*\[submitted to BSSA Special Issue (August 10, 2000)\]*

Site Specific Issues on Strong Ground Motion during the Kocaeli, Turkey
Earthquake of August 17, 1999, as Inferred from Array Observations of
Microtremors and Aftershocks

by

Kazuyoshi Kudo, Tatsuo Kanno, Hiroshi Okada, Oguz Özel, Mustafa Erdik,

Masayoshi Takahashi, Tsutomu Sasatani, Sadanori Higashi, and Kunikazu
Yoshida

# 

ABSTRACT Array observations of microtremors and aftershocks were carried
out near permanent strong motion observation sites run by Kandilli
Observatory & Earthquake Research Institute as well as Ministry of
Public Works and Settlement and damaged areas after the Kocaeli, Turkey
earthquake of August 17, 1999．Major objectives are to determine S-wave
velocity structures and to understand the site effects relatively on
strong motion or damage. Most strong motion sites near the fault are
classified to stiff and/or very hard soil. The records cannot directly
be used for interpreting damage of buildings in sedimentary basin.

A large and long duration of strong motion records at ATS, near Avcilar,
west of Istanbul are closely related to the low velocity (Vs\~200 m/sec)
of surface layers. The S-wave velocity structure at Avcilar where
severely damaged during the mainshock is similar to the lowland (ATS)
and it differs significantly from that of CNA where the strong motion
record from the mainshock was obtained and it locates 4 km northeast
from Avcilar. The strong ground motion at Avcilar during the mainshock
is estimated to be similar to that at ATS. The strong motion sites,
Sakarya (SKR) is located on very hard soil, while thick and soft
sediments covers downtown Adapazari. It is plausible that strong ground
motions during the mainshock at damaged area, ADC, were significantly
different from those of SKR. A large difference of strong motions
between a hillside and Izmit Bay area in and around Golcuk is also
indicated by the comparison of aftershock records.

# Introduction

The major damage of buildings and loss of lives during the Kocaeli
(Izmit), Turkey earthquake of August 17, 1999, were concentrated in a
narrow band along the ground surface fault zone. Therefore, the primary
reasons of the damage will be attributed to the near earthquake source
effects, leaving aside the quality of buildings. However, the following
examples will be out of above category. One example is the heavy damage
of buildings in Avcilar, west of Istanbul, even relatively further
(\~150 km) from the source (e.g., Cranswick et al., 2000). In addition,
the strong contrast of damage ratios between the strong motion
observation site; Sakarya (SKR), and the downtown Adapazari, and wide
variation of the damage ratios even in a relatively narrow area of
Golcuk (the Architectural Institute of Japan Reconnaissance Team et al.,
2000) are also similar issues. We have to remind poor construction of
collapsed buildings, however, site effects on earthquake ground motion
were plausibly significant on the damage in these areas.

A strong motion record is a key for understanding the damage and the
earthquake source dynamics. The strong motion records from the Kocaeli
earthquake were successfully recovered and quickly distributed through
the Internet by the Earthquake Research Department, Directorate for
Disaster Affairs of the Ministry of Public Works and Settlement (ERD)
(1999), and Kandilli Observatory and Earthquake Research Institute
(KOERI) (1999). The efforts were very valuable for scientists and
engineers, as they could quickly understand the severity of ground
motion during the earthquake. However, because of sparse network in
Turkey (Celebi et al., 2000), no strong motion record was obtained at
severely damaged areas, except Duzce (DZC). The surface geological
and/or the geotechnical data near the strong motion observation sites
and damaged areas are very limited, therefore, it is very hard to
quantitatively interpret the variation of ground motion severity and its
relation to earthquake damage.

It has been suggested that an S-wave velocity structure at a site is a
most valuable data for understanding the site effects on ground motion.
However, the methods for determining S-wave velocity structure for deep
sedimentary basin are quite limited and any conventional geophysical
exploration are hardly applicable to damaged areas just after a
devastating earthquake. We, therefore, carried out array observations of
microtremors aiming to determine the S-wave velocity structures beneath
the strong motion observation sites and damaged areas. Additionally, we
also conducted quite temporal array observations of aftershocks to
understand the relative difference of site effects on strong motion or
damage in and around Golcuk, near epicentral area, on September 1999,
and at the Istanbul University in Avcilar, west of Istanbul.

