) of third channel (16-bit) data
7
Least significant byte (LSB) of third channel (16-bit) data
8
Auxiliary data byte for timing and error checking
This realtime digital data stream output must be 100% compatible with the USGS
Realtime Digital Telemetry System when the XRTPDB program (published in the
IASPEI Software Library Volume 1; See Sub-Section 6. “Required Software” of
Section IV above) is used for realtime data acquisition of the accelerograph.
NOTE 1: The Auxiliary data byte (8 bits) should be used as follows: (1) the 0th to 5th
bit are used for parity error checking of the six data bytes, (2) the 6th bit may be used
for message if necessary, and (3) the 7th bit may be used for timing if necessary.
NOTE 2: The realtime digital stream output must not be interrupted when the
accelerograph is performing its normal functions.
NOTE 3: IASPEI Software (executable code and source code) packages are published
jointly by the International Association of Seismology and Physics of the Earth's
Interior and the Seismological Society of America. They are available for sale from the
112
Seismological Society of America, 201 Plaza Professional Building, El Cerrito, CA
94530, USA (Phone: 1-510-525-5474; Fax: 1-510-525-7204).
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Appendix 1.
Technical Tests to be Conducted by a Bidder for a Proposed Accelerograph
“Technical Tests” for a proposed accelerograph must be conducted in an
appropriate laboratory by the bidder at their own expenses and must include the
following tests. The shaking table system used for the Section 1 tests must be at or
exceed the CWB specifications [see Note 1 below]. Otherwise, the bidder will be
automatically disqualified.
A report describing the “technical tests” and results must be included in the
bidder's proposal. In addition, the recorded acceleration data, the recorded displacement
data if applicable, and the required software [see Section IV.6] must be provided as
computer readable files on floppy disks or CD-ROM for personal computers running
under Microsoft Windows or DOS).
Failure to submit the technical test report
(including the specified data files on floppy disks or CD-ROM) with the bid proposal
will lead to automatic rejection of the bidder's proposal. However, bidders whose
proposed accelerographs had been qualified in the 2002 CWB bidding of the 24-bit
digital accelerographs are exempted from these required technical tests.
1. System Response to Vibration
An accelerograph must be subjected to the shaking table tests using a proper
shaking table system [see Note 1 below]. The accelerometers used to monitor the shake
table (which must be separate from that in the accelerograph) may be used as the
reference. The bidder must also record the time history of the shake-table displacement
with a suitable displacement sensor (+/- 1% accuracy or better) for test (7) below. The
recorded data must be submitted as computer readable files on floppy disks or CDROM, with software to convert the recorded files to the standard PC-SUD format.
Input signals for the shake table are:
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(1) 1 Hz, 0.1 g sine waves for 60 seconds in x-direction,
(2) 1 Hz, 0.1 g sine waves for 60 seconds in y-direction,
(3) 1 Hz, 0.1 g sine waves for 60 seconds in z-direction,
(4) 10 Hz, 0.1 g sine waves for 60 seconds in x-direction,
(5) 10 Hz, 0.1 g sine waves for 60 seconds in y-direction,
(6) 10 Hz, 0.1 g sine waves for 60 seconds in z-direction,
(7) 1 Hz, 3 mm displacement "steps" in one direction (with 25 msec to 30-msec
rise time for “rounding” the step corners) for 60 seconds.
2. System Static Accuracy
The static accuracy of an accelerograph can be determined by a tilt test of the
accelerograph on a tilt table. A precision tilt table (with better than 0.1 degree tilt
control) must be used. Data must be recorded for 60 seconds each for the following tilt
angles: 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, and 360 degrees, and
submitted as computer readable files on floppy disks or CD-ROM.
3. Digitizer Performance
A bidder may choose one of the following two choices for testing digitizer
performance: either 3A. Sandia Test, or 3B. The CWB 2002 Test.
3A. Sandia Test
Digitizer performance is to be tested according to the Modified Noise Power Ratio
test as described in Sandia National Laboratories technical report SAND 94-0221,
"Modified Noise Power Ratio Testing of High Resolution Digitizers", by T. S.
McDonald, 1994. This report is available as SANDIA94.PDF from Mr. Chien-Fu Wu
upon request.
The test involves driving two identical digitizer channels with pseudo random,
band limited, Gaussian noise and measuring the noise power ratio (NPR), defined as the
ratio of the RMS input noise to the RMS non-coherent noise floor (both averaged over
115
the digitizer pass band). The resolution is estimated indirectly by comparing the NPR as
a function of RMS input noise against ideal digitizers.
Vendors are required to provide a plot of NPR in decibels versus loading factor in
decibels compared with theoretical curves for ideal digitizers of varying dynamic ranges
(i.e., number of bits). The loading factor is the ratio of the digitizer clip level to the
RMS input noise. The NPR must be determined at RMS input levels between the RMS
shorted input and clipping in 10 dB steps. Vendors are also required to provide a plot of
shorted input power in decibels versus frequency and at least one plot of the phase of
the non-coherent noise in degrees versus frequency. Both plots must including at least
the frequency band 0 < f < 50 Hz.
3B. The CWB 2002 Test
(1) Noise Test: The inputs to the digitizer are shorted and the system noise is recorded
for 300 seconds by the accelerograph as a computer readable file and to be
submitted with the report. The recorded system noise should be less than the
equivalent of 1 digital count of a 20-bit system on a RMS basis in the frequency
range of 0.01 to 50 Hz.
(2) Full-Scale Clip: A voltage calibrator is connected to the inputs and the full-scale clip
level of the digitizer on each channel be recorded for 10 seconds each by the
accelerograph as a computer readable file (to be submitted with the report). This
allows the full-scale accuracy to be verified.
(3) Filter Performance Verification: A swept sine is applied to the inputs of the digitizer
to test the amplitude and phase response of the digitizer and be recorded for 60
seconds by the accelerograph as a computer readable file (to be submitted with the
report). Accelerographs using over sampling techniques will demonstrate the
performance of the DSP filter, while more classical digitizers will demonstrate the
performance of the analog anti-aliasing filter.
(4) Frequency Response Spot Tests: Apply a sine wave of very high spectral purity,
record 60 seconds by the accelerograph as a computer readable file (to be submitted
116
with the report). CWB will examine the recorded data for noise that should not
degrade a 20-bit system to less than 114-dB dynamic range.
4. Utility Software
The manufacturers must provide utility software perform the following functions
for their proposed accelerograph with their bid proposal:
(1) Operate the unit and set the instrument parameters, including the timing system.
(2) Retrieve data from the accelerograph.
(3) Display the retrieved data.
(4) If the accelerograph does not write data in the PC-SUDS format directly, then
conversion software must be supplied to convert the data to the PC-SUDS format
for test of IASPEI software compatibility.
NOTE 1: The bidder must perform the test for “system response to vibration” using a
proper shaking table system that must meet the following specifications:
(1) The shaking table system must be able to carry the load of the 24-bit accelerograph
to be tested plus the weight of all other monitoring sensors on the shake table, and
must be capable of shaking up to +/- 0.2g at 1 Hz.
(2) The shaking table system must be equipped with a reference 3-component
accelerometer and at least one displacement gauge (e.g., a LVDT displacement
transducer) along the active shaking axis to monitor the shake table motion.
(3) The shaking table system must have a data logger of 24-bit resolution and capable of
sampling at 200 samples per second. We recommend that the bidder uses another
unit of the bid 24-bit accelerograph as the data logger to record the output of the
reference accelerometer and displacement gauge.
(4) The shaking table system must be capable of faithfully carrying out the 7 specified
input signals as specified in Section 1 of this Appendix.
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(5) The shaking table system must be able to faithfully record the time history of the
displacement of the shake table using a proper displacement gauge with an accuracy
of better than +/-1% for small displacements in the millimeter range.
(6) The time history of the reference accelerometers and of the displacement gauge must
be recorded with a data logger that is time synchronized with the accelerograph
under test. If a 3-channel data logger is used for the reference 3-component
accelerometer and the displacement gauge, the bidder may substitute one channel of
the accelerometer output (in the direction that is not active in shaking) by the out put
of the displacement gauge.
(7) If a uni-axial shaking table system is used, then the accelerograph must be mounted
so that every axis (i.e., x, y, or z) can be tested along the active axis in turn.
(8) A detailed description of the shaking table systems used for the technical tests must
be provided by the bidder in their technical report, including specs of major subsystems (i.e., the manufacturer, the model number, and its technical performance
specifications). Failure to include this information will lead to automatic rejection of
the bid.
Please note that any bidder not using a proper shaking table system (i.e., whose
performance does not meet the above CWB specifications) will be automatically
disqualified.
NOTE 2: IASPEI Software (executable code and source code) packages are published
jointly by the International Association of Seismology and Physics of the Earth's
Interior and the Seismological Society of America. They are available for sale from the
Seismological Society of America, 201 Plaza Professional Building, El Cerrito, CA
94530,
USA
(Phone:
1-510-525-5474;
118
Fax:
1-510-525-7204).
Appendix 2.
Test at the CWB Headquarters and at the CWB Hualien Station
Bidder must contact CWB 10 days before the closing date for bidding to arrange a
schedule for testing at the CWB Headquarters (64 Kung Yuan Road, Taipei), and at the
CWB Hualien Station (24 ______ Street, Hualien).
Bidder must transport the proposed accelerograph to the CWB Headquarters and to
the CWB Hualien Station at their own expenses. All accelerograph operations must be
conducted by the bidder, under monitoring by CWB. A copy of all the recorded data
must be provided to CWB immediately after the test.
However, bidders whose proposed accelerographs had been qualified in the CWB
2002 bidding of the 24-bit digital accelerographs are exempted from these required
technical tests.
I. Test at the CWB Headquarters (1 day):
(1) Tilt table test: at tilt angles of 0, 30, 60, 90, 135, 180, 210, 270, 315, and 360
degrees. Record at least one minute when the accelerograph is at each tilt angle.
(2) RTD (16-bit realtime data stream output) test: Bidder must provide a 2-meter or
longer RS-232 output cable with a 25-pin connector for connecting to CWB’s
realtime time system. Record at least 5 minutes with occasionally shaking of the
accelerograph to simulate an earthquake.
II. Test at the CWB Hualian Station (14 days):
(1) The bidder must set up their proposed accelerograph with GPS timing on the
seismic pier at the CWB Haulien station for a field test of approximately 14 days.
(2) Continuous recording for at least 3 hours or until the memory is full. Provide a copy
of the recorded data to CWB immediately.
119
(3) Set trigger level at 0.0005g for all 3 seismic channels, and set trigger recording
whenever any one of the 3 seismic channels exceeds 0.0005g, with 30 seconds of
pre-event and 30 seconds of post-event recording, and synchronize the
accelerograph clock with GPS timing. Leave the accelerograph at the Hualien
seismic pier for approximately 14 days, and provide a copy of the recorded data to
CWB at the end of the test.
120
Appendix 3.
Post Award Performance Acceptance Tests
Criteria for Acceptance
The basic question is: how does one know that an accelerograph is functioning
properly and meets the technical specifications? By shaking an accelerograph on a
shake table, one can find out if it is functioning correctly and by analyzing the recorded
data, one can determine if it meets the important technical specifications.
Tests for All Accelerographs
If any accelerographs fail to meet any one of the following tests, besides any
applicable penalty clause in the contract, it will be returned to the supplier for repair or
replacement until it passes all the tests.
1. Visual Inspection
All accelerographs will be visually inspected for damage and other imperfections:
(i) Verify that there is no damage to the case, with particular attention to the connectors
and latches; (ii) Generally inspect the visible portions of the accelerograph for evidence
of damage; and (iii) Verify that all items on the packing list are included in the shipment.
An acceptable unit must not have any obvious imperfections. Report any damage
or discrepancies to the supplier's representative. Make notes of any damage during
shipment for use in preparing possible claims against the shipping carrier.
2. Power/Charger Test
Each accelerograph will be connected to its AC power charger and allowed to
charge the internal backup battery for a period of 24 hours with the accelerograph
turned off.
121
After the charging period, the accelerograph will be turned off and its battery cable
disconnected. For an acceptable unit, its open circuit voltage should be 12.9 Vdc +/- 1.3
V at 24 hours later.
3. Tilt Test
The accelerograph to be tested will be mounted flat on a precision tilt table, and the
accelerograph will be tilted to various angles. An error of not more than +/- 0.03 g for
the sensitive axis aligned with gravity is required. An additional error of +/- 0.03 g is
allowed for cross-axis effects if applicable.
4.Shake Table Test
All accelerographs (after charging 24 hours) will be placed on a shake table for
shaking tests. The CWB shake table can accommodate 1 accelerograph at a time and
shake along one horizontal direction. Input signals for the shake table are: (1) 1 Hz, 0.1
g sine waves for 60 seconds, (2) 10 Hz, 0.1 g sine waves for 60 seconds, and (3) 1 Hz, 3
mm rounded displacement "steps" (with 25 msec to 30 msec rise time).
An acceptable accelerograph must be able to record all the input test signals, and
must record a time history for any test signal that is within +/- 3% of the signal recorded
by the reference accelerometer for the sine waves (on an RMS basis and adjusted for
sampling time difference), and within +/- 10% of the displacement measured by the
displacement gauge.
III. Tests for Randomly Selected Accelerographs
If any randomly selected accelerograph fails to meet any one of the following tests,
besides any applicable penalty clause in the contract, the supplier is required to correct
the problem(s) for all accelerographs.
1. Power Consumption
122
Three randomly selected accelerographs will be charged for 24 hours with the units
turned OFF. The units will then be disconnected from their AC power chargers and
placed in their acquisition mode. After being allowed to operate for a period of 48hours
(with GPS timing device off) from the backup battery, the accelerographs will be
triggered to record for 90 minutes.
