Historic earthquake relocation

investigation
Historic earthquake relocation
Yunong Nina Lin
Institute of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei, Taiwan, R.O.C.
The related subsurface fault planes of the Hsinchu-Taichung earthquake, Taiwan in 1935 remains a puzzle
till now. Although according to some previous literature, they were recommended to be a dextral fault
with almost vertical fault plane and a back thrust dipping to the west, we have no strong seismic proof of
the existence of the back thrust fault so far. In this study, we tried to relocate the micro-earthquakes
recorded in 1935, and figured out what the possible fault plane geometry of the back thrust is. The result
shows a west-dipping and deep-seated fault plane. This fault plane may have a gradually changing dip
from 60° at the top to 45° at the bottom (~10 km).
A big earthquake (MGR=7.1)1 took place in northwestern
Taiwan on April 21st, 1935. This earthquake daylighted two
surface ruptures: The Tuntzuchiao Fault in the south, which is a
dextral strike-slip fault, and the Siko Fault in the north, which is
a thrust fault. Based on the some wave-form and numerical
modelings done by Sheu et al. (1982) 2, Lin (1987) 3 and Huang
& Yeh (1994)4, the fault plane geometry of the Siko Fault is
west dipping, concave upward and deep-seated (up to 10 km in
depth).
In Taiwan, a west-dipping fault plane infers a
hinterland sequence under regional tectonic setting, which is not
a unique phenomenon here. However, a “west dipping” and
“deep-seated” fault plane in Taiwan is rare, for most west
dipping fault planes in Taiwan are shallow-seated. Therefore,
the existence of the Siko fault has a strong influence on the
reconstruction of the subsurface structural geology, if the
fault-plane geometry is real.
Nevertheless, except those
modeling results, there are no any other strong physical proofs,
such as seismicities, standing for this fault plane geometry so far.
The purpose of this study is to re-image the fault plane of the
Siko Fault via seismic data.
Data
A
B
Chinshui
Miaoli
Tunglo
Chuhuangkeng
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We searched the catalog of Central Weather Bureau (CWB) in
Taiwan, through a period from 1973 to 2003. Unfortunately, in
these latest two decades this once seismically active area seems
going through a quiescence stage (Fig. 1). Since it is not
possible to re-image the fault plane via such few earthquakes,
we turned to the historic seismic record. In 1936, Nasu (1936)5
and his colleagues came to Taiwan doing some
micro-earthquake observations from August to December.
They set up 4 seismic stations in Miaoli, Chuhaungkeng, Tunglo
and Chinshui. Totally 55 earthquakes with better quality were
published by ERI, including the P-S travel time residual,
maximum acceleration and initially-calculated depth. We took
advantage of these recorded P-S travel time residual and tried to
relocate these earthquakes.
Velocity model
Because only P-S travel time residual was provided, we need
to have the P- and S- wave velocity in order to do the relocation.
Besides of the 1D P-wave velocity structure published by CWB,
we also tried the velocity model specific for the
Hsinchu-Taichung area done by Lin et al. (1989, with both
P-wave and S-wave velocity model)6, Lee (1998, S-wave
velocity model) 7 and Sheng (1999, P-wave velocity
mode) 8. We combined the models of Lee and
Sheng to be ShL model, and then evaluated these
three models of CWB, Lin and ShL. A FORTRAN
program VELEST was introduced here to do this
evaluation. This freeware could be obtained on the
website
http://quake.usgs.gov/research/software/index.html
Figure 1. The map showing the regional
tectonic setting and all the earthquakes
from 1973 to 2003 in CWB catalog. A: The
regional tectonic feature. Our study area
is located at the northwestern Taiwan. B:
The catalog earthquakes relocated by
hypoDD. The big blue circle represents
the 1935 mainshock.
Two blue lines
represent the Tuntzuchiao Fault in the south
and the Siko Fault in the north. Obviously,
only very few earthquakes happened near
the Siko Fault in these two decades.
©2004 NTU publishing Group
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investigation
2.4 and 3.4 seconds at distance = 0 km). Finally we decided to
take the CWB model for further relocation job.
A
Relocation method & result
The method we used is the most basic inversion due to the
lack of other information necessary for complicated inversion.
A FORTRAN subroutine called CPTIME was engaged here,
with velocity model, depth and distance as the inputs and travel
time as the output. We applied the CWB velocity model,
initially-calculated depth and travel time residuals to make the
distance as output. We also tried to use a uniform P-S velocity
difference to do the relocation. The result shows very similar
patterns but different scattering diameters of these
micro-earthquakes (Fig. 4). We made a cross-section from
(120.5°E, 25.25°N) to (121.05°E, 24.75°N) and projected all the
B
A
B
Figure 2. A: The velocity structure models tested in
this study. Solid lines: the original models; dashed
lines: the modified models after running with the
program VELEST. B: Model RMS testing. The
. average RMS values for the models from top to bottom
are 0.13, 0.19 and 0.19. The upper one (CWB) model
has the lowest proportion of higher RMS values.
For more usage and details, please also refer to the user guide in
the package.