# 

# Array Observation of Microtremors

## Method

Pioneer works for determining velocity structure by using microtremors
or microseisms were carried out by Aki (1957), Toksoz (1964), and Lacoss
et al. (1969). The methods are provided for estimating velocity
structures by using the phase velocity dispersion of surface waves
included in microtremors. Asten and Henstridge (1984), Horike (1985),
and Matsushima and Okada (1990) made efforts to increase the resolution
of phase velocity determination by using array observation of
microtremors, applying the frequency-wavenumber (f-k) analysis (Capon,
1969). Horike (1985) developed also an inversion method of obtained
phase velocity of Rayleigh waves for determining S-wave velocity
structure. Aki (1957) gave a theoretical basis of the spatial
autocorrelation coefficient defined for microtremors data and developed
a method to estimate phase velocity dispersion of surface waves
contained in microtremors by using a specially designed circular array.
Henstridge (1979) also introduced a licit expression of a relationship
between the spatial autocorrelation coefficient and the phase velocity
of fundamental-mode Rayleigh waves. The method has been extended to an
exploration method by Okada et al. (1990), Matsuoka et al. (1996), and
Okada (1998), that is currently called SPAC method. The SPAC method has
been employed throughout the present study. A flow of observation and
analysis in the method is shown in Figure 1, and a principle of the
method is given in Appendix.

We have carried out array microtremors measurements at the strong motion
observation sites (ATS, CNA, YPT, SKR, and DZC) and damaged areas, such
as Avcilar (ISU and AVC), Adapazari (ADC and ADU), Golcuk (GLF and GLH),
and Degirmendere (DMD) at two different periods (September and December
in 1999). Figure 2 shows the location map of the surveyed areas.

A circular array consisting of four (optionally seven) stations was
used, which is a regular triangle-shape with a center, as shown in
Figure 1. A size of an array was selected associated with the
availability of space at a site. As shown in Table 1, plural arrays by
changing sizes at different time were deployed, except SKR site, to
cover wide wavelength or phase velocity change as a function of
frequency. Generally speaking, measurements were restricted with narrow
areas of observation rather than we expected, therefore, the resolution
of determined phase velocities at low frequencies would be limited. We
made measurements in daytime and an observation time duration of
microtremors for one measurement was 30 minutes for small (L\<100 m)
arrays and 45 or 60 minutes for larger ones, respectively.

## Instruments 

Five (optionally seven) sets of the portable seismographs (Kudo et al.,
1998) were used for array microtremors measurements. The seismograph is
originally developed for temporal observation of strong and weak
earthquake motions. They are composed of a tri-axial accelerometer
(Akashi Co. Ltd.) of highly damped (h\~26) moving coil type (natural
frequency of 3 Hz), signal conditioner (amplifier and filter), and a
data logger (24 bits digitizer, 20 megabytes flash memory, and GPS time
synchronization; Hakusan-Kogyo Co. Ltd.). A flat response (-3 dB) of
ground acceleration is attained from 0.1 Hz to an aliasing frequency.
The sensitivity of the sensor is 1 Volt/g and optionally 5 Volt/g; g
denotes the gravity. Instruments were also used for aftershock
observation without amplifying. The clipping level of sensor is 150
cm/sec; the maximum observable acceleration is 1 g at 1 Hz or 10 g at 10
Hz. The allowable input level of the data logger is selectable for 1 or
5 Volts. Total weight including an inner battery (2 kg) is 7 kg. The
low-pass filter of cut-off frequencies at 2, 5, and 30 Hz is provided.
We used the filter at cutoff frequency of 2 Hz for large arrays and 5 Hz
for small ones in microtremors observation.

## Analysis and Results

We use only vertical motion of microtremors for aiming to extract
Rayleigh waves. Figure 3 shows an example of the velocity time histories
integrated from acceleration of microtremors that were simultaneously
recorded at one place, their power spectra, and coherence. It is
apparent that there is no use for instrumental correction in the higher
frequency range than 0.1 Hz.

