Microtremor Analysis of Marsa Matrouh Industrial Area Using

Microtremor Analysis of Marsa Matrouh
Industrial Area Using Horizontal to
Vertical Spectral Ratio Method
Sayed SR Moustafa
Geology and Geophysics Dept., Faculty of Science, King Saud University, Saudi
Arabia
Seismology Dept., National Research Institute of Astronomy and Geophysics
(NRIAG), Cairo, Egypt
e-mail: [email protected]
ABSTRACT
Understanding how sedimentary basins respond to seismic-wave energy generated by large
earthquake events is a significant concern for seismic-hazard estimation. Current study
explores the use of microtremors for evaluating site effects. The microtremor horizontal-tovertical spectral ratio (HVSR) method was utilized to assess the predominant frequency and
amplification factor of the sediments in the Marsa Matrouh industrial area in the northwestern
portion of Egypt. Free-field microtremor data collection were carried out for 20 minutes using
three component seismograph measurements at 33 sites. Clear HVSR peaks were obtained in
the majority of the surveyed area. The central and western parts of the area are characterized
by two well separated peaks which indicate distinct shallow and deep impedance contrasts.
The predominant frequency map of sediments shows a distribution in a broad range of 4.5–
12.5 Hz. The observed frequencies can be related to the total thickness of Quaternary
sediments of the area. They are deposited over bedrock built of Miocene limestone. The
results of the study suggest that the microtremor method could be helpful in identifying those
areas most vulnerable to ground amplification.
KEYWORDS:
amplification factor, dominant frequency, HVSR, Egypt, Matrouh.
INTRODUCTION
Surface geology and soil properties have significant influences on the intensity of earthquake
ground motion (Power et al. 2004). Understanding site conditions are of great importance in site
effect evaluation (Idriss 1990, Borcherdt 1994, Bard 1998). The ambient noise or microtremor
survey method provides a cost effective and alternative tool for site exploration as it is simple and
non-invasive. The predominant frequency of sediments of a surveyed site can be determined by
applying the Horizontal-to-Vertical Spectral Ratio (HVSR) of three component recording without
the need to know their thickness and shear wave velocity structure (Bard 1998). In the last
decade, this method has been widely utilized for various applications, including seismic
microzonation (e.g., Tuladhar et al. 2004), bedrock depth mapping (e.g., Ibs-von Seht and
Wohlenberg 1999), shallow soil shear-wave velocity structure profiling (e.g., Parolai et al. 2002)
and soil liquefaction potential (e.g., Huang and Tseng 2002).
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The majority of the buildings in the northwestern region of Egypt are not designed to resist
earthquakes, therefore, relatively small events can be the source of huge socioeconomic disasters
(El-Sayed et al. 1998). The geological structure, seismicity, active tectonics, topography of the
Mediterranean region has also been frequently subjecting the area to natural disasters which may
ensue in a large loss of life and property (Ibrahim 1994, El-Araby and Sultan 2000, Yousif et al.
2014). The economic and social effects of potential earthquake disasters for the northwestern
coast of Egypt can be reduced through a comprehensive assessment of seismic hazard in affected
areas (El-Araby and Sultan 2000). Predominant frequency and amplification factors are
considered of the most important seismic hazard parameters assessments to be defined. The
ground motion amplification and high level of damages over soft soil and unconsolidated deposits
have been shown to be responsible for increasing intensity than over the consolidated and hard
sediments (Theodoulidis et al. 2008). Numerous studies (e.g., Field and Jacob 1993, Borcherdt
1994) have demonstrated the ability of local geologic conditions to alter the seismic motion. The
ambient seismic noise could be efficiently utilized for estimating ground amplification (Kanai
1957). The primary premise is that the response of sediments to noise sources is related to that for
incident seismic waves. The calculated theoretical response of a horizontally stratified
sedimentary layer to ambient noise sources has supported that indication (Field and Jacob 1993).
Hence, in the current study, the HVSR (Nakamura 1989) is utilized to estimate the predominant
frequency and site amplification from microtremor records.