An acceptable accelerograph (with GPS timing device off) must be able to operate
for 48 hours off the backup battery and then record for 90 minutes. Similar test may be
performed on selected accelerographs with the GPS timing device. In this case, these
accelerographs must be able to operate for 36 hours off the backup battery and then
record for 90 minutes.
2. GPS Timing
Three randomly selected accelerographs will be checked for GPS timing against an
external UTC timing device for several times during a day according to the supplier's
procedure. An acceptable accelerograph must be able to maintain time within +/-5
milliseconds of UTC at all the times. In the event of losing the external GPS timing
signal, the accelerograph must be capable of maintaining absolute timing with a drift of
less than +/- 26 milliseconds per day.
3. DC-Level Drift
Three randomly selected accelerographs will be set up for DC-level drift test. The
auto-zeroing feature will be turned off and data will be collected several times every day
for 5 days in an outdoor environment. After temperature effects are removed, an
acceptable accelerograph must have an average DC-level drift (with respect to a 20-bit
system) of less than +/- 240 counts per day and a cumulative drift of less than +/- 720
digital counts over a period of 5 days in a typical field environment for the 2g full-scale
accelerograph when auto-zeroing of DC level is turned off.
4. Trigger Level
123
Three randomly selected accelerographs will be placed on the CWB's small shake
table. Verify that the trigger level is within +/-10% of the technical specifications.
5. Interconnection
The supplier will demonstrate that data can be download via direct wire and
telephone/modem (supplied by the user) connection, and that the software performs as
specified in the technical specifications.
6. Recording Sub-Unit Noise
The technical specifications for the recording sub-unit call for “noise less than 3
dB with respect to a 120 dB system (on a RMS basis) when the signal input is shorted”.
To test this requirement, three randomly selected accelerographs will be subjected to the
following test. By disconnecting the sensors from the analog input board and shorting
the input pins together, the noise of the recording unit will be recorded for 10 minutes.
The noise should be less than 1 LSB as measured on a RMS basis in the frequency
range 0.01 to 50 Hz for a 20-bit system.
7. Other Tests
CWB may choose to perform additional tests for some randomly selected
accelerographs to verify that the units meet the technical specifications.
124
Appendix B2.
A Preliminary Evaluation of Technical Compliance Test of a Reftek Model
130-SMA/01 Accelerograph (S/N 9080; S/N 9164)
by
W. H. K. Lee
Submitted to the CWB Instrumentation Committee
March 29, 2004
125
1. Introduction
This evaluation report summarizes a preliminary analysis of the data obtained in a
technical compliance test conducted on a Reftek model 130-SMA/01 accelerograph
(S/N 9080; S/N 9164).
A technical report was submitted to CWB by Datatek, Inc., Reftek’s representative
in Taiwan. It contains a description of the technical tests conducted by the bidder
according to the “2004 CWB Specifications for 24-bit Digital Earthquake StrongMotion Accelerographs”, with data files and software given on a CD-ROM, as well as
some results with plots.
Due to time limitation, I concentrated on analysis of two sets of data: (1) the shake
table tests conducted on the S/N 9080 unit at the ANCO Engineers, Inc., Boulder,
Colorado, USA, and (2) the tilt table tests conducted on the S/N 9164 unit at the CWB
Headquarters, Taipei, Taiwan.
2. Shake Table Tests
According to the ANCO report given in the technical report submitted by the
bidder, shake table tests were conducted on the ANCO R-1 vector biaxial hydraulic
shake table with the following characteristics:
Maximum load: 1500 pounds
Table size: 24 inches by 24 inches
Maximum displacement: ± 5 inches
Maximum velocity: 30 inches/sec
Maximum acceleration: Up to 2 g; ±0.5 g at 1 Hz.
The R-1 tabletop motion is guided by high-precision, linear bearings and is
controlled by a Shore-Western analog servo-controller. For the vibration tests of the
Reftek accelerograph, the shake table was oriented for single axis, horizontal motion.
Motion on the shaking table is monitored by three Dytran accelerometers and a Celesco
126
displacement transducer. These reference sensors were recorded on a separate Reftek
accelerograph unit (S/N 9180).
A total of 7 shaking tests were conducted (1 Hz and 10 Hz sine waves along the X-,
Y-, and Z-component, respectively, and 1 Hz, 3mm displacement steps). Because the
step displacement test was not clearly written in the CWB Specifications, a cumulative
step-motion test was conducted for 60 seconds, instead of repeating the same 1-sec step
for 60 times. Actually, a cumulative step test is more demanding on the accelerograph
than a repeated step test.
IASPEI Software (Lee, 1994a, b, c) were used in processing and analyzing the
recorded data.
In particular, the program SeisGram (Lomax, 1994) was used to
compute and plot all the figures shown below. Since the shake motions are large, I
analyzed the data for both 24-bit and 16-bit formats (the 16-bit files were created by
truncating the least significant 8 bits). The results are essentially the same, and for
plotting convenience, I will show the results from using the 16-bit files. Furthermore, I
also used a totally different program written by Doug Dodge (based on the Livermore’s
SAC software) to verify the result of a double integration of acceleration to
displacement.
2.1. Shake Test with 1-Hz Sine Waves
1 Hz sine waves (amplitude of about 0.1 g) were used as input motions to the
shake table. Figures 1, 2, and 3 show the recorded data and their corresponding spectra
of the Reftek accelerograph for the X-, Y-, and Z-component, respectively. Due to
mechanical noises on the shake table and the fact that the frequency response of the
accelerograph extends to 100 Hz (instead of 50 Hz), the 1Hz sine waves were not purely
recorded. Nevertheless, the Reftek accelerograph recorded similar spectra as other
accelerographs that CWB has purchased (see CWB Instrumentation Committee, 1993).
2.2. Shake Test with 10-Hz Sine Waves
127
10 Hz sine waves (amplitude of about 0.1 g) were used as input motions to the
shak table. Figures 4, 5, and 6 show the recorded data and their corresponding spectra
for the Reftek accelerograph for the X-, Y-, and Z-component, respectively. Due to
mechanical noises on the shake table and the fact that the frequency response of the
accelerograph extends to 100 Hz (instead of 50 Hz), the 1Hz sine waves were not purely
recorded. Nevertheless, the Reftek accelerograph recorded similar spectra as other
accelerographs that CWB has purchased (see CWB Instrumentation Committee, 1993).
128
Figure 1 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 1-Hz sine-waves shake test along the X-component.
129
Figure 2 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 1-Hz sine-waves shake test along the Y-component.
130
Figure 3 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 1-Hz sine-waves shake test along the Z-component.
131
Figure 4 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 10-Hz sine-waves shake test along the X-component.
132
Figure 5 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 10-Hz sine-waves shake test along the Y-component.
133
Figure 6 : Recorded acceleration data and the corresponding FFT spectrum of the
Reftek accelerograph under a 10-Hz sine-waves shake test along the Z-component.
134
2.3. Shake Test with a Cumulative 1-Hz 3 mm Steps
1-Hz 3 mm displacement steps was applied to the shake table horizontally (along
the Y-component of the Reftek accelerograaph) cumulatively for 60 seconds. This
cumulative step test is more demanding than a simple repeated 1-Hz step test for 60
times. Therefore, it is not surprising that a simple double integration (with just mean
removal) of the recorded acceleration data does not produce the displacement steps
adequately.
After discussing this issue with John Evans and Doug Dodge, I applied a band-pass
filtering to the recorded data with a very broad frequency band, from 0.001 Hz to 50 Hz.
The filter used is a 4-pole Buttesworth filter available in the SeisGram program (Lomax,
1994).
The top frame in Figure 7 shows a 2-second segment of the recorded
acceleration (about 30 second after the test started). The second frame shows the
filtered acceleration.
The third frame shows the velocity obtained by a simple
integration, and the fourth frame shows the displacement obtained by one more
integration.
Figure 8 shows a comparison between the displacement obtained by double
integration (top frame) and the displacement recorded by the Celesco displacement
transducer (recorded on Channel 4 of a separate Reftek unit S/N #9181) in the bottom
frame. Although the time scale is the same for both frames, the displacement scales in
these two plots are not identical. The reason is that I did not have enough time to
convert the recorded data to the proper physical units.
If I take the bit weight and sensitivity data given in the bidder’s report for Channel
3 (Y-axis) of Unit S/N 9081: 818x10^-9 volts/count, and 1.18 volts/g, then the
displacement from double integration for the step is 3.02 mm. Similarly, for Channel 4
(Celesco displacement transducer) of Unit S/N 9181: 1.632x10-6 volts/count, and 1.06
inches/volt, I obtained 2.87 mm for the step displacement. Please note that the plots in
Figure 8 are from the 16-bit files (i.e., the 16 most significant bits from the original 24bit files), so that there is multiplying factor of 2^8 = 256 for the digital counts.
135
The
difference between these two displacement values is 0.15 mm, well within the 10%
required.
Figure 9 and 10 show the same results for a 10-second segment of the recorded
acceleration as those shown in Figures 7 and 8. Please note that the double integrated
displacement (bottom frame in Figure 9, and top frame in Figure 10) shows the step
with increasing upward slope as time increases. A plausible explanation is that the
horizontal motion of the shake table has some tilt.
136
Figure 7 : Recorded acceleration data and integration results of the Reftek
accelerograph under a 1-Hz step displacement shake test for 2 seconds (see text for
explanation).
137
Figure 8 : Comparison of displacement obtained by double integration of recorded
acceleration for 2 seconds with direct displacement obtained by the displacement
transducer (see text for explanation).
138
Figure 9 : Recorded acceleration data and integration results of the Reftek
accelerograph under a 1-Hz step displacement shake test for 10 seconds (see text for
explanation).
139
Figure 10 : Comparison of displacement obtained by double integration of recorded
acceleration for 10 seconds with direct displacement obtained by the displacement
transducer (see text for explanation).
140
3. Tile Table Tests
Ten files were recorded for the tilt tests of a Reftek 24-bit accelerograph conducted
at the CWB Headquarters. The original data files in Reftek format were converted to the
PC-SUDS format by the program (REF2SUDS.EXE) supplied by Reftek. I then used
the SGRAM program of Lomax (1994) to view and compute the average for each
channel of the 10 data files. Normally, the entire record was used. However, five data
files showed large disturbances in the early part of the record. Consequently, I selected
a portion that is free of disturbances to compute the average. These selected data used
are:
Tilt_60.dmx: Starting at 07:38:35 for 20 seconds
Tilt_90.dmx: Starting at 07:39:40 for 40 seconds
Tilt_210.dmx: Starting at 07:44:30 for 30 seconds
Tilt_270.dmx: Starting at 07:45:40 for 40 seconds
Tilt_360.dmx: Starting at 07:49:10 for 20 seconds
From the Reftek Report, the bit weight is 818x10^-9 Volts/Count, and from an email of Jared Raczka (dated March 25, 2004), the sensitivies are:
Ch. 1 = 1.22 V/g
Ch. 2 = 1.20 V/g
Ch. 3 = 1.18 V/g
From these data, I deduced the following conversion from digital counts recorded
to g units:
Ch. 1 (Z-component) 1 Count = 0.67049 micro g
Ch. 2 (Y-component) 1 Count = 0.68167 micro g
Ch. 3 (X-component) 1 Count = 0.69322 micro g
The Reftek accelerograph was rotated with the x-axis as the active axis for tilt angles of
0, 30, 60, 90, 135, 180, 210, 270, 315, and 360 degrees in clockwise direction.
3.1. Analysis of Recorded Data for the Active X-component (Ch. 3)
141
The average accelerations recorded for Ch. 3 (X-component) are shown in the
following table in digital counts (Column 2), and in g (Column 3). If the bias at 0
degree tilt angle is removed, the corrected accelerations are shown in Column 4. The
expected accelerations from theoretical consideration for various tilt angles are given in
Column 5. Finally, the differences between corrected accelerations and the expected
accelerations are given in last column.
Table 1. Acceleration of the Active X-component (Ch. 3) at Various Tilts
---------------------------------------------------------------------Tilt
Acceleration Acceleration Acceleration Acceleration
Angle
in Counts
in g
(degrees) (x10^6)
corrected
(g)
expected
(g)
Difference
(g)
---------------------------------------------------------------------0
+0.034957
+0.02423
0.00000
0.00000
0.00000
30
-0.69722
-0.48333
-0.50756
-0.50000
-0.00756
60
-1.2341
-0.85550
-0.87973
-0.86603
-0.01053
90
-1.4299
-0.99124
-1.01547
-1.00000
-0.01547
135
-1.0027
-0.69509
-0.71932
-0.70711
-0.01221
180
+0.029418
+0.02039
-0.00384
0.00000
-0.00384
210
+0.75963
+0.52659
+0.50236
+0.50000
+0.00236
270
+1.4880
+1.03151
+1.00728
+1.00000
+0.00728
315
+1.0621
+0.73627
+0.71204
+0.70711
+0.00493
360
+0.031231
+0.02165
-0.00258
0.00000
-0.00258
----------------------------------------------------------------------
3.2 Analysis of Recorded Data for the Z-Component (Ch. 1)
The average accelerations recorded for Ch. 1 (Z-component) are shown in the
following table in digital counts (Column 2), and in g (Column 3). If the bias at 0
degree tilt angle is removed, the corrected accelerations are shown in Column 4. The
142
expected accelerations from theoretical consideration for various tilt angles are given in
Column 5. Finally, the differences between corrected acclerations and the expected
accelerations are given in last column.