The integrated velocity computed from original acceleration was used in
the following analysis. The SPAC coefficients at a frequency are
calculated for each time blocks divided into every 81.92 seconds. The
time block was chosen for over-rapping 21.92 seconds to the next block.
Finally the SPAC coefficients were averaged. One example of the results
for representative frequencies obtained at ATS is shown in Figure 4. The
error bar in the figure shows the standard deviation of the SPAC
coefficients obtained from 20 to 40 blocks. The quality of determined
phase velocity is controlled by the degree of matching between the
Bessel function and the observed SPAC coefficients against distances.
The results are not so excellent, but satisfactory. Next, we will find a
dispersion curve or S-wave velocity structure that fits to observed data
by using the genetic algorithm (GA) (Yamanaka and Ishida, 1996). Circles
as well as triangles in Figure 5 show the phase velocities determined by
the SPAC method, while continuos lines show dispersion curves estimated
by using the genetic algorithm with some minor manual corrections.
P-wave velocities are assumed corresponding to S-waves by using the
empirical relation by Ludwig et al. (1970). Thus determined phase
velocity dispersions at the sites and the estimated S-wave velocity (Vs)
structures are shown in Figure 5 and Table 2. They are summarized as
follows:

### West Istanbul

ATS: Strong motion observation site run by the Kandilli Observatory and
Earthquake Research Institute (KOERI). No significant damage was found
in the building where strong motion instrument has been installed.
S-wave velocities of surface layers increase from 200 m/sec to 1200
m/sec with depth (from surface down to 550 m). The high level and
relatively long period accelerations from the mainshock can be
attributed to those soft surface layers (Vs\<400 m/sec).

ISU and AVC: These sites are close to the damaged area of Avcilar and
their velocity structures are similar to that of ATS. Therefore, their
ground motions during mainshock are supposed to be similar to that of
ATS. This will be discussed later with the aftershock data.

CNA: Strong motion observation site run by KOERI. A thin (\~15 m) low
velocity (Vs\~300 m/sec) layers exist near the surface, but high
velocity (Vs\>600 m/sec) layers are underlying at a shallow depth. The
reason of high acceleration at CNA can be qualitatively interpreted by
the existence of shallow thin surface layers of low S-wave velocity with
high velocity contrast.

### Izmit Bay area

YPT: Strong motion observation site run by KOERI. There was no
distinctive damage in the buildings where the strong motion instrument
has been installed. The S-wave velocity of surface layer is estimated to
be 340 m/s and it increases with depth. The observed deepest layer
having S-wave velocity of 950 m/sec is estimated at a depth of about 500
m.

GLF: The site is located in the eastern part of Golcuk where a big
factory is under-construction. The area sank about two meters during or
after the mainshock. The site has very thick (\~80 m) sedimentary layers
of which S-wave velocity of 150 and 260 m/sec. A rather high velocity
(950 m/sec) layer is estimated at a depth of about 350 m.

DMD: The site is located near west of the Naval base and close to the
west margin of the inland where surface fault was observed. Large number
of medium-rise (higher than four stories) buildings were heavily damaged
or collapsed near the site. Soft sediments (Vs\<400 m/sec) are very thin
compared with GLF and YPT.

GLH: The site is located on the hill of the south of damaged area in
Golcuk. This area consisting of mostly low-rise buildings had slight
damage. The S-wave velocities of layers to a depth of about 100 m are
higher than the other sites near Golcuk.

### Adapazari City

SKR: Strong motion observation site run by the Earthquake Research
Department (ERD). The available area for array observation of
microtremors was so narrow that one small array observation could be
carried out. Dispersions of phase velocity were slight but the velocity
itself was relatively high. The S-wave velocity of near surface is
estimated to be approximately 1000 m/sec or higher.

ADC: The site is located on the heavily damaged area in Adapazari City,
three kilometers northeast from SKR. A soft layer (Vs\~230 m/sec)
extends to a depth of about 40m. A thick (\~100 m) and an intermediately
hard layer (Vs\~440 m/sec) underlie this soft layer.

ADU: The site is located on a few kilometers northeast from the heavily
damaged area in downtown Adapazari City. The buildings at the surveyed
area were mostly one or two stories and the damage was very light
compared with downtown Adapazari. Very soft and thick layers with Vs of
170 and 330 m/sec cover the surface. It is not so definite, but total
thickness of sedimentary layers (Vs\<1000 m/sec) is estimated to be 470
m.

The strong motion data from the mainshock cannot be directly used for
interpretation of damage in the sedimentary basin extending in Adapazari
City. This will be discussed later.

### Duzce

DZC: Because there was no enough space to carry out the array
observation of microtremors at the ERD strong motion site, we moved to
one kilometer east from the site. Duzce suffered severe damage from the
August earthquake as well as the November 12 earthquake (Mw7.2). The
site is a sedimentary basin consists of thick and soft sediments at
surface (Vs\~260 m/sec) and intermediate depths (Vs\~460-510 m/sec). We
may also say that the ground motions during two large events were
strongly affected by surface soils.