We input the earthquakes from CWB catalog to be the testing
dataset (1501 earthquakes), and tested the three velocity models
respectively (Fig. 2). In the Figure 2-(A), the solid line means
the original model while the dash line means the modified
model. Figure 2-(B) shows the RMS distribution of the three
models. Obviously after the VELEST, each model shows
extremely low RMS level, which means good fits to the CWB
dataset. Among these three, the CWB model has the lowest
average (~0.13) and the smallest proportion of the higher RMS
(which means, a smaller tail in the higher RMS).
We also plotted the travel time curve of the three models (Fig.
3). We found that the P-S travel time residual at smaller
distance of the CWB model fits our the historic data best (travel
time residual = 0.8 second at distance = 0 km), for most of the
recorded travel time residuals ranging from 2.0-5.5 seconds,
which are already beyond the resolution of the other two models
(the smallest travel time residuals of the other two models are
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C
Figure 3. The travel time curve derived from the three
velocity models by CPTIME calculation. Among these
three, only the CWB model has the P-curve (blue) and
the S-curve (red) get very close to each other at the
distance of 0 km. For the other two, the time residual
between P and S at distance = 0 km is already too large
to be applied to those historic earthquakes.
©2004 NTU publishing Group
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investigation
earthquakes onto a vertical plane (Fig. 5). It is clearly seen
that in Fig. 5-(A), a west-dipping plane gradually changing dip
form nearly 60° at the top and 45° at the bottom (~ 10 km deep)
is imaged. From Fig. 5-(B) to 5-(F), a similar west-dipping
deep-seated plane is also recognized, while the character of
dipping may slightly differ. This allows us to assure the
existence of such a special fault plane.
Discussion
The purpose of this short study, to re-image the fault plane of
the Siko Fault, seems to be satisfied here. However, some
sources of errors should be taken into consideration. First, the
exact locations of the 4 seismic stations remain unknown. The
X, Y, and Z coordinates of the stations are arbitrarily read from
the 1/25000 map. Second, the CWB velocity model only
provide with the P-wave velocity. As for the S-wave velocity,
we simply calculated by the formula Vs = 3Vp . This may
represent the true S-wave velocity model and cause some
potential errors. Third, the quality of the data itself. The
recorded precision of the P-S time residual is up to 0.01 second,
which might already be the best available precision in 1935.
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However, it is still not precise enough to reduce the relocation
RMS to an extremely small extent. We finally set the RMS
threshold for the summation of the X, Y, and Z dimension to be
1.5 km, and consequently lost 14 earthquakes after setting this
RMS threshold.
Even though, we still could not deny the imaged geometry
presented by these earthquakes. The result reminds us to
re-examine the role and importance of this deep-seated back
thrust in the northwester Taiwan.
1.
2.
3.
4.
5.
6.
7.
8.
Gutenberg, B. & Riichter C.F. Seismicity of the earth and associated phenomena.
Priceton University Press. 310p (1949).
Sheu, H.C., Kosuga, M., & Sato, H. Mechanism and fault model of the Hsinchu-Taichung
(Taiwan) earthquake of 1935. Zisin II 35, 567-574 (1982).
Lin, D.H. Mechanism of the Hsinchu-Taichung, Taiwan, earthquake of 1935. M.S. Thesis,
National Central Univ. 88p (1987)
Huang, B.S. & Yeh, Y.T. Source geometry and slip distribution of the April 21, 1935
Hsinchu-Taichung, Taiwan earthquake. Tectonophysics 210, 77-90 (1992).
Nasu, N. The after-shocks of the Formosa earthquake of 1935. Bulletin of Earthquake
Research Institute Suppl. 3, 10-21 (1936).
Lin, C.H., Yeh, Y.H., & Roecker, S.W. Seismic velocity structures in the Sanyi-Fengyuan
area, central Taiwan. Proc. Geol. Soc. China 32, 1, 101-120 (1989).
Sheng, C.H. The study of the velocity structure of the northwestern linear seismic zone
by ray tracing. M.S. Thesis, National Central Univ. 103p (1999).
Lee, H.C. The S-wave velocity structure under center-west Taiwan by strong ground
motion waveform modeling. M.S. Thesis, National Central Univ. 138p (1998).
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A
B
C
D
E
F
Figure 4 The relocated earthquakes. A: CWB velocity structures. B:
uniform velocity of 4.0 km/s. C: uniform velocity of 4.5 km/s. D: uniform
velocity of 5.0 km/s. E: uniform velocity of 5.5 km/s. F: uniform velocity of
6.0 km/s. Blue lines: the Tuntzuchiao Fault (south) and the Siko Fault
(north). The coordinate system is the Taiwan TM-2 Coordinate system, not
the longitude and latitude system.
Figure 5 The earthquakes projected onto the vertical plane with two end
points as (120.5°E, 25.25°N, left) and (121.05°E, 24.75°N, right). Notice that
from A to F, a west-dipping fault plane is imaged and has a concave upward
geometry. This fault plane extends to about 10 km in depth.
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A
B
C
D
E
F
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