Table 2. Acceleration of the Z-component (Ch. 1) at Various Tilts
---------------------------------------------------------------------Tilt
Acceleration Acceleration Acceleration Acceleration
Angle
in Counts
in g
(degrees) (x10^6)
corrected
(g)
expected
(g)
Difference
(g)
---------------------------------------------------------------------0
+0.16566
+0.11107
0.00000
0.00000
0.00000
30
-0.031090
-0.02085
-0.13192
-0.13397
+0.00205
60
-0.58035
-0.38912
-0.50019
-0.50000
-0.00019
90
-1.3287
-0.89088
-1.00195
-1.00000
-0.00195
135
-2.3948
-1.60569
-1.71676
-1.70711
-0.00965
180
-2.8458
-1.90808
-2.01915
-2.00000
-0.01915
210
-2.6464
-1.77438
-1.88545
-1.86602
-0.01943
270
-1.3459
-0.90241
-1.01348
-1.00000
-0.01348
315
-2.7887
-1.86980
-1.98087
-2.00000
+0.01913
360
+1.6571
+0.11111
+0.00004
0.00000
+0.00004
---------------------------------------------------------------------3.3. Analysis of Recorded Data for the Y-Component (Ch. 2)
The average accelerations recorded for Ch. 2 (Y-component) are shown in the
following table in digital counts (Column 2), and in g (Column 3). If the bias at 0
degree tilt angle is removed, the corrected accelerations are shown in Column 4. The
expected accelerations from theoretical consideration for various tilt angles are given in
143
Column 5. Finally, the differences between corrected accelerations and the expected
accelerations are given in last column.
Table 3. Acceleration of the Y-component (Ch. 2) at Various Tilts
---------------------------------------------------------------------Tilt
Acceleration Acceleration Acceleration Acceleration
Angle
in Counts
in g
(degrees) (x10^6)
corrected
(g)
expected
(g)
Difference
(g)
---------------------------------------------------------------------0 +0.004552
+0.00310
0.00000
0.00000
0.00000
30 +0.004760
+0.00324
+0.00014
0.00000
+0.00014
60 +0.008876
+0.00605
+0.00295
0.00000
+0.00295
90 +0.015613
+0.01064
+0.00754
0.00000
+0.00754
135 +0.027242
+0.01857
+0.01547
0.00000
+0.01547
180 +0.033859
+0.02308
+0.01998
0.00000
+0.01998
210 +0.033380
+0.02275
+0.01965
0.00000
+0.01965
270 +0.022492
+0.01533
+0.01223
0.00000
+0.01223
315 +0.013223
+0.00901
+0.00591
0.00000
+0.00591
360 +0.006485
+0.00442
+0.00132
0.00000
+0.00132
----------------------------------------------------------------------
4. Conclusions and Recommendations
The shake table tests show that the Reftek accelerograph meets the CWB
specifications for vibrations.
144
A tilt test is perhaps the simplest test to find out the "static" accuracy of an
accelerograph. Since the acceleration due to gravity is a constant at a given location,
tilting an accelerograph along an active axis (e.g., X-component) will change its
recorded acceleration according to the effect of gravity on that axis. Since the Zcomponent measures the vertical-component of gravity, tilting the X-component will
also produce a known effect on the recorded acceleration of the Z-component. However,
tilting the X-component should have no gravity effective on the Y-component, but will
indicate the cross-axis error for the acceleration recorded by the Y-component.
The CWB specs call for a static accuracy of +- 1% of the full scale, or +- 0.035 g
for the submitted accelerograph for testing, which has a full scale of 3.5g. According to
Tables 1, 2, and 3, if the bias at 0 degree tilt is removed, then the worse case for the
active X-component is -0.015 g, for the Z-component is -0.019 g, and for the Ycomponent is +0.020 g.
Therefore, the submitted accelerograph meets the CWB specs for the tilt test.
To avoid confusion in the field, the bias at 0 degree tilt for all components should
be removed by the accelerograph so that no corrections are needed for the recorded data.
The manufacturer supplied the bit weights and sensitivities with only 3 significant
figures or digits. Due to round-off errors, it is desirable to have these values given with
4 or 5 significant digits.
If the bidder wins the bid, technical tests as given in Appendix 1 of the CWB specs
must be repeated under an observer designated by CWB. Since the bidder did not
submitted a test unit with a 2g full-scale (a 3.5g, DC-100 Hz unit instead), the bidder
must repeat their tests with a 2g full-scale (DC-50 Hz) unit.
Furthermore, the
REF2SUDS program to convert the recorded data files to PC-SUDS requires some
modifications so that some data fields are properly specified.
I recommend that a revised Appendix 1 of the CWB specs, and a more detailed
guideline for the PC-SUDS conversion program be prepared by the CWB
Instrumentation Committee. If Reftek wins the bid, then CWB should request that the
145
technical tests be conducted according to the revised Appendix 1 and the REF2SUDS
be modified according to the new guideline.
References
CWB Instrumentation Committee, (1993). A preliminary report on testing
accelerographs and accelerometers (dated October 9, 1993), reproduced as
Appendix 1 of the Annual Report to the Central Weather Bureau by T. L. Teng et al.,
CWB Seismology Center Report No. 7, 1994.
Lee, W. H. K. (Editor), (1994a). “Realtime Seismic Data Acquisition and Processing”,
IASPEI Software Library Volume 1, Second Edition, Seism. Soc. Am., El Cerrito,
CA.
Lee, W. H. K. (Editor), (1994b). “Plotting and Displaying Seismic Data”, IASPEI
Software Library Volume 2, Second Edition, Seism. Soc. Am., El Cerrito, CA.
Lee, W. H. K. (Editor), (1994c). “Digital Seismogram Analysis and Waveform
Inversion”, IASPEI Software Library Volume 3, Updated Edition, Seism. Soc. Am.,
El Cerrito, CA.
Lomax, A., (1994). User manual for SeisGram, IASPEI Software Library Volume 3
(Updated Edition), 13-80.
146
Section C. Strong-Motion Data Processing and Software Development
Willie Lee and Doug Dodge
Contents
I. Introduction ................................................................................................................148
II. Software Development .............................................................................................148
III. Coherence Analysis of Multiple Co-Located, “24-bit” Strong-Motion Instruments
.......................................................................................................................................149
References .....................................................................................................................150
Appendix C1. Script Code for Coherence Analysis ......................................................151
Appendix C2. A Coherence Analysis of Data Recorded by Multiple Co-Located “24bit” Strong-Motion Instruments at the Hualien Seismic Station, Taiwan .....................170
Abstract..........................................................................................................................170
1. Introduction ...............................................................................................................171
2. Recorded Earthquakes ...............................................................................................176
3. Coherence Analysis ...................................................................................................179
4. Comparisons of “24-bit” Strong-Motion Instruments ...............................................180
4.1. Earthquake at 09:03 on April 3, 2004 (Event #1)...............................................181
4.2. Earthquake at 05:33 on April 9, 2004 (Event #2)...............................................182
4.3. Earthquake at 02:26 on April 23, 2004 (Event #3).............................................185
4.4. Earthquake at 02:27 on April 23, 2004 (Event #4).............................................190
4.5. Earthquake at 15:20 on April 24, 2004 (Event #5).............................................195
4.6. Earthquake at 19:26 on April 24, 2004 (Event #6).............................................199
4.7. Earthquake at 22:29 on April 24, 2004 (Event #7).............................................204
4.8. Earthquake at 14:28 on April 25, 2004 (Event #8).............................................209
4.9. Earthquake at 07:56 on May 1, 2004 (Event #9)................................................214
4.11. Earthquake at 20:06 on May 9, 2004 (Event #11)............................................219
4.12. Earthquake at 15:28 on May 13, 2004 (Event #12)..........................................224
4.13. Earthquake at 06:04 on May 16, 2004 (Event #13)..........................................229
4.14. Earthquake at 07:04 on May 19, 2004 (Event #14)..........................................234
147
4.15. Earthquake at 20:25 on May 22, 2004 (Event #15)..........................................239
4.16. Earthquake at 16:56 on June 2, 2004 (Event #16)............................................244
Discussions ....................................................................................................................249
Acknowledgements .......................................................................................................250
References .....................................................................................................................250
I. Introduction
During 2004, we made slow but steady progress in systematic processing of the
strong-motion data recorded by the Central Weather Bureau (CWB).
The basic
computer program for performing quality assurance, SmBrowser, has been described in
the 2003 Annual Report (Dodge and Lee, 2004), and will not be repeated here.
Enhancement has been made to SmBrowser to improve processing efficiency, and
considerable efforts have been devoted to verify station coordinates, which is still now
underway.
II. Software Development
A field test of multiple co-located strong-motion instruments was conducted in
Hualien from the end of April 1 to June 3, 2004. To analyze the recorded the data, we
developed some pre-processing software and application code for coherence analysis.
In particular, computing the coherence function between two time-series signals was
implemented, and over 1,000 correlation pairs had been computed for up to 16
earthquakes recorded by the six deployed instruments.
A standard method to quantitatively compare two time-series signals, x(t) and y(t),
where t is time, is to compute the coherence function between these two signals in the
frequency domain. The magnitude squared coherence function, Cxy(f) is defined by:
Cxy(f) = │Pxy(f) │2
⁄ [Pxx(f) Pyy(f)]
(1)
where f is frequency; Pxy(f) is the cross spectral density (CSD) function of x(t) and y(t);
and Pxx(f), and Pyy(f) are power spectral density (PSD) function of x(t) and y(t),
respectively. The goal of computing power spectra is to describe the distribution over
148
frequency of the power contained in a signal, based on a finite set of sampled data. PSD
function is actually a special case of CSD function when x(t) = y(t). The cross spectral
density function is defined by:
Pxy(ω) = Σ Rxy(m) exp(-iωm)
(2)
where the summation is over m, ω = 2πf /fs, fs is the sampling frequency, and Rxy(m) is
the cross correlation sequence:
Rxy(m) = E{x(n) y*(n+m)}
(3)
where E{·} is the expected value operator, and x(n) and y(n) are the discrete time series
of x(t) and y(t), respectively.
Fortunately, coherence analysis can be implemented by using the software package
MATLAB with its Signal Processing Toolbox (Math Works Inc., 2000). In particular,
we use “cohere(x,y)” to compute the magnitude squared coherence function (between
two length n signal vectors x and y) as a function of frequency.
The maximum
frequency is 100 Hz because our data are sampled at 200 samples per second. The
minimum frequency is limited by the length of the two sampled signals (i.e., the lesser
length of the two). Readers are referred to MathWorks (2000) for more details.
The script code for performing coherence analysis using MATLAB is given in
Appendix C1.
III. Coherence Analysis of Multiple Co-Located, “24-bit” StrongMotion Instruments
Although strong-motion instruments had been deployed in the field for several
decades around the world, we are not aware of any published reports of testing multiple
co-located instruments in the field with the recorded earthquake data presented. Under
the sponsorship of the Seismology Center of the Central Weather Bureau (CWB), six
strong-motion instruments from four different manufacturers [two Geotech A900As,
one Reftek (130-SMA/01), one Kinemetrics (K2), and two Tokyo-Sokushins (TS-575
and TS-G3)] had been deployed on the seismic pier (2 x 3 meters surface) at the Hualien
Seismic Station (HWA) during a testing period from April 1 to June 3, 2004. We will
149
use “accelerograph” interchangeable with “strong-motion instrument” in this report,
although the TS-G3 instrument is not an accelerograph. The TS-G3 instrument has a
broadband velocity sensor that is capable of functioning up to 2g ground motions, and
therefore, it is qualified to be called a strong-motion instrument.
A total of 16 earthquakes were recorded during the test period by the permanent
accelerograph at HWA (Model A900A by Geotech). Among these 16 earthquakes, the
temporary A900A accelerograph recorded 13 (but it was not deployed until after the
first 2 events had occurred), the K2 accelerograph recorded 15, the Reftek
accelerograph recorded only the first two earthquakes (due to an operator error of not
connecting to an AC power source, and was taken back after about a 2-week
deployment). The two Tokyo-Sokushin instruments were not deployed until after the
first two earthquakes had occurred, and recorded 13 and 14 of these earthquakes,
respectively.
We developed some pre-processing software and application scripts for coherence
analysis using the MATLAB and its Signal Processing Toolbox (see section above). In
particular, computing the coherence function between two time-series signals was
implemented, and over 1,000 correlation pairs had been computed.
A detailed report analyzing the “24-bit” strong-motion instruments is given in
Appendix C2.
Coherence plots from data recorded by various accelerographs are
shown by earthquakes. The results indicate that these accelerographs performed not as
99% perfect (judging from their coherences between each other) as we would like, but
not as bad as we might have feared.
References
Dodge, D. and W. Lee (2004). Strong-motion data processing and software
development. In Annual Report to the Central Weather Bureau by T. L. Teng and
W. H. K. Lee, CWB Seismology Center Report No. 35, p. 202-448, April, 2004.
MathWorks (2000). User’s Guide of Signal Processing Toolbox for use with MATLAB.
The MathWorks, Inc., Natick, MA.
150
Appendix C1.
Script Code for Coherence Analysis
The Matlab code in this file was used to produce the coherence plots shown in
Appendix C2 of this report. It runs under Matlab 6.1 with the signal-processing toolbox.
The code reads a driver file containing the names of waveform data files (in PC-SUDS
format) for each instrument type organized by earthquake. For each earthquake, the
code computes all-possible pairings of the available instruments, and for each pair
produces a plot showing the overlapped seismograms channel-by-channel and the
coherence function for each channel pairing. The output plots are output as encapsulated
postscript files.