MATERIALS AND METHODS
Study Area
The Marsa Matrouh industrial area occupies a small portion of the Northwestern coastal zone
of Egypt. It is an accessible area attaining promising lands for industrial expansion beyond the
Nile Valley and Delta (Figure 1a) (White 2002). The climatic conditions of the area are typically
arid, characterized by a long hot dry summer, mild winter with little rainfall, high evaporation
with moderately to high relative humidity (Ali et al. 2007). Geomorphollogically, the study area
can be considered as a part of the coastal plain which occupies a narrow strip of land stretching
adjacent to the Mediterranean Sea, its elevation ranges between 5 and 60 m above sea level with
northwest ward slope (Figure 1b). The maximum inland extension of this plain is about 5 km
from the sea, north to south direction (Yousif et al. 2014). The coastal plain displays different
landforms along the different segments, being influenced by the local structures (El Shazly and
Shata 1969). The landscape of the coastal plain is influenced by several landforms, which include
elongated ridges, dunes and shallow depressions (Raslan 1995).
Geologically, the area under investigation is located in an unstable shelf which differs
stratigraphically from the stable one in being thinner and more distributed (Said 1962). The
exposed rocks in the study area are entirely of sedimentary origin ranging in age from the Middle
Miocene to Quaternary (Figure 1c). The Middle Miocene is represented by Marmarica limestone
formation which covers the most parts of the study area (Klitzsch et al. 1988). This formation is
made up of an upper white limestone fossiliferous member, middle snow white chalk member
and a lower member of alternating cross bedded carbonates, shale and marls (Raslan 1995). It
changes laterally from chalky marly limestone to sandy and clay facies at the approach of the
headland (Shata 1955). The Pliocene sediments are of limited distribution in the area of
investigation. The lower part of Pliocene succession is formed of creamy limestone and its upper
part of brown calcareous sandstone (Raslan 1995). The Quaternary sediments include Pleistocene
and Holocene deposits. Pleistocene sediments are widely distributed in the study area along the
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Mediterranean Coast. They are mainly represented by oolitic limestone, which constitutes the
main bulk of the Pleistocene sediments and is almost developed in the form of elongated ridges
running parallel to the present coast (Hassouba 1995). The Holocene non-consolidated deposits
have a widespread and consist, essentially, of weathering products of the Miocene and
Pleistocene deposits, which are represented by several varieties of different lithological
characters, i.e., beach, alluvial and Aeolian deposits (El-Shazly 1964). The structure of the
northwestern coastal zone is dominated by folds and faults, with most folds formed during the
Late Cretaceous - Early Tertiary, and have a NE-SW direction that agrees with the Syrian Arc
system trend (Shata 1955, Said 1962). As a result of this trend, local plunging anticlines were
developed consisting of headland protruded into the sea along the coast in the study area. The
headlands are separated by a series of synclinal basins having the same trend (Shata 1955, El
Shazly and Shata 1969). The structures were formed primarily by rotational stresses during
several stages, starting since the Middle Cretaceous, and continued throughout the Tertiary and
probably through the Quaternary (Shata 1955, Said 1962, El Shazly and Shata 1969).
Free-Field Microtremor Measurements
Microtremor measurements were performed in approximately 3500 km2 large area which
extends across the whole width of the area in NW-SE direction (Figure 2). About 33 free-field
measurements were performed. The measuring locations were carefully selected to avoid as much
as possible the influence of buildings, underground structures and traffic as these features may
significantly alter the recordings. Weather conditions were also noticed and hence windy or rainy
days were avoided to upsurge the reliability of HVSR analysis. Data acquisition was performed
using portable Orion Nanometrics digital seismograph and s three-component 1 Hz Lennartz Le3C sensor. This sensor has a flat frequency response between 1 and 80 Hz. Below 1 Hz, the
response of the instrument decays by approximately 10 dB/octave so that a conservative threshold
for marking the frequency below which the data contain mainly instrumental noise (i.e.,
combined seismograph and sensor self-noise) is 0.2 Hz. The good ground coupling was obtained
by using long spikes mounted at the base of the sensor. At each location, recording duration was
at least 20 minutes of continuous recording time with a sampling rate of 100 Hz, which
guaranteeing reliable spectral estimates up to 30 Hz. To assure data quality, measurements were
generally carried out following the standard experimental conditions of microtremor
measurements (Chatelain et al. 2008). For each surveyed site and to remove intensive artificial
disturbances, all signals were band-pass filtered in a pass band of 0.5-15 Hz and recorded time
series were afterwards visually inspected to identify possible erroneous measurements and
stronger transient noise. Each record was then split into 30 seconds long non-overlapping
windows, for which amplitude spectra in the range 0.5–15 Hz were computed using a triangular
window with 5% smoothing and corrected for sensor transfer function. For each window, the
amplitude spectra of the three components were computed using a fast Fourier transform (FFT)
algorithm. Two orthogonal horizontal spectral components (H1, H2) were merged by the
geometric mean before dividing the vertical component (V). The average HVSR microtremor
components were thus calculated using:
HVSR = H 1 ⋅ H 2 / V
From the calculated HVSR plot for all windows, the windows, including strong transient
noise was further identified in order to be excluded from further computation. Finally, the average
HVSR curve of all windows with the corresponding 95% confidence interval was computed.