# Array Observation of Aftershocks in Golcuk

Quite temporal aftershock observations were carried out for
understanding the site effects on ground motions in Golcuk City, where
buildings were heavily damaged. The first array consists of three sites:
on a hill in southeastern part of the city (GLS: no damage), near the
Ataturk street of eastern Golcuk (GLA: heavy damage), and at lowland in
Golcuk City (GLF: heavy damage). GLS is taken as hard soil or rock site
and the others are sediment sites. The observation period was one night
for north-south array and four days for the station YPT, where is a
reference site. The second array observation was carried out at three
stations (GLF, GLJ, and GLN) along the east-west direction near the
coast of Golcuk. The intervals between stations in Golcuk were
approximately 2 or 3 km. Table 3 shows the sites and observed events
that were recovered at plural sites. The location map of stations is
shown in Figure 6. The instruments are the same as those used for array
observation of microtremors, but without an amplifier and filter.

An example of the observed record section is shown in Figure 7. The
station GLF recorded most of the common events. Therefore, we used GLF
as a reference; nevertheless the site effects at GLF are the largest
among the observation sites. Spectral ratio method is our first
approach. To apply the method, focal distance and radiation pattern
corrections will be required. The source locations of the recorded
events have not necessarily been determined. Therefore, we used S-P time
for distance correction. A linear relation is obtained between focal
distances computed using the source locations determined by KOERI (1999)
and S-P times measured by our observation at GLS and GLF where the
delays of arrival time due to sedimentary layers were corrected, as
shown in Figure 8. To determine the focal distances at sediment sites,
the arrival time was corrected by using the S-wave velocity structure
determined by array observation of microtremors and the associated
P-wave velocity (Vp) was assumed by the empirical Vp/Vs relation by
Ludwig et al. (1970). Because structure models at GLJ, GLA, and GLS are
not available, we assumed that the focal distance at GLJ was averaged
using GLF and GLN, and the same ones of GLS were used for GLA. A
radiation pattern effect is hardly removed, however, we assumed that the
effect becomes small by vectorial summation of two horizontal
components. Spectral ratios obtained through these observations are
shown in Figure 9. Solid and gray lines show the average spectral ratios
and the individuals, respectively (Figure 9a). If we take YPT as a
reference site, the site responses in and around Golcuk are summarized
as shown in Figure 9b. GLF and GLJ exhibit large responses relative to
YPT at low frequency (0.5-0.6 Hz) and GLA shows very large site
amplification at 1.5 and 4 Hz. On the other hand, site responses at GLN
and GLS were smaller than YPT below 3.5 Hz. A large response of GLS at
high frequency would be caused by shallow surface soil. It is not easy
to interpret the relation between site amplification and damage ratio.
However, based on the detailed inventory survey of buildings in Golcuk
and Degirmendere by Architectural Institute of Japan Reconnaissance Team
et al. (2000), the high collapse ratios of medium-rise buildings near
GLF and GLJ correlate with the aftershock and microtremors data.

# Estimation of Ground Motion at Avcilar

One of the significant questions about the present earthquake is why
severe damage of buildings in Avcilar, west of Istanbul occurred
(Cranswick et al., 2000). One of the possible reasons will be fragility
of buildings; however, is that an only reason? The strong ground motion
during the mainshock at ATS, near Avcilar, showed large amplification
compared with other sites in west of Istanbul, e.g. CNA. Their velocity
seismograms and acceleration spectra are compared in Figure 10 and we
can identify the large difference between them, nevertheless the
epicentral distances are almost the same.

We have temporally installed a portable accelerograph without amplifier
on a basement of building in the campus of Istanbul University (ISU).
The building was moderately damaged during the mainshock. The ground
motions from the Marmara Sea earthquake of October 20 (M=4.4, 40.76N,
28.94E, Depth=9.6 km determined by KOERI) were recorded at this site as
well as at both CNA and ATS. The acceleration time histories are shown
in Figure 11. Figure 12(a) shows the Fourier spectral amplitudes at CNA
and ATS during the mainshock on August 17, 1999 along with those at CNA,
ATS, and ISU during the event (M 4.4) on October 20, 1999. Figure 12(b)
shows the spectral ratios of CNA to ATS for the mainshock and those of
CNA and ISU to ATS for the event on October 20, 1999. These figures show
that the spectral ratios (CNA/ATS) have no significant difference
between the strong motion from the mainshock and the weak motion from
the M4.4 event; this implies that the effects of distance, azimuth, and
non-linearity of soils are small or negligible at CNA and ATS. Figures
10 and 12(b) indicate that ATS is amplified by a factor of 5-10 compared
with CNA at lower frequency than 4 Hz, on the other hand, the relation
becomes reverse in the high frequency range. Gray lines in Figure 12(b)
indicate that the ground motions at ISU and ATS are comparable.
Cranswick et al. (2000) has suggested a large amplification of ground
motion in the longer period than 4 sec in Avcilar, however, they
obtained similar result with us in shorter periods than 1 sec.