This is the driving function. It reads a text file that contains for each earthquake to
be used in the analysis, the origin time to the nearest minute and the file names of the
recordings from each triggered instrument. For each earthquake all the data are read and
accumulated into a cell array. After all the data have been accumulated, the plots are
produced for the earthquake.
function makeCorrelationPlots
fid=fopen('correlation.driver.txt');
fgetl(fid);
while 1
tline = fgetl(fid);
if ~ischar(tline), break, end
[token,remainder] = strtok( tline );
[EventTime,remainder] = strtok( remainder );
% For this event get all data from instruments that recorded the event...
% This information goes into the 'Data' structure
j = 0;
151
Data = {};
[remainder, data] = getInstrumentData( remainder, 'A900Perm' );
if ~isempty( data )
j = j + 1;
Data{j} = data;
end
[remainder, data] = getInstrumentData( remainder, 'A900Temp' );
if ~isempty( data )
j = j + 1;
Data{j} = data;
end
[remainder, data] = getInstrumentData( remainder, 'K2' );
if ~isempty( data )
j = j + 1;
Data{j} = data;
end
[remainder, data] = getInstrumentData( remainder, 'Reftek' );
if ~isempty( data )
j = j + 1;
Data{j} = data;
end
[remainder, data] = getInstrumentData( remainder, 'TS575' );
if ~isempty( data )
j = j + 1;
Data{j} = data;
end
[remainder, data] = getInstrumentData( remainder, 'TSG3' );
if ~isempty( data )
j = j + 1;
152
Data{j} = data;
end
plotAllPairs( EventTime, Data );
end
fclose(fid);
%---------------------------------------------------------------------------------------
The PlotAllPairs function produces a plot for each instrument against every other
instrument with a recording in this dataset.
function plotAllPairs( EventTime, Data )
N = length( Data );
for j = 1 : N - 1
for k = j + 1 : N
PlotPair( EventTime, Data{j}, Data{k} );
end
end
%---------------------------------------------------------------------------------------
This function produces a plot for a single comparison of one instrument against another.
The plot is constrained to use data from one second before the nominal P-arrival to 10
seconds after. All of the data meet this criterion, and it is a short enough time segment
that the structure of the P- and S-coda are clearly visible. The two seismograms are
cross correlated to find the best alignment. By aligning them it becomes easier to
visually compare the two traces. Also, the coherence function degrades if the traces are
misaligned by more than a few samples. Each component is plotted for the pair followed
153
by a plot that shows the coherence for all three components superimposed on one
another.
function PlotPair( EventTime, Data1, Data2 )
tStart = -1;
tEnd = 10;
clf
set(gcf, 'PaperPosition', [0.5 0.5 7.5 10]);
for j = 1 : 3
idx = getMatchingChannel( Data1{1}, Data2{1}, j );
[time1, trace1, flipped1, time2, trace2, flipped2] = getAlignedSeismograms( Data1,
Data2, j, idx );
[time1, trace1, time2, trace2] = getWindowedSeismograms( time1, trace1, time2,
trace2, tStart, tEnd );
hAxis(j) = subplot( 4,1,j );
plot(time1, trace1, 'g', time2, trace2, 'm-.')
if j == 3
xlabel( 'Time relative to P-pick (s)')
else
set(gca,'XTickLabel',[]);
end
ylabel( 'Acceleration (cm/s^2)' )
hSeisLegend(j) = addLegend( Data1, Data2 );
addTitle( j, Data1, Data2, j, idx, flipped1, flipped2 );
154
[F, cxy] = getCoherence( trace1, trace2, Data1{2}(j).Rate );
coherenceData(j).freq = F;
coherenceData(j).coherence = cxy;
coherenceData(j).label = sprintf( 'Pair %d', j );
end
hAxis(4) = subplot(4,1,4);
plot( coherenceData(1).freq, coherenceData(1).coherence, 'r',coherenceData(2).freq,
coherenceData(2).coherence, 'b--', coherenceData(3).freq, coherenceData(3).coherence,
'k-.' );
set(gca, 'xscale', 'log');
xlabel('Frequency (Hz)');
ylabel('Coherence');
hCohLegend
=
legend(
coherenceData(1).label,
coherenceData(2).label,
coherenceData(3).label );
title( 'Coherence (All Pairs)' );
hSupTitle = makeSuperTitle( EventTime, Data1, Data2 );
set(gcf,'paperunits','inches')
set(gcf,'units','inches')
set(gcf,'position',get(gcf,'paperposition'))
ymax = 0.9319; % puts top of top plot 1.25 inches from top of page.
for j = 1 : 3
ymax = shiftSubplotPosition( hAxis(j), ymax, hSeisLegend(j) );
end
setCoherencePosition( hAxis(4), ymax, hCohLegend)
printPlot(EventTime, Data1, Data2 )
155
%---------------------------------------------------------------------------------------
This function renders the generated plot into a Postscript file names after the event and
instrument pair.
function printPlot(EventTime, Data1, Data2 )
EventTime(find(EventTime == '/')) = [];
EventTime(find(EventTime == '-')) = [];
EventTime(find(EventTime == ':')) = [];
str = sprintf( 'print -depsc -tiff %s_to_%s_eq_%s', Data1{1}, Data2{1}, EventTime );
eval( str );
%---------------------------------------------------------------------------------------
This function makes a super title for the pair plot.
function hSupTitle = makeSuperTitle( EventTime, Data1, Data2 )
str = sprintf( 'Comparison of %s and %s For
Earthquake (2004) %s', Data1{1},
Data2{1}, EventTime );
hSupTitle = suptitle( str );
set(hSupTitle, 'fontsize', 14);
%---------------------------------------------------------------------------------------
This function is part of the page-layout code. It places the coherence plot in the correct
position on the printed page.
function setCoherencePosition( hAxis, ymax, hCohLegend)
pos = get(hAxis,'position');
156
pos(4) = ymax - pos(2) - 0.0727; %0.8 inch spacing
set(hAxis,'position', pos );
legendPos = get(hCohLegend,'position');
legendPos(2) = pos(2) + .01;
legendPos(1) = pos(1) + 0.0182;
set(hCohLegend,'position', legendPos);
%---------------------------------------------------------------------------------------
This function is part of the page layout code. It places each time-domain subplot in its
correct position on the page.
function ymax = shiftSubplotPosition( hAxis, ymax, hLegend )
pos = get(hAxis,'position');
pos(2) = ymax - pos(4) - 0.0182; %0.2 inch spacing
set(hAxis,'position', pos );
legendPos = get(hLegend,'position');
legendPos(2) = pos(2) + .01;
set(hLegend,'position', legendPos);
ymax = pos(2);
%---------------------------------------------------------------------------------------
This function adds a legend to each component comparison plot. The legend associates
each line style and color with the appropriate instrument name.
function hLegend = addLegend( Data1, Data2 )
hLegend = legend(Data1{1}, Data2{1} );
axisPos = get(gca, 'position');
157
legendPos = get(hLegend,'position');
legendPos(2) = axisPos(2) + .01;
set(hLegend,'position', legendPos);
%---------------------------------------------------------------------------------------
This function windows out appropriate parts of each seismogram to be compared.
function [time1, trace1, time2, trace2] = getWindowedSeismograms( time1, trace1,
time2, trace2, tStart, tEnd )
idx11 = getIndex( time1, tStart);
idx12 = getIndex( time1, tEnd );
idx21 = getIndex( time2, tStart);
idx22 = getIndex( time2, tEnd );
time1 = time1(idx11:idx12);
trace1 = trace1(idx11:idx12);
time2 = time2(idx21:idx22);
trace2 = trace2(idx21:idx22);
%---------------------------------------------------------------------------------------
This function reads in the pair of seismograms to be compared, aligning the retrieved
traces on the P-wave arrival presumed to be set in the file. It then correlates the traces,
and uses the correlation-derived shift to align the traces to the nearest sample. In some
cases, a trace has to be flipped because of channel reversal, and that status is reported
in the output.
function
[time1,
trace1,
flipped1,
time2,
getAlignedSeismograms( Data1, Data2, j, idx )
[ time1, trace1, flipped1 ] = getPlotData( Data1, j );
[ time2, trace2, flipped2 ] = getPlotData( Data2, idx );
158
trace2,
flipped2]
=
timeshift = getTimeshift( trace1, trace2, time1, time2, Data1{2}(j).Rate );
time2 = time2 + timeshift;
%---------------------------------------------------------------------------------------
This function cross-correlates two traces to determine the best alignment.
function timeshift = getTimeshift( trace1, trace2, time1, time2, rate )
[c,lags] = xcorr( trace1, trace2 );
[y,i] = max(c);
timeshift = lags(i) / rate - (time2(1) - time1(1) );
%---------------------------------------------------------------------------------------
This function cross-correlates two traces to determine the best alignment.
function addTitle( pairNumber, Data1, Data2, j, idx, flipped1, flipped2 )
hTitle = title( getTitle( pairNumber, Data1, Data2, j, idx, flipped1, flipped2 ) );
set(hTitle,'units','normalized');
pos = get(hTitle,'position' );
pos(2) = .85;
set(hTitle,'position',pos);
%---------------------------------------------------------------------------------------
This function produces a title string based on the contents of the Data structure arrays
and indices into the structure arrays.
function theTitle = getTitle( pairNumber, Data1, Data2, idx1, idx2, flipped1, flipped2 )
chan1Name = getChanName( Data1, idx1 );
159
chan2Name = getChanName( Data2, idx2 );
if strcmp(flipped1, 'yes' )
flip1Text = ' (flipped)';
else
flip1Text = '';
end
if strcmp(flipped2, 'yes' )
flip2Text = ' (flipped)';
else
flip2Text = '';
end
theTitle = sprintf( 'Pair %d:
%s chan (%d) (%s)%s,
%s chan (%d) (%s)%s',
pairNumber, Data1{1}, idx1, chan1Name, flip1Text, Data2{1}, idx2, chan2Name,
flip2Text );
%---------------------------------------------------------------------------------------
This function provides a channel name to be used in plot annotation.
function chanName = getChanName( Data, idx )
if length( Data{3} ) < 3
chanName = 'Name not available';
elseif strcmp( Data{3}(idx).Chan, '_' )
chanName = 'Name not set';
else
chanName = sprintf( 'Name = %s', Data{3}(idx).Chan );
160
end
%---------------------------------------------------------------------------------------
This function extracts a time-series from a Data structure array. The time series is
intended to be used as input to the plot function.
function [ time, trace, flipped ] = getPlotData( Data, idx )
flipped = 'no';
trace = Data{2}(idx).Data;
% Channels 2 and 3 for TS575 need to be flipped to be consistent with other
instruments.
if strcmp( Data{1}, 'TS575') & idx > 1
trace = -trace;
flipped = 'yes';
end
N = length( trace );
dt = 1 / Data{2}(idx).Rate;
time = linspace(Data{2}(idx).Time, Data{2}(idx).Time + (N-1) * dt, N ) Data{5}.Time;
time = time';
%---------------------------------------------------------------------------------------
This function returns the index of a particular time value from a time array.
function idx = getIndex( timeArray, time )
idx = 1;
[y,i] = min( abs( timeArray - time) );
idx = round(i);
161
%---------------------------------------------------------------------------------------
This function computes the coherence between two aligned traces. It returns both a
frequency and a coherence vector.
function [F, cxy] = getCoherence( trace1, trace2, sampRate )
WindowLength = 256;
[cxyraw, F] = cohere( trace1, trace2, WindowLength, sampRate );
cxy = smooth(cxyraw, 2 );
%--------------------------------------------------------------------------------------This function smooths the coherence vector.
function c = smooth( c, ns )
% Applies a ns points smoothing operator to vector c
M = length(c);
for j = ns + 1 : M - ns
c(j) = mean( c(j-ns:j+ns) );
end
%---------------------------------------------------------------------------------------
This function looks up the conversion factor between counts and ground-motion units by
instrument name.
function factor = getCountsToGmUnits( instname )
factor = 1;
if strcmp( instname, 'A900Perm' ) | strcmp( instname, 'A900Temp' )
maxAcc = 1960; % cm/s^2
maxCounts = 2^15;
162
factor = maxAcc / maxCounts;
elseif strcmp( instname, 'K2' )
maxAcc = 1960; % cm/s^2
maxCounts = 2^23;
factor = maxAcc / maxCounts;
elseif strcmp( instname, 'Reftek' )
bitWeight = 8.18e-7; % volts/count
sensitivity = 1.2; % volts/g
toCmSecSqrd = 980; %cm/s^2 per g
factor = bitWeight / sensitivity * toCmSecSqrd;
elseif strcmp( instname, 'TS575' )
maxAcc = 2000; % cm/s^2
maxCounts = 8192000;
factor = maxAcc / maxCounts;
elseif strcmp( instname, 'TSG3' )
maxVel = 200; % cm/sec
maxCounts = 8192000;
factor = maxVel / maxCounts;
end
%---------------------------------------------------------------------------------------
This function reads a suds file assumed to have a P-pick set on one of the channels. It
retrieves data from PrePSeconds before the pick extending for BufferLength seconds.
function [data, samprate] = getDataBuffer( fname, PrePSeconds, BufferLength )
% Read the file (assumed to have a P-pick set, get the pick time and use that
% to cut the file from PrePSeconds in front of P to a length of BufferLength.
% If BufferLength points are not available, then buffer goes to end of traces.