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Figure 1: Geomorphology and geology of the study area. a- Location of the study area (red
square). b- Aster digital elevation model (DEM) of the area c- General geological setting and the
main geological units of the North West Coastal Zone of Egypt (Klitzsch et al. 1988). A
rectangle indicates study area.
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In addition, a directional HVSR analysis was performed in 10◦ angular steps to identify
possible directions of noise sources, but no preferential directions were recognized. The
geometric average of the HVSRs from all segments at one site was assigned as the H/V value
(Nakamura 1989) of that site, from which the predominant frequency and corresponding
amplitude can be estimated. All data processing and analysis were performed using Grilla
software package (Micromed 2012). An example of the whole data processing and analysis
process is depicted in Figure 3 for site MM16 shown in Figure 2.
Figure 2: Location map of the employed free-field microtremor measurements
in the study area.
RESULTS AND DISCUSSIONS
Microtremor measurements have been performed in the industrial area of the town of Marsa
Matrouh for a preliminary evaluation of site response. The signals recorded have been analyzed
utilizing the HVSR method (Nakamura 1989, Lachetl and Bard 1994). The scrutiny of the HVSR
data set ensures reliable data for mapping predominant frequencies in the mapped area. The
HVSR analyses of free-field measurements showed that most of the recorded data fulfil the
proposed SESAME criteria for reliable measurements (Duval et al. 2004). The criteria
recommended for a reliable HVSR curve, which are based on the relation of the predominant
frequency to the window length, the number of significant cycles and the standard deviation of
the peak amplitude were fulfilled in all collected dataset. The next criteria for a clear peak which
are based on the relation of the peak amplitude to the level of the HVSR curve, and standard
deviations of the predominant frequency and of its amplitude were fulfilled in about thirty sites.
Hence we can confirm that the frequency of observed the peak is considered to be the
predominant frequency of sediments down to the first strong impedance contrast (Duval et al.
2004, Bard 2005). Only three sites did not fulfill reliability peak. The main reasons for this failure
are mainly due to high levels of noise or too small amplitude of the peak and flat spectral ratio.
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Figure 3: Representative HVSR data collected for site MM16. (a) Three-component ambient
noise seismic record. (b) Amplitude spectra of individual components. (c) Corresponding HVSR
plot. Thin lines represent the 95% confidence interval. The main peak seismic resonance
frequency is about 7.19 Hz. Graphs were generated using the Grilla software package (Micromed
2012).
A representative example of the estimated HVSR curves is depicted in Figure 4. In general,
clear peaks were obtained showing the broad range of predominant frequencies between 3 and 12
Hz. The narrow 95% confidence interval of the average HVSR curves (shown as thin lines in
Figure 4) clearly portrays the good temporal stability of the analyzed signals.
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Figure 4: Examples of microtremor measurements (HVSR analyses) with clear peaks (left)
and with a more complex shape (middle) or flat spectral ratio (right). Locations of measurements
are shown in Figure 2. Thin lines represent the 95% confidence interval.