We estimated strong ground motion during the mainshock in damaged area
using underground structures determined by array observations of
microtremors and earthquake motion observed near the damaged areas.
First, we estimated the incident wave at ATS by deconvolving the
responses of the surface layers from the record of the M4.4 event based
on 1D S-wave propagation theory by using the velocity structure model
estimated by microtremors (see Table 2 or Fig. 5). Next, the estimated
input motion is convolved with the velocity structure model at ISU,
assuming the common input motion at the layer of S-wave velocity of 1230
m/sec. Thus obtained synthetic transverse motion is compared with the
observation as shown in Figure 13. Later arrivals are not well
simulated, but major early S-wave arrivals are satisfactory. The S-wave
velocity models with assuming constant Q are shown in Table 4. As we
could not find any significant effects of non-linearity at ATS during
mainshock (Fig. 12), a similar approach for the mainshock will be
permitted. The estimated transverse motions during the mainshock at ISU
and AVC are compared with that of ATS in Figure 14. The ground
acceleration at AVC is lowest among them; however, the differences are
not so significant.

# Estimation of Ground Motion in the Adapazari Basin

The strong ground motion at downtown Adapazari is also a matter of our
concern, because no strong motion record was obtained in the
fast-growing urban/industrial area of the Adapazari basin where
buildings were severely damaged (Celebi et al., 2000). Principally, a
similar approach to the case of Avcilar may not be suitable for
estimating ground motion at Adapazari area, because it plausibly
includes near source and non-linear effects of soft soil during strong
shaking. First, we estimated ground motions during the mainshock at ADC
and ADU where S-wave velocity structure models have been estimated (Fig.
5 and Table 4) similar to the case of Avcilar. That is, the EW component
of the ground motion at SKR deconvolved by the response of the surface
layer is used for an input motion at ADC and ADU and the synthetic
motions at ADC and ADU are estimated by convolving the sedimentary
layers determined by microtremors. The velocity models and Q factors
used in the computation are shown in Table 4. Figure 15 shows the
synthetic ground accelerations and velocities at ADC and ADU, assuming
1D propagation of S-wave without any corrections of distance and
non-linearity. The synthetic motions will only be valuable as an upper
band of estimation at the sites. The distances at SKR, ADC, and ADU from
the nearest surface fault line were 3.4, 6.4, and 9.6 km, respectively.
The peak acceleration (582 cm/sec^2^) and velocity (108 cm/sec) at ADC
will be reduced to 488 cm/sec^2^ and 74 cm/sec, respectively, using the
relative distance relation with the help of the empirical formula
(Joyner and Boore, 1982). In case of ADU, those will become to 368
cm/sec^2^ and 61 cm/sec, respectively.

Our next concern is non-linear effects on strong ground motion.
Liquefaction was observed in downtown Adapazari; however, it was not
evident at ADC and ADU. During the 1995 Hyogo-ken Nambu earthquake,
nonlinear effects on ground motion were not so significant at least
several seconds after S-wave arrival or before the occurrence of
liquefaction (Aguirre and Irikura, 1997). In addition, the non-linearity
of ground was not an essential effect for the long period ground motions
recorded at Niigata City where liquefaction was observed during the 1964
Niigata earthquake (Kudo et al, 2000). The estimated ground motions at
ADC and ADU dominate in longer period than 1 second; therefore, we are
not necessary to make drastic corrections, at least for long period
motions in ADC and ADU. Our discussion is restricted in qualitative due
to non-availability of soil dynamic and other necessary data; therefore,
we have to despair of further corrections especially for short period
motion.

Although the effects of 2D or 3D effects are not taken into account,
because of too sparse of geotechnical data in the Adapazari basin, the
estimation of ground motions as an upper band will possibly be
corrected.