163
[ waveforms, stations, origins, picks ] = readsuds(fname);
picktime = picks.Time - waveforms(1).Time;
samprate = waveforms(1).Rate;
idx1 = round( picktime * samprate ) + 1;
idx2 = round( (picktime + BufferLength ) * samprate ) + 1;
[nchannels,m] = size( waveforms );
for l = 1 : nchannels
D = waveforms(l).Data;
D = D - mean(D);
if idx2 > length(D)
idx2 = length(D);
end;
tmp = D(idx1:idx2);
data(:,l) = tmp;
end
function [remainder, Data] = getInstrumentData( remainder, label )
[filename,remainder] = strtok( remainder );
if ~strcmp( filename, '-' )
[ waveforms, stations, origins, picks ] = readsuds(['../' filename]);
waveforms = convertToGmUnits( waveforms, label );
if strcmp(label, 'TSG3' )
waveforms = differentiateData( waveforms );
end
Data = { label, waveforms, stations, origins, picks };
else
164
Data = {};
end
%---------------------------------------------------------------------------------------
This function is used to differentiate velocity data to acceleration. It estimates the
derivative using a first-order difference.
function waveforms = differentiateData( waveforms )
tmp = diff( waveforms(1).Data ) * waveforms(1).Rate;
waveforms(1).Data = [tmp(1); tmp];
tmp = diff( waveforms(2).Data ) * waveforms(2).Rate;
waveforms(2).Data = [tmp(2); tmp];
tmp = diff( waveforms(3).Data ) * waveforms(3).Rate;
waveforms(3).Data = [tmp(3); tmp];
%---------------------------------------------------------------------------------------
This function converts the raw seismogram data for a single Suds file from counts to
ground-motion units. In the process of the conversion, it also removes the mean from
the traces.
function waveforms = convertToGmUnits( waveforms, instname )
factor = getCountsToGmUnits( instname );
waveforms(1).Data = ( waveforms(1).Data - mean( waveforms(1).Data ) ) * factor;
waveforms(2).Data = ( waveforms(2).Data - mean( waveforms(2).Data ) ) * factor;
waveforms(3).Data = ( waveforms(3).Data - mean( waveforms(3).Data ) ) * factor;
165
%---------------------------------------------------------------------------------------
This function is used to determine the correct channel match between two instruments.
This step is necessary because the different instruments do not have a common standard
for associating physical components with channels.
function chan = getMatchingChannel( thisInstrument, thatInstrument, thisChan )
chan = -1;
names = {'A900Perm', 'K2', 'Reftek', 'A900Temp', 'TS575', 'TSG3'};
identity = [1,1;2,2;3,3];
v{1,1} = identity;
v{2,2} = identity;
v{3,3} = identity;
v{4,4} = identity;
v{5,5} = identity;
v{6,6} = identity;
%A900Perm to K2
v{1,2} = [1,3;2,2;3,1];
v{2,1} = v{1,2};
%A900Perm to Reftek
v{1,3} = [1,1;2,3;3,3];
v{3,1} = v{1,3};
%A900Perm to A900Temp
v{1,4} = identity;
v{4,1} = identity;
166
%A900Perm to TS575
v{1,5} = identity;
v{5,1} = identity;
%A900 Perm to TSG3
v{1,6} = identity;
v{6,1} = identity;
%K2 to Reftek
v{2,3} = [1,2;2,3;3,1];
v{3,2} = v{2,3};
%K2 to A900Temp
v{2,4} = [1,3;2,2;3,1];
v{4,2} = v{2,4};
%K2 to TS575
v{2,5} = [1,3;2,2;3,1];
v{5,2} = v{2,5};
%K2 to TSG3
v{2,6} = [1,3;2,2;3,1];
v{6,2} = v{2,6};
%Reftek to A900Temp ( equivalent to Reftek to A900Perm )
v{3,4} = v{1,3};
v{4,3} = v{3,4};
167
%Reftek to TS575
v{3,5} = [1,2;2,3;3,2];
v{5,3} = v{3,5};
%Reftek to TSG3
v{3,6} = [1,1;2,3;3,2];
v{6,3} = v{3,6};
%A900Temp to TS575
v{4,5} = identity;
v{5,4} = identity;
%A900Temp to TSG3
v{4,6} = identity;
v{6,4} = identity;
%TS575 to TSG3
v{5,6} = identity;
v{6,5} = identity;
idx1 = getIndex( names, thisInstrument );
idx2 = getIndex( names, thatInstrument );
if idx1 > 0 & idx2 > 0
mapper = v{idx1, idx2 };
chan = mapper( thisChan, 2 );
end
%---------------------------------------------------------------------------------------
168
This function is used by the previous channel-association function to get the index for a
lookup into the association array.
function idx = getIndex( names, thisName )
idx = -1;
for j = 1 : length( names )
if strcmp(thisName, names{j} )
idx = j;
return;
end
end
%---------------------------------------------------------------------------------------
169
Appendix C2.
A Coherence Analysis of Data Recorded by Multiple Co-Located “24-bit”
Strong-Motion Instruments at the Hualien Seismic Station, Taiwan
Willie Lee, Chien-Fu Wu, and Chun-Chi Liu
Nov. 14, 2004
Abstract
Although strong-motion instruments had been deployed in the field for several
decades around the world, we are not aware of any published reports of testing multiple
co-located instruments in the field with the recorded earthquake data presented. Under
the sponsorship of the Seismology Center of the Central Weather Bureau (CWB), six
accelerographs from four different manufacturers [two Geotechs (A900A), one Reftek
(130-SMA/01), one Kinemetrics (K2), and two Tokyo-Sokushins (TS-575 and TS-G3)]
had been deployed on the seismic pier (2 x 3 meters surface) at the Hualien Seismic
Station (HWA) during a testing period from April 1 to June 3, 2004. Since the two
Geotech A900As are 16-bit instruments, we will not include them here in the analysis.
The other four instruments are the so called “24-bit” type, because they use 24-bit A/D
chips and record data in 24-bit integers.
A total of 16 earthquakes were recorded during the test period by the permanent
accelerograph at HWA (Model A900A by Geotech). Among these 16 earthquakes, the
temporary A900A unit recorded 13 (but it was not deployed until after the first 2 events
had occurred), the K2 accelerograph recorded 15, the Reftek accelerograph recorded
only the first two earthquakes (due to an operator error and was taken back after about a
2-week deployment). The two Tokyo-Sokushin instruments were not deployed until
after the first two earthquakes had occurred, and recorded 13 and 14 of these
earthquakes, respectively.
170
We developed some pre-processing software and application scripts for coherence
analysis using the MATLAB and its Signal Processing Toolbox (MathWorks, 2000). In
particular, computing the coherence function between two time-series signals was
implemented, and over 1,000 correlation pairs had been computed. In this report, we
present the results of our coherence analysis on the data recorded by multiple co-located
strong-motion instruments. Coherence plots from data recorded by various instruments
for up to fifteen earthquakes are shown in this report. The results indicate that these
strong-motion instruments performed not as 99% perfect (judging from their coherences
between each other) as we would like, but not as bad as we might have feared.
1. Introduction
Under the sponsorship of the Seismology Center of the Central Weather Bureau
(CWB), six strong-motion instruments from four different manufacturers [two Geotechs
(A900A), one Reftek (130-SMA/01), one Kinemetrics (K2), and two Tokyo-Sokushins
(TS-575 and TS-G3)] had been deployed on the seismic pier (2 x 3 meters surface) at
the Hualien Seismic Station (HWA) during a testing period from April 1 to June 3, 2004.
We will use “accelerograph” interchangeable with “strong-motion instrument” in
this report, although the TS-G3 instrument is not an accelerograph, strictly speaking.
The TS-G3 instrument has a broadband velocity sensor that is capable of functioning up
to 2g ground motions, and therefore, it is qualified to be called a strong-motion
instrument. We will process the recorded velocity data by the TS-G3 instrument into
acceleration data, so that they can be used for coherence analysis with the recorded data
from the accelerographs.
Although some comparisons of strong-motion instruments had been undertaken
since the dawn of strong-motion seismology in the 1930s, we are not aware of any
published results comparing earthquake data recorded by multiple instruments in the
field. When the first digital strong-motion accelerograph appeared, Iwan et al. (1985)
conducted laboratory tests on the background noise level of the instrument and
compared the results with previously reported observations for the analog (optical)
171
instruments. For procurement purposes, testing multiple accelerographs (and sometime
accelerometers) had been conducted by the CWB Instrumentation Committee on a few
occasions in the laboratory, including using a shake table (e.g., CWB, 1994; 1997).
Digital accelerographs from individual manufacturers had been tested annually (if
necessary) for more than 10 years as an essential part of the CWB procurement
procedure. In addition, new accelerographs submitted for bidding are required to be
deployed in Hualien for a short period of time to see if the instruments can record
earthquakes. However, the primary purpose of these tests is to ensure that the CWB
specifications are met by the bidders, and over the years, a few accelerographs has been
rejected for mostly failing the shake table tests. Due to tight procurement schedule and
limited resources, we have not undertaken tests designed to compare the performance of
co-located accelerographs in the field until recently.
Such investigation is important in order to define the instrumental limits of field
observations of strong earthquakes. Because of variations of component parts and
manufacturing procedures, we do not expect two co-located (less than two meters apart)
accelerographs (even of the same model) will record identical ground motions for the
same earthquake. The question is how variable will the results be, and what is its
impacts on the estimates of parameters of earthquake engineering interests.
In early 2004, an invitation was sent to manufacturers requesting their voluntary
participations of a field test of multiple accelerographs in Hualien. We are grateful that
three manufacturers agreed: Kinemetrics, Reftek, and Tokyo-Sokushin. Furthermore,
we are particularly interested in the new broadband velocity senor made by TokyoSokushin that is capable of functioning up to 2g ground motions. The Hualien Seismic
Station already had a permanent accelerograph (an A900A accelerograph by Geotech).
Also, an A900A accelerograph from a nearby school site had to be temporarily
relocated (due to school remodeling) and was thus available for the comparison study.
Figure 1 shows a photo and a location map of the instruments on the seismic pier
of the Hualien Seismic Station. The seismic pier is about 2 by 3 meters in surface area,
and the spatial separation between any two instruments varies from about 0.6 to 2
172
meters. The HWA site is chosen for reasons of logistics and of abundance of strong
earthquakes occurring nearby. The Kinemetrics and the Reftek accelerographs were
deployed at the end of March, 2004. However, due to shipping delay, the two TokyoSokushin instruments were deployed in mid-April.
173
Figure 1 : Instruments deployed on the seismic pier (2 by 3 meters) at the
入 口
Figure 1. A photo of the instruments on the seismic pier (2 by 3 meters) at the Hualien
Seismic Station (HWA) is shown (top), and a map showing the instrument locations
174
with (x, y) coordinates in centimeters (bottom). This photo and map were made after
the Reftek accelerograph was removed.
The Reftek accelerograph was deployed
between the A900-427 (A900A permanent) and the VSE-355G3 (Tokyo-Sokushin G3)
instruments. Please note that the photo and the map are not oriented in the same
direction.
175
2. Recorded Earthquakes
After about 2 months, the recorded earthquakes by various strong-motion
instruments are shown in Table 1. The permanent accelerograph at Hualien (a Geotech
A900A accelerograph) recorded a total of 16 earthquakes from April 3 to June 2,
ranging from local magnitude of 2.3 to 6.5, and epicentral distance of about 2 to 144
km. Moment magnitudes of these earthquakes are also given whenever available, and
their values are smaller than the local magnitudes.
The Reftek accelerograph recorded only the first two earthquakes. It appeared that
due to an operator error, the instrument was not charged by the AC-DC converter, and
the accelerograph was taken back by the manufacturer after a deployment of only about
2 weeks.
The Kinemetrics K2 accelerograph recorded all but 1 of the earthquake. Event #10
is a very small earthquake and only the TS-G3 instrument recorded it besides the
permanent A900A accelerograph. This may simply due to the setting of the trigger
level.
The Tokyo-Sokushin 575 accelerograph and a similar instrument with a G3
(broadband velocity sensor) recorded 13, and 14 earthquakes, respectively. Since these
two instruments were deployed after the first two earthquakes had occurred, they
recorded the same earthquakes as the Kinemetrics K2 accelerograph for the same time
period. The TS-G3 instrument also recorded Event #10, probably due to a little more
sensitive triggering level.
The temporary A900A accelerograph did not join the other instruments until after
the first two earthquakes had occurred. It also recorded the same set of earthquakes as
the Kinemetrics K2 accelerograph and the two Tokyo-Sokushin instruments for the
same time period.
Table 1 summarizes the relevant information about the earthquakes recorded by
these 6 instruments.
The origin times (in Year, MMDD, Hr:Mn:Sec), hypocenter
(Latitude in degrees N, Longitude in degrees E, and Depth in km), local magnitude (ML)
are based mostly on the preliminary CWB Autoloc solutions. Mw values are taken
176
from the BATS moment tensor solutions by the Institute of Earth Sciences, Academia
Sinica.
We then computed the epicentral distances (epdist in km).
“Y” in Table 1
indicates that a data file is available. The strong-motion instruments are designated as:
AP = A900A (permanent accelerograph at HWA).
AT = A900A (temporary accelerograph at HWA).
K2 = Kinemetrics K2 accelerograph at HWA.
RK = Reftek 130-SMA/01 accelerograph at HWA.
TS = Tokyo-Sokushin 575 accelerograph (accelerometer sensor) at HWA.
G3 = Tokyo-Sokushin G3 instrument (broadband velocity sensor) at HWA.