From our analysis, three types of site conditions were observed. The first type represents
bedrock sites where the Marmarica limestone is observed along the surface (e.g. MM08, MM14,
MM18, and MM27). On the bedrock outcrop flat spectral ratios were obtained Second site
condition type detected is the hard and compact sand and marl soil or weathered and fractured
limestone (e.g., MM03, MM19, MM21 and MM25). The last observed site condition is
corresponding to loose sand and marl soft soils (e.g., MM02, MM04, MM06 and MM07). In most
cases there is a sharp peak in HVSR which is rather symmetrical. An asymmetric shape with
additional side peaks at frequencies higher than the frequency of the main peak is also observed.
Some peaks are less sharp with some features which can be an indication of two closely spaced
peaks. Among more complex shapes two peaks well separated in frequency are depicted in some
sites. They indicate two impedances contrasts in the subsurface, the first, shallow is most
probably related to the Quaternary deposits and the second, deep to the Miocene Marmarica
bedrock. Some measurements show several peaks which are not well separated, indicating
complex setting, or a very broad peak, which is characterized by wider 95% confidence interval.
In general temporal stability is better at higher frequencies while 95% confidence interval is often
wider at lower frequencies.
The amplitudes of HVSR peaks are mainly in the range 3–20, only in three cases, they reach
values between 25 and 30 (Figure 5). In general, there are more high peak amplitudes (above 10)
at low predominant frequencies (3–8 Hz). Since low predominant frequencies are related to
deeper parts of the basin, this can be an indication of high impedance contrast between
Quaternary sediments and Miocene Marmarica bedrock. On the other hand, high predominant
frequencies are presumably related to the parts of the basin where microtremor HVSR method
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detected shallower stiffer rocks within Quaternary sediments. It is likely that the impedance
contrast is lower in this case. Nevertheless, more than one distinct impedance contrasts as
observed in the survey area (two peaks in the HVSR curve) affect also the amplitude of the
individual peaks.
Figure 5: Amplitude vs. frequency graph of HVSR peaks.
The collected data from the 33 measuring points were further used for drawing two maps: a
predominant frequency map showing resonance frequencies of sediments (Figure 6) and an
amplitude map (Figure 7) showing the peak amplitudes of HVSR peaks. For measurements which
shows two clear HVSR peaks, we used the value of the peak with higher amplitude. The
predominant frequency of sediments shows a distribution in a range of 3.5–12.5 Hz. High
frequencies (up to 13 Hz) are visible also close to the E and SE margin of the basin. This is
expected due to the thin deposits close to the border of the basin.
On the amplitude map (Figure 7) the highest values (up to 27) are located in the northwestern
part. In the majority of the town area the HVSR peak amplitudes are between 5 and 17. However,
it is known that the amplitude of HVSR peak is a less reliable parameter of microtremor
measurements (Bard 2005). It can be therefore used only as a rough indicator of impedance
contrast between surface sediments and bedrock.
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Figure 6: Map of sediments predominant frequency derived from free-field microtremor data.
Figure 7: Map of microtremor HVSR peak amplitudes.
CONCLUSIONS
Since the geotechnical characteristics of soft sediments and their thickness are not known in
the Marsa Matrouh basin, due to the lack of geotechnical or geophysical data, microtremor
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investigations have proved to be an effective tool for assessing the predominant frequency of the
sediments. The basin was surveyed with a 33 free-field measurements. This is especially
important because the variations of the predominant frequencies are considerable within the study
area. The predominant frequency map of sediments shows a distribution in a wide range of 3.5–
12.5 Hz. The observed frequencies can be related to the total thickness of Quaternary sediments,
deposited on the bedrock built of Miocene limestone. Two distinct peaks in HVSR curve were
obtained, with considerably different frequencies, indicating shallow and deep impedance
contrasts. Low frequencies were obtained in a few measurements. The highest HVSR peak
amplitudes were obtained in the NW part of Marsa Matrouh, indicating high impedance contrast
between sediments and the bedrock.
ACKNOWLEDGEMENTS
I highly appreciate the help and support I get from the Seismology Department at the
National Research Institute of Astronomy and Geophysics. I am very grateful to Professor A.
Khairy and all the staff of the Egyptian National Seismological Network for providing the data.
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