# Discussion

It is a matter of our concern to understand the severity of the
estimated ground motions at Avcilar (AVC and ISU) and in Adapazari basin
(ADC and ADU) relative to the strong motions that have ever been
recorded. Figure 16 compares the pseudo velocity response spectra (h=5%)
of simulated motions with those of some records that have been used in
earthquake engineering.

The left of Figure 16 shows the response spectra calculated from the
simulated motions at ISU and AVC and those obtained from the recorded
motions at ATS. They are compared with those of El Centro from the 1940
Imperial Valley earthquake (Ms 7.1) and Hachinohe from the 1968
Tokachi-Oki earthquake (M 7.8). The response spectra of ISU, AVC, and
ATS are comparable in amplitude with those of El Centro and Hachinohe at
periods shorter than 2 sec. The amplitudes of the response spectra in
the period range are most significant for low- and medium- rise
buildings. In fact many reinforced concrete buildings near the strong
notion site were severely damaged during the Tokachi-oki earthquake. As
a result, this earthquake became trigger for the revision of the
building code in Japan. The buildings with similar quality are still
used in Japan as well as everywhere in the world.

The right of Figure 16 shows the response spectra calculated from the
simulated motions at ADC as well as ADU and the recorded motions at SKR
together with those of Kobe (JMA) from the 1995 Hyogo-ken Nambu
earthquake. The thick broken and solid lines indicate the response
spectra at ADC with and without distance correction, respectively. In
the distance correction, the empirical relationship by Joyner and Boore
(1982) was referred to. Similarly, the gray broken and solid lines also
indicate those at ADU with and without distance correction. The
simulated velocity responses of ADC after a distance correction are
comparable to those of Kobe at a period around 2 sec. Those in the
period range from 1 to 1.5 sec. are almost 50 % of Kobe but exceeding
100 cm/sec. Roughly saying, the velocity responses of ADC are two times
larger than those of SKR in the frequency range from 0.5 to 3 sec. Such
ground motions at ADC would be very serious for medium- and high-rise
buildings. A very high contrast of building damage between SKR and ADC
or downtown Adapazari could probably be attributed to the difference of
the site response.

# Conclusion

We conclude that the array observation of microtremors is very promising
method for determining S-wave velocity structures from shallow to
intermediate depths, especially at damaged areas in sediment sites,
while the array observation of aftershocks allow us to directly compare
site responses. A linkage of S-wave velocity structure and aftershock
data provides for quantitative evaluation of site responses. A large and
long duration of strong motion records at ATS are closely related to the
low velocity (Vs=200 m/sec) of soft surface layers. The sedimentary
layers in Avcilar (ISU and AVC) are similar to those of ATS, so that the
ground motion at Avcilar during the mainshock may be similar to that of
ATS. Aftershock data prove actually this estimation. The ground motion
at a period around 1 sec (response of 60-80 cm/sec) is comparable to
those of El Centro (1940) and Hachinohe (1968) that have been used for
building design in Japan. At Hachinohe, the damage of RC-building was
especially severe during the 1968 Tokachi-oki earthquake.

Through the array observations of microtremors, it becomes evident that
SKR is located on very hard soil (Vs \>1000 m/sec), while thick soft
sediments cover the downtown and its extent (ADC and ADU). The ground
motion at ADC in downtown Adapazari during the mainshock is estimated to
be two or three times larger than that of SKR. Especially large
amplifications in the period range from 1 to 2 sec were very severe to
medium-rise buildings near ADC.

A large difference of strong motions between a hillside and Izmit Bay
area in and around Golcuk is also indicated by the comparison of
aftershock records. However, it is still necessary to investigate more
detailed underground structures for understanding the difference of
damage ratios in the downtown of Golcuk.

# Acknowledgements

Strong motion records distributed by ERD and KOERI were used. We wish to
express our gratitude to all efforts for acquiring the data. We also
express our thanks to all organizations and people in Turkey that
permitted and helped us to carry out the microtremors and aftershock
observations. A part of this study was conducted as a member of the
Architectural Institute of Japan Reconnaissance Team (Leader: Prof. T.
Kabeyazawa). The supports from the members were valuable. This work was
in part financially supported by the Grant-in-Aid for Scientific
Research No.11694134 from Ministry of Education, Science and Culture of
Japan and the Kajima Foundation.

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# Appendix

Array observation of microtremors is potentially a