177
Table 1. Summary of Earthquake Information and Available Data Files
--------------------------------------------------------------------------------------------------------No Year MMDD Hr:Mn:Sec Lat_N Lon_E Dep ML/Mw epdist AP AT K2 RK TS G3
--------------------------------------------------------------------------------------------------------1 2004 0403 09:03:28.0 24.05 121.59 11.6 3.9/
8.0 Y
2 2004 0409 05:33:38.3 24.10 121.53 23.5 4.8/4.0 15.6 Y
Y Y
Y Y
3 2004 0423 02:26:38.5 23.97 121.59 4.8 2.3/
2.3 Y Y Y
Y Y
4 2004 0423 02:27:34.6 23.97 121.58 4.0 2.5/
3.2 Y Y Y
Y Y
5 2004 0424 15:20:31.2 23.95 121.48 20.7 5.3/4.4 13.6 Y Y Y
Y Y
6 2004 0424 19:26:02.7 23.96 121.46 15.2
/3.4 15.4 Y Y Y
Y Y
7 2004 0424 22:29:01.8 23.96 121.48 16.2
/3.5 13.4 Y Y Y
Y Y
8 2004 0425 14:28:35.4 23.95 121.46 14.2 3.3/
9 2004 0501 07:56:13.3 24.08 121.52 17.8
15.6 Y Y Y
Y Y
/5.0 14.4 Y Y Y
Y Y
10 2004 0509 07:46:07.8 23.97 121.60 6.5 2.5/
1.5 Y
Y
11 2004 0509 20:06:48.3 24.58 121.75 67.1 5.7/4.8 68.0 Y Y Y
Y Y
12 2004 0513 15:28:47.4 24.05 121.51 18.9 4.6/3.7 12.8 Y Y Y
Y Y
13 2004 0516 06:04:08.3 23.09 121.99 12.5 6.0/5.3 105.9 Y Y Y
Y Y
14 2004 0519 07:04:12.0 22.70 121.39 8.7 6.5/6.0 143.5 Y Y Y
Y Y
15 2004 0522 20:25:38.5 24.04 121.51 19.2 4.2/
12.2 Y Y Y
Y Y
16 2004 0602 16:56:28.8 23.64 121.33 9.7 5.2/4.5 47.2 Y Y Y
Y Y
----------------------------------------------------------------------------------------------------------
178
3. Coherence Analysis
A standard method to quantitatively compare two time-series signals, x(t) and y(t),
where t is time, is to compute the coherence function between these two signals in the
frequency domain. The magnitude squared coherence function, Cxy(f) is defined by:
Cxy(f) = │Pxy(f) │2
⁄ [Pxx(f) Pyy(f)]
(1)
where f is frequency; Pxy(f) is the cross spectral density (CSD) function of x(t) and y(t);
and Pxx(f), and Pyy(f) are power spectral density (PSD) function of x(t) and y(t),
respectively. The goal of computing power spectra is to describe the distribution over
frequency of the power contained in a signal, based on a finite set of sampled data. PSD
function is actually a special case of CSD function when x(t) = y(t). The cross spectral
density function is defined by:
Pxy(ω) = Σ Rxy(m) exp(-iωm)
(2)
where the summation is over m, ω = 2πf /fs, fs is the sampling frequency, and Rxy(m) is
the cross correlation sequence:
Rxy(m) = E{x(n) y*(n+m)}
(3)
where E{·} is the expected value operator, and x(n) and y(n) are the discrete time series
of x(t) and y(t), respectively.
Fortunately, coherence analysis can be implemented by using the software package
MATLAB with its Signal Processing Toolbox. In particular, we use “cohere(x,y)” to
compute the magnitude squared coherence function (between two length n signal
vectors x and y) as a function of frequency. The maximum frequency is 100 Hz
because our data are sampled at 200 samples per second. The minimum frequency is
179
limited by the length of our sampled signals. Readers are referred to the User’s Guide
of Signal Processing Toolbox for use with MATLAB for more details (MathWorks,
2000).
Since we have a total of 73 data files and each file has three components (vertical,
north-south, and east-west), we have a total of 219 individual time series. Thus we can
compute over 20,000 coherence functions from possible pairs of available time series.
Since correlating between different earthquakes and between different components of
the instruments are of secondary importance, we concentrated on comparing the same
component between different instruments for the same earthquake. Even then, we have
over 1,000 coherence functions to study.
The A900A accelerographs (permanent and temporary) are 16-bit instruments,
whereas the Kinemetrics, the Reftek and the two Tokyo-Sokushin instruments produce
24-bit data. Although these latter instruments do not have perfect 24-bit resolution from
0 to 50 Hz, we call them “24-bit” instruments because they use 24-bit analog-to-digital
chips and record data in 24-bit integers.
4. Comparisons of “24-bit” Strong-Motion Instruments
In this section, we will compare the “24-bit” strong-motion instruments, i.e.,
Kinemetrics K2, Reftek 130-SMA/01, Tokyo-Sokushin 575, and Tokyo-Sokushin G3
(with the broadband velocity sensor), earthquake by earthquake. Since the recorded data
are labeled by channel numbers, and different manufacturers assign components to
channels differently, we selected the proper component pairs and the components are
noted in the figure captions. We have standardized the polarity in all the figures of this
report.
Please note that in all the plots, the order is by channel number of the first selected
instrument and the corresponding components are labeled according to the legends
shown.
Since the first instrument is selected alphabetically, Kinemetrics K2
accelerograph becomes the first “24-bit” instrument chosen, the component order is
East-West (EW), North-South (NS), and Vertical (V). However, when we compare TS180
575 and TS-G3, the component order is Vertical (V), North-South (NS), and East-West
(EW).
All the plots are in color, and therefore, do not show up well in black-and-white
printing. This is not a serious problem for the coherence functions by components in
superposition, because the curves can be distinguished by (1) solid, (2) dash, and (3)
dash and dot, in addition to different colors. However, the waveform plots of the two
instruments are superposed for each component. Although they are in two different
colors, the superposition makes it difficult to distinguish, especially when the plot is
compressed to the limited space within a page. They are included in the plots to show
the data used for computing the coherence functions.
In general, the earthquake
waveforms from two co-located instruments appear to be nearly the same and hardly
distinguishable from each other. Visual comparison of two signals is very subjective
and consequently, we compute coherence functions for objective comparisons.
4.1. Earthquake at 09:03 on April 3, 2004 (Event #1)
Event #1 is a very small earthquake located about 8 km from the accelerographs
and has a local magnitude of 3.9, and no Mw value is available. Only 3 instruments
were in operation: AP, K2, and RK, and all recorded it.
Figure 2 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the Reftek
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded Reftek time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. For the NorthSouth component, the coherence is nearly perfect from about 0.8 Hz to 40 Hz. However,
the coherence functions for the other two components are not that good for no obvious
reasons.
181
4.2. Earthquake at 05:33 on April 9, 2004 (Event #2)
Event #2 is a small earthquake located about 16 km from the accelerographs and
has a local magnitude of 4.8, and Mw = 4.0. Only 3 instruments were in operation: AP,
K2, and RK, and all recorded it.
Figure 3 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the Reftek
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded Reftek time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. For the NorthSouth component, the coherence is nearly perfect from about 0.8 Hz to 40 Hz. However,
the coherence functions for other two components are not that good, especially for the
vertical component. Nevertheless, coherence is a little better in this case than in Event
#1, probably because the earthquake is a little larger.
182
Comparison of K2 and Reftek For Earthquake (2004) 04030903
10
Acceleration (cm/s2)
Pair 1: K2 (Component = EW),
Reftek (Component = EW)
5
0
−5
K2
Reftek
−10
10
Acceleration (cm/s2)
Pair 2: K2 (Component = NS),
Reftek (Component = NS)
5
0
−5
K2
Reftek
−10
10
Acceleration (cm/s2)
Pair 3: K2 (Component = V),
Reftek (Component = V)
5
0
−5
−10
−5
K2
Reftek
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 2 : Coherence functions between recorded data of K2 and Reftek for Event #1.
See text for explanations.
183
Comparison of K2 and Reftek For Earthquake (2004) 04090533
Acceleration (cm/s2)
40
Pair 1: K2 (Component = EW),
Reftek (Component = EW)
20
0
−20
K2
Reftek
Acceleration (cm/s2)
−40
60
Pair 2: K2 (Component = NS),
Reftek (Component = NS)
40
20
0
K2
Reftek
−20
Acceleration (cm/s2)
−40
20
Pair 3: K2 (Component = V),
Reftek (Component = V)
10
0
−10
−20
−5
K2
Reftek
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 3 : Coherence functions between recorded data of K2 and Reftek for Event #2.
See text for explanations.
184
4.3. Earthquake at 02:26 on April 23, 2004 (Event #3)
Event #3 is a microearthquake located about 2 km from the accelerographs and has
a local magnitude of 2.3 and no Mw estimate. Five instruments were in operation: AP,
AT, K2, TS, and G3, and all recorded it.
Figure 4 shows the coherence functions between the recorded time series data of
East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 5 Hz to 20 Hz. This earthquake is too small
to have sufficient waves in longer periods, and thus, the coherence functions are fair to
poor below 5 Hz.
Figure 5 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the horizontal components from about 2 Hz to 10 Hz, and fair for the vertical
component. The general the coherence in this case is not as good as the case between
K2 and the TS-575 instrument. One possible explanation is that the TS-G3 instrument
has a broadband velocity sensor, and the recorded data have to be differentiated
numerically to yield acceleration data for comparison. See comment about the
coherence of the vertical-component pair below.
Figure 6 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
185
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 2 Hz to 20 Hz, but the coherence
for the vertical component is rather poor. One possible explanation is that the two
instruments were located about 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
186
Comparison of K2 and TS575 For Earthquake (2004) 04230226
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
10
TS575 (Component = EW) (flipped)
5
0
K2
TS575
−5
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
2
TS575 (Component = V)
1
0
−1
−2
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 4 : Coherence functions between recorded data of K2 and TS-575 for Event #3.
See text for explanations.
187
Comparison of K2 and TSG3 For Earthquake (2004) 04230226
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
10
TSG3 (Component = EW)
5
0
K2
TSG3
−5
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
2
TSG3 (Component = V)
1
0
−1
−2
−5
K2
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 5 : Coherence functions between recorded data of K2 and TS-G3 for Event #3.
See text for explanations.
188
Comparison of TS575 and TSG3 For Earthquake (2004) 04230226
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
2
TSG3 (Component = V)
1
0
−1
TS575
TSG3
−2
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−10
10
TSG3 (Component = EW)
5
0
TS575
TSG3
−5
−5
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 6 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#3. See text for explanations.
189
4.4. Earthquake at 02:27 on April 23, 2004 (Event #4)
Event #4 is a microearthquake located about 3 km from the accelerographs and has
a local magnitude of 2.5 and no Mw estimate. It occurred in less than one minute after
Event #3. Five instruments were in operation: AP, AT, K2, TS, and G3, and all
recorded it.
Figure 7 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 17-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 2 Hz to 30 Hz. The coherence results are
better than that for Event #3, probably due to a smaller time window (17 vs 28 sec) of
the available data for analysis.
Figure 8 shows the coherence functions between the recorded time series data of
East-West, North-South, and Vertical components of the K2 and the TS-G3 instruments.
The top 3 frames shows the recorded K2 time series in green and the recorded TS-G3
time series in purple. The bottom frame shows the coherence functions between these
three pairs of recorded time series. We selected a 17-second window of the recorded
data, starting just one second before the first P-arrival. Coherence is nearly perfect for
all three components from about 1 Hz to 10 Hz, but not as good as the coherence
between K2 and the TS-575 instrument. One possible explanation is that the TS-G3
instrument has a broadband velocity sensor and the recorded data have to be
differentiated numerically to yield acceleration data for comparison.
Figure 9 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West, components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 17-second window of
190
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 1 Hz to 20 Hz, but the coherence
for the vertical component is poor beyond 10 Hz. One possible explanation is that the
two instruments were located about 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
191
Comparison of K2 and TS575 For Earthquake (2004) 04230227
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
6
TS575 (Component = EW) (flipped)
4
2
0
K2
TS575
−2
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−4
4
TS575 (Component = NS) (flipped)
2
0
−2
K2
TS575
Acceleration (cm/s2)
−4
1
Pair 3: K2 (Component = V),
TS575 (Component = V)
0.5
0
−0.5
K2
TS575
−1
−1.5
−2
0
2
4
6
8
10
Time relative to P−pick (s)
12
14
16
18
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 7 : Coherence functions between recorded data of K2 and TS-575 for Event #4.
See text for explanations.
192
Comparison of K2 and TSG3 For Earthquake (2004) 04230227
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
6
TSG3 (Component = EW)
4
2
0
K2
TSG3
−2
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−4
4
TSG3 (Component = NS)
2
0
−2
K2
TSG3
Acceleration (cm/s2)
−4
1
Pair 3: K2 (Component = V),
TSG3 (Component = V)
0.5
0
−0.5
K2
TSG3
−1
−1.5
−2
0
2
4
6
8
10
Time relative to P−pick (s)
12
14
16
18
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 8 : Coherence functions between recorded data of K2 and TS-G3 for Event #4.
See text for explanations.
193
Comparison of TS575 and TSG3 For Earthquake (2004) 04230227
Acceleration (cm/s2)
1
Pair 1: TS575 (Component = V),
TSG3 (Component = V)
0.5
0
−0.5
TS575
TSG3
−1
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−1.5
4
TSG3 (Component = NS)
2
0
−2
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−4
6
TSG3 (Component = EW)
4
2
0
TS575
TSG3
−2
−4
−5
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 9 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#4. See text for explanations.
194
4.5. Earthquake at 15:20 on April 24, 2004 (Event #5)
Event #5 is a small earthquake located about 14 km from the accelerographs and
has a local magnitude of 5.3 and Mw = 4.4. Five instruments were in operation: AP,
AT, K2, TS, and G3, and all recorded it.
Figure 10 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 1 Hz to 30 Hz.
Figure 11 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 1 Hz to 40 Hz, but the coherence
for the vertical component is poor beyond 4 Hz. One possible explanation is that the
One possible explanation is that the two instruments were located a little over one meter
apart on the seismic pier and the vertical component may be more sensitive to location
on the seismic pier.
Figure 12 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 1 Hz to 40 Hz, but the coherence
195
for the vertical component is poor beyond 4 Hz. One possible explanation is that the
two instruments were located about 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
Comparison of K2 and TS575 For Earthquake (2004) 04241520
100
Acceleration (cm/s2)
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
50
0
K2
TS575
−50
100
Acceleration (cm/s2)
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
50
0
−50
K2
TS575
Acceleration (cm/s2)
−100
40
Pair 3: K2 (Component = V),
TS575 (Component = V)
20
0
−20
−40
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 10 : Coherence functions between recorded data of K2 and TS-575 for Event #5.
See text for explanations.
196
Comparison of K2 and TSG3 For Earthquake (2004) 04241520
100
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
50
0
K2
TSG3
−50
100
Acceleration (cm/s2)
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
50
0
−50
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−100
40
TSG3 (Component = V)
20
0
−20
−40
−5
K2
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 11 : Coherence functions between recorded data of K2 and TS-G3 for Event #5.
See text for explanations.
197
Comparison of TS575 and TSG3 For Earthquake (2004) 04241520
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
40
TSG3 (Component = V)
20
0
−20
TS575
TSG3
−40
100
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
50
0
−50
TS575
TSG3
−100
100
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
50
0
TS575
TSG3
−50
−5
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 12 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#5. See text for explanations.
198
4.6. Earthquake at 19:26 on April 24, 2004 (Event #6)
Event #6 is a very small earthquake located about 15 km from the accelerographs
and has a moment magnitude of 3.4. Five instruments were in operation: AP, AT, K2,
TS, and G3, and all recorded it.
Figure 13 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 4Hz to 50 Hz.
Figure 14 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is very
good for the two horizontal components from about 2 Hz to 30 Hz, but the coherence
for the vertical component is fair to poor in the same frequency band. One possible
explanation is that the two instruments were located a little over one meter apart on the
seismic pier and the vertical component may be more sensitive to location on the
seismic pier.
Figure 15 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 3 Hz to 30 Hz, but the coherence
199
for the vertical component is fair to poor in the same frequency band. One possible
explanation is that the two instruments were located 1.3 meters apart on the seismic pier
and the vertical component may be more sensitive to location on the seismic pier.
200
Comparison of K2 and TS575 For Earthquake (2004) 04241925
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
5
0
−5
K2
TS575
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
5
TS575 (Component = V)
0
K2
TS575
−5
−5
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 13 : Coherence functions between recorded data of K2 and TS-575 for Event #6.
See text for explanations.
201
Comparison of K2 and TSG3 For Earthquake (2004) 04241925
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
5
0
−5
K2
TSG3
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TSG3 (Component = V)
2
0
−2
−4
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 14 : Coherence functions between recorded data of K2 and TS-G3 for Event #6.
See text for explanations.
202
Comparison of TS575 and TSG3 For Earthquake (2004) 04241925
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
5
TSG3 (Component = V)
0
TS575
TSG3
−5
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
−10
10
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
5
0
−5
−10
−5
TS575
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 15 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#6. See text for explanations.
203
4.7. Earthquake at 22:29 on April 24, 2004 (Event #7)
Event #7 is a very small earthquake located about 13 km from the accelerographs
and has a moment magnitude of 3.5. Five instruments were in operation: AP, AT, K2,
TS, and G3, and all recorded it. This earthquake occurred very near Event #6, and has
nearly the same moment magnitude.
Figure 16 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 2 Hz to 40 Hz.
Figure 17 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 2 Hz to 30 Hz, but the coherence
for the vertical component is fair to poor in the same frequency band. One possible
explanation is that the two instruments were located a little over one meter apart on the
seismic pier and the vertical component may be more sensitive to location on the
seismic pier.
Figure 18 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
204
perfect for the two horizontal components from about 2 Hz to 30 Hz, but the coherence
for the vertical component is fair to poor in the same frequency band. One possible
explanation is that the two instruments were located 1.3 meters apart on the seismic pier
and the vertical component may be more sensitive to location on the seismic pier.
205
Comparison of K2 and TS575 For Earthquake (2004) 04242228
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
20
TS575 (Component = EW) (flipped)
10
0
−10
K2
TS575
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−20
20
TS575 (Component = NS) (flipped)
10
0
−10
K2
TS575
−20
10
2
Acceleration (cm/s )
Pair 3: K2 (Component = V),
TS575 (Component = V)
5
0
−5
−10
−5
K2
TS575
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 16 : Coherence functions between recorded data of K2 and TS-575 for Event #7.
See text for explanations.
206
Comparison of K2 and TSG3 For Earthquake (2004) 04242228
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
20
TSG3 (Component = EW)
10
0
−10
K2
TSG3
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = NS)
10
0
−10
K2
TSG3
−20
10
2
Acceleration (cm/s )
Pair 3: K2 (Component = V),
TSG3 (Component = V)
5
0
−5
−10
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 17 : Coherence functions between recorded data of K2 and TS-G3 for Event #7.
See text for explanations.
207
Comparison of TS575 and TSG3 For Earthquake (2004) 04242228
10
2
Acceleration (cm/s )
Pair 1: TS575 (Component = V),
TSG3 (Component = V)
5
0
−5
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−10
20
TSG3 (Component = NS)
10
0
−10
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = EW)
10
0
−10
−20
−5
TS575
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 18 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#7. See text for explanations.
208
4.8. Earthquake at 14:28 on April 25, 2004 (Event #8)
Event #8 is a very small earthquake located about 16 km from the accelerographs.
It has a local magnitude of 3.3 and no moment magnitude estimate. Five instruments
were in operation: AP, AT, K2, TS, and G3, and all recorded it. This earthquake
occurred very near Event #6, and has nearly the same moment magnitude.
Figure 19 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. From about 5 Hz to
50 Hz, coherence is nearly perfect for the two horizontal components, and is very good
for the vertical component. However, coherence of each of the component pairs is fair
to poor below 5 Hz for no obvious reasons.
Figure 20 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. From about 5 Hz to
30 Hz, coherence is nearly perfect for the two horizontal components, but is poor for the
vertical component. Coherence of each of the component pairs is fair to poor below 5
Hz for no obvious reasons.
Figure 21 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 28-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
209
perfect for the two horizontal components from about 5 Hz to 30 Hz, but the coherence
for the vertical component is poor in general. One possible explanation is that the two
instruments were located 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
210
Comparison of K2 and TS575 For Earthquake (2004) 04251428
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
4
TS575 (Component = EW) (flipped)
2
0
−2
K2
TS575
−4
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−6
5
TS575 (Component = NS) (flipped)
0
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−5
2
TS575 (Component = V)
1
0
−1
−2
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 19 : Coherence functions between recorded data of K2 and TS-575 for Event #8.
See text for explanations.
211
Comparison of K2 and TSG3 For Earthquake (2004) 04251428
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
5
TSG3 (Component = EW)
0
K2
TSG3
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−5
5
TSG3 (Component = NS)
0
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−5
2
TSG3 (Component = V)
1
0
−1
−2
−5
K2
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 20 : Coherence functions between recorded data of K2 and TS-G3 for Event #8.
See text for explanations.
212
Comparison of TS575 and TSG3 For Earthquake (2004) 04251428
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
2
TSG3 (Component = V)
1
0
−1
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−2
5
TSG3 (Component = NS)
0
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−5
5
TSG3 (Component = EW)
0
TS575
TSG3
−5
−5
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 21 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#8. See text for explanations.
213
4.9. Earthquake at 07:56 on May 1, 2004 (Event #9)
Event #9 is a moderate size earthquake located about 14 km from the
accelerographs. It has a moment magnitude of 5.0. Five instruments were in operation:
AP, AT, K2, TS, and G3, and all recorded it. This earthquake occurred very near Event
#6, but is larger in magnitude by 1.6.
Figure 22 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 0.8 Hz to 20 Hz.
Figure 23 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components, but the coherence for
the East-West component becomes poor beyond about 30 Hz.
Coherence for the
vertical component is very good from about 0.8 Hz to 10 Hz, and becomes poor beyond
10 Hz.
Figure 24 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 0.8 Hz to 40 Hz, but the
214
coherence for the vertical component is poorer in general in comparison with the
coherence for the horizontal components. One possible explanation is that the two
instruments were located 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
215
Comparison of K2 and TS575 For Earthquake (2004) 05010755
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
400
TS575 (Component = EW) (flipped)
200
0
−200
K2
TS575
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−400
400
TS575 (Component = NS) (flipped)
200
0
−200
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−400
100
TS575 (Component = V)
50
0
−50
−100
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 22 : Coherence functions between recorded data of K2 and TS-575 for Event #9.
See text for explanations.
216
Comparison of TS575 and TSG3 For Earthquake (2004) 05010755
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
100
TSG3 (Component = V)
50
0
−50
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−100
400
TSG3 (Component = NS)
200
0
−200
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−400
400
TSG3 (Component = EW)
200
0
−200
−400
−5
TS575
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 23 : Coherence functions between recorded data of K2 and TS-G3 for Event #9.
See text for explanations.
217
Comparison of TS575 and TSG3 For Earthquake (2004) 05010755
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
100
TSG3 (Component = V)
50
0
−50
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−100
400
TSG3 (Component = NS)
200
0
−200
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−400
400
TSG3 (Component = EW)
200
0
−200
−400
−5
TS575
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 24 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#9. See text for explanations.
218
4.10. Earthquake at 07:46 on May 9, 2004 (Event #10)
Event #10 is a microearthquake located about 2 km from the accelerographs. It has
a local magnitude of 2.5. Although five instruments were in operation, but only AP and
G3 recorded it, probably due to slightly more sensitive triggering. Since we do not have
two “24-bit” instruments recorded this earthquake, no comparison can be made.
4.11. Earthquake at 20:06 on May 9, 2004 (Event #11)
Event #11 is a moderate size earthquake located about 68 km from the
accelerographs. It has a local magnitude of 5.7 and a moment magnitude of 4.8. Five
instruments were in operation: AP, AT, K2, TS, and G3, and all recorded it.
Figure 25 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is
almost perfect for all three components from about 0.8 Hz to nearly 50 Hz, although
there is a decrease in coherence for the vertical component around 30 Hz.
Figure 26 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 3 Hz, fair to 20 Hz, and poor
beyond.
Figure 27 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
219
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 3 Hz, but becomes poor beyond.
One possible explanation is that the two instruments were located about 1.3 m apart on
the seismic pier and the vertical component may be more sensitive to location on the
seismic pier.
220
Comparison of K2 and TS575 For Earthquake (2004) 05092006
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
5
0
−5
K2
TS575
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TS575 (Component = V)
2
0
−2
−4
−5
K2
TS575
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 25 : Coherence functions between recorded data of K2 and TS-575 for Event
#11. See text for explanations.
221
Comparison of K2 and TSG3 For Earthquake (2004) 05092006
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
5
0
−5
K2
TSG3
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TSG3 (Component = V)
2
0
−2
−4
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 26 : Coherence functions between recorded data of K2 and TS-G3 for Event #11.
See text for explanations.
222
Comparison of TS575 and TSG3 For Earthquake (2004) 05092006
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
4
TSG3 (Component = V)
2
0
−2
TS575
TSG3
−4
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
−10
10
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
5
0
−5
−10
−5
TS575
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 27 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#11. See text for explanations.
223
4.12. Earthquake at 15:28 on May 13, 2004 (Event #12)
Event #12 is a small earthquake located about 13 km from the accelerographs. It
has a local magnitude of 4.6 and a moment magnitude of 3.7. Five instruments were in
operation: AP, AT, K2, TS, and G3, and all recorded it.
Figure 28 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 0.8 Hz to 40 Hz, although the coherence is a
little poorer for the vertical component in comparison with the horizontal components.
Figure 29 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 2 Hz, but becomes poor beyond.
One possible explanation is that the two instruments were located over a little over one
meter apart on the seismic pier and the vertical component may be more sensitive to
location on the seismic pier.
Figure 30 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
224
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 2 Hz, but becomes poor beyond.
One possible explanation is that the two instruments were located 1.3 meters apart on
the seismic pier and the vertical component may be more sensitive to location on the
seismic pier.
225
Comparison of K2 and TS575 For Earthquake (2004) 05131528
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
30
TS575 (Component = EW) (flipped)
20
10
0
K2
TS575
−10
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−20
40
TS575 (Component = NS) (flipped)
20
0
−20
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−40
20
TS575 (Component = V)
10
0
−10
−20
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 28 : Coherence functions between recorded data of K2 and TS-575 for Event
#12. See text for explanations.
226
Comparison of K2 and TSG3 For Earthquake (2004) 05131528
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
40
TSG3 (Component = EW)
20
0
−20
K2
TSG3
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−40
40
TSG3 (Component = NS)
20
0
−20
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−40
20
TSG3 (Component = V)
10
0
−10
−20
−5
K2
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 29 : Coherence functions between recorded data of K2 and TS-G3 for Event #12.
See text for explanations.
227
Comparison of TS575 and TSG3 For Earthquake (2004) 05131528
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
20
TSG3 (Component = V)
10
0
−10
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−20
40
TSG3 (Component = NS)
20
0
−20
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−40
40
TSG3 (Component = EW)
20
0
−20
−40
−5
TS575
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 30 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#12. See text for explanations.
228
4.13. Earthquake at 06:04 on May 16, 2004 (Event #13)
Event #13 is a moderate size earthquake located about 106 km from the
accelerographs. It has a local magnitude of 6.0 and a moment magnitude of 5.3. Five
instruments were in operation: AP, AT, K2, TS, and G3, and all recorded it.
Figure 31 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 0.8 Hz to nearly 40 Hz, although the
coherence is a little poorer for the vertical component in comparison with the horizontal
components.
Figure 32 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 3 Hz, but becomes fair and then
poor beyond. One possible explanation is that the two instruments were located over a
little over one meter apart on the seismic pier and the vertical component may be more
sensitive to location on the seismic pier.
Figure 33 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
229
the recorded data, starting just one second before the first P-arrival. From 0.8 Hz to 30
Hz, coherence is nearly perfect for the two horizontal components. Coherence for the
vertical component is very good from about 0.8 Hz to 3 Hz, but becomes fair and then
poor beyond. One possible explanation is that the two instruments were located 1.3
meters apart on the seismic pier and the vertical component may be more sensitive to
location on the seismic pier.
230
Comparison of K2 and TS575 For Earthquake (2004) 05160604
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
5
0
−5
K2
TS575
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TS575 (Component = V)
2
0
−2
−4
−5
K2
TS575
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 31 : Coherence functions between recorded data of K2 and TS-575 for Event
#13. See text for explanations.
231
Comparison of K2 and TSG3 For Earthquake (2004) 05160604
15
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
10
5
0
K2
TSG3
−5
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TSG3 (Component = V)
2
0
−2
−4
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 32 : Coherence functions between recorded data of K2 and TS-G3 for Event #13.
See text for explanations.
232
Comparison of TS575 and TSG3 For Earthquake (2004) 05160604
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
4
TSG3 (Component = V)
2
0
−2
TS575
TSG3
−4
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
−10
15
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
10
5
0
TS575
TSG3
−5
−10
−5
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 33 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#13. See text for explanations.
233
4.14. Earthquake at 07:04 on May 19, 2004 (Event #14)
Event #14 is the largest event recorded during the test period. This earthquake is
located about 144 km from the accelerographs. It has a local magnitude of 6.5 and a
moment magnitude of 6.0. Five instruments were in operation: AP, AT, K2, TS, and
G3, and all recorded it.
Figure 34 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting as early as possible (this event was probably triggered late
and because it is far away, it was not recorded in its entirety). Coherence is nearly
perfect for all three components from about 0.8 Hz to 30 Hz, although the coherence is a
little poorer for the vertical component in comparison with the horizontal components.
Figure 35 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from 0.8 Hz to 10 Hz, but becomes fair
beyond. Coherence for the vertical component is very good from about 0.8 Hz to 3 Hz,
but becomes fair and then poor beyond. One possible explanation is that the two
instruments were located a little over one meter apart on the seismic pier and the
vertical component may be sensitive to location on the seismic pier.
Figure 36 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
234
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from 0.8 Hz to 10 Hz, but becomes fair
beyond. Coherence for the vertical component is very good from about 0.8 Hz to 3 Hz,
but becomes poor beyond. One possible explanation is that the two instruments were
located about 1.3 meters apart on the seismic pier and the vertical component may be
sensitive to location on the seismic pier.
235
Comparison of K2 and TS575 For Earthquake (2004) 05190704
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
5
0
−5
K2
TS575
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TS575 (Component = V)
2
0
−2
−4
−5
K2
TS575
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 34 : Coherence functions between recorded data of K2 and TS-575 for Event
#14. See text for explanations.
236
Comparison of K2 and TSG3 For Earthquake (2004) 05190704
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
5
0
−5
K2
TSG3
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
4
TSG3 (Component = V)
2
0
−2
−4
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 35 : Coherence functions between recorded data of K2 and TS-G3 for Event #14.
See text for explanations.
237
Comparison of TS575 and TSG3 For Earthquake (2004) 05190704
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
4
TSG3 (Component = V)
2
0
−2
TS575
TSG3
−4
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
−10
10
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
5
0
−5
−10
−5
TS575
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 36 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#14. See text for explanations.
238
4.15. Earthquake at 20:25 on May 22, 2004 (Event #15)
Event #15 is a small earthquake located about 12 km from the accelerographs. It
has a local magnitude of 4.2 and no moment magnitude estimate. Five instruments were
in operation: AP, AT, K2, TS, and G3, and all recorded it.
Figure 37 shows the coherence functions between the recorded time series data of
the recorded time series data of the East-West, North-South, and Vertical components of
the K2 and the TS-575 accelerographs. The top 3 frames shows the recorded K2 time
series in green and the recorded TS-575 time series in purple. The bottom frame shows
the coherence functions between these three pairs of recorded time series. We selected a
30-second window of the recorded data, starting just one second before the first Parrival. Coherence is nearly perfect for all three components from about 3 Hz to 40 Hz,
although the coherence is a little poorer for the vertical component in comparison with
the horizontal components.
Figure 38 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 3 Hz to 30 Hz. Coherence for the
vertical component is much poorer in comparison with that for the horizontal
components. One possible explanation is that the two instruments were located a little
over one meter apart on the seismic pier and the vertical component may be sensitive to
location on the seismic pier.
Figure 39 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
239
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from 2 Hz to 30 Hz. Coherence for the
vertical component is poor in general. One possible explanation is that the two
instruments were located about 1.3 meters apart on the seismic pier and the vertical
component may be more sensitive to location on the seismic pier.
240
Comparison of K2 and TS575 For Earthquake (2004) 05222025
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
20
TS575 (Component = EW) (flipped)
10
0
−10
K2
TS575
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−20
20
TS575 (Component = NS) (flipped)
10
0
−10
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−20
20
TS575 (Component = V)
10
0
−10
−20
−5
K2
TS575
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 37 : Coherence functions between recorded data of K2 and TS-575 for Event
#15. See text for explanations.
241
Comparison of K2 and TSG3 For Earthquake (2004) 05222025
Pair 1: K2 (Component = EW),
2
Acceleration (cm/s )
20
TSG3 (Component = EW)
10
0
−10
K2
TSG3
Pair 2: K2 (Component = NS),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = NS)
10
0
−10
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = V)
10
0
−10
−20
−5
K2
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 38 : Coherence functions between recorded data of K2 and TS-G3 for Event #15.
See text for explanations.
242
Comparison of TS575 and TSG3 For Earthquake (2004) 05222025
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
20
TSG3 (Component = V)
10
0
−10
TS575
TSG3
Pair 2: TS575 (Component = NS) (flipped),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = NS)
10
0
−10
TS575
TSG3
Pair 3: TS575 (Component = EW) (flipped),
2
Acceleration (cm/s )
−20
20
TSG3 (Component = EW)
10
0
−10
−20
−5
TS575
TSG3
0
5
10
15
Time relative to P−pick (s)
20
25
30
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 39 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#15. See text for explanations.
243
4.16. Earthquake at 16:56 on June 2, 2004 (Event #16)
Event #16 is a small earthquake located about 47 km from the accelerographs. It
has a local magnitude of 5.2 and a moment magnitude of 4.5. Five instruments were in
operation: AP, AT, K2, TS, and G3, and all recorded it.
Figure 40 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-575
accelerographs. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-575 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 30-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for all three components from about 0.8 Hz to 40 Hz, although the coherence is a
little poorer at 30 Hz for the vertical component in comparison with the horizontal
components.
Figure 41 shows the coherence functions between the recorded time series data of
the East-West, North-South, and Vertical components of the K2 and the TS-G3
instruments. The top 3 frames shows the recorded K2 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 0.8 Hz to 30 Hz. Coherence for
the vertical component is almost perfect from 0.8 Hz to 3 Hz, but becomes fair and then
poor beyond. One possible explanation is that the two instruments were located a little
over one meter apart on the seismic pier and the vertical component may be more
sensitive to location on the seismic pier.
Figure 42 shows the coherence functions between the recorded time series data of
the Vertical, North-South, and East-West components of the TS-575 and the TS-G3
instruments. The top 3 frames shows the recorded TS-575 time series in green and the
recorded TS-G3 time series in purple. The bottom frame shows the coherence functions
between these three pairs of recorded time series. We selected a 31-second window of
244
the recorded data, starting just one second before the first P-arrival. Coherence is nearly
perfect for the two horizontal components from about 0.8 Hz to 30 Hz. Coherence for
the vertical component is almost perfect from about 0.8 Hz to 3 Hz, but becomes poor in
higher frequencies. One possible explanation is that the two instruments were located
about 1.3 meters apart on the seismic pier and the vertical component may be more
sensitive to location on the seismic pier.
245
Comparison of K2 and TS575 For Earthquake (2004) 06021656
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TS575 (Component = EW) (flipped)
5
0
−5
K2
TS575
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TS575 (Component = NS) (flipped)
5
0
−5
K2
TS575
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
2
TS575 (Component = V)
1
0
−1
K2
TS575
−2
−3
−5
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 40 : Coherence functions between recorded data of K2 and TS-575 for Event
#16. See text for explanations.
246
Comparison of K2 and TSG3 For Earthquake (2004) 06021656
10
2
Acceleration (cm/s )
Pair 1: K2 (Component = EW),
TSG3 (Component = EW)
5
0
−5
K2
TSG3
−10
10
2
Acceleration (cm/s )
Pair 2: K2 (Component = NS),
TSG3 (Component = NS)
5
0
−5
K2
TSG3
Pair 3: K2 (Component = V),
2
Acceleration (cm/s )
−10
2
TSG3 (Component = V)
1
0
−1
K2
TSG3
−2
−3
−5
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = EW
Component = NS
Component = V
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 41 : Coherence functions between recorded data of K2 and TS-G3 for Event #16.
See text for explanations.
247
Comparison of TS575 and TSG3 For Earthquake (2004) 06021656
Pair 1: TS575 (Component = V),
2
Acceleration (cm/s )
2
TSG3 (Component = V)
1
0
−1
TS575
TSG3
−2
−3
10
2
Acceleration (cm/s )
Pair 2: TS575 (Component = NS) (flipped),
TSG3 (Component = NS)
5
0
−5
TS575
TSG3
−10
10
2
Acceleration (cm/s )
Pair 3: TS575 (Component = EW) (flipped),
TSG3 (Component = EW)
5
0
−5
−10
−5
TS575
TSG3
0
5
10
15
20
Time relative to P−pick (s)
25
30
35
Coherence (All Pairs)
1
Coherence
0.8
0.6
0.4
0.2
Component = V
Component = NS
Component = EW
0
−1
10
0
1
10
10
2
10
Frequency (Hz)
Figure 42 : Coherence functions between recorded data of TS-575 and TS-G3 for Event
#16. See text for explanations.
248
Discussions
The Hualien field test of accelerographs is still ongoing at present, but with only 3
co-located accelerographs, because the Reftek and the two Tokyo-Sokushin instruments
had been removed. The first author has urged CWB to deploy two of its own TokyoSokushin 575 accelerographs to HWA in order to study the coherence between two 24bit accelerographs of identical model.
In order to capture larger earthquakes, we need to continue this type of experiment
to answer the question about coherence at frequency below 1 Hz, since the frequency
band of engineering interest extends to about 0.1 Hz.
At present, another experiment is being carried out under the leadership of C. C.
Liu to compare sensors at a broadband station in Taiwan (as far away from the coast as
practical). All signals are being digitized and continuously recorded by the same model
of Quanterra data loggers.
We have generated far more results (over 1,000 coherence plots) than we can
digest in a short period of time.
We also need to figure out a better method to
summarize the coherence analysis results. In addition, we need to investigate the
impacts of non-perfect coherence on parameters of engineering interest (e.g., peak
ground acceleration, peak ground velocity, peak ground displacement, response spectra,
etc.).
The preliminary comparison results presented here indicated that the 24-bit
accelerographs (that CWB has purchased) performed quite well in the field. Coherence
for the horizontal components is nearly perfect in many of the recorded earthquakes.
But the coherence for the vertical component is generally poorer, in comparison with
the coherence for the horizontal components. We suspect that this may have something
to do with the seismic pier, and the vertical component may be more sensitive to where
the instrument is placed on the seismic pier.
Since the sensor of the Reftek accelerograph is based on the new solid-state MEM
design and is very different than the traditional FBA design used for A900A, K2, and
Tokyo-Sokushin 575. We now have some field evidence that these two different type
249
sensors recorded very similar ground accelerations. However, we definitely need more
earthquakes recorded by a co-located Reftek accelerograph to have more confidence.
Coherence of recorded data between the Tokyo-Sokushin G3 instrument (with
broadband velocity sensor) and the K2 accelerograph is slightly poorer, in comparison
with coherence of recorded data between Tokyo-Sokushin 575 accelerograph and the
K2 accelerograph. However, more study is needed to find some explanations for the
difference, e.g., where or not numerical differentiation of velocity data introduces
incoherence in the acceleration data.
Finally, due to practical limitations of manufacturing and deployment in the field,
it will be very difficult to record the same earthquake to better than 99% in coherence
by two co-located instruments (about 1 meter apart) in general. This result is perhaps
not as good as we like, but not as bad as we may have feared. We may never know the
“true” ground motions, but the present study suggests that we could “reproduce” nearly
the same recorded data by instruments of different manufacturers in the field.
Acknowledgements
We wish to thank Mr. Outhay Viengkhow (Kinemetrics), Mr. Paul Passmore
(Reftek), and Mr. Isamu Yokoi (Tokyo-Sokushin) and their respective Taiwan
representatives for participating in this field test of multiple accelerographs in Hualien.
References
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CWB Seismology Center Report No. 7, June, 2004.
CWB (1997). A preliminary report on technical compliance test of a Model Etna
accelerograph by CWB Instrumentation Committee. In Annual Report to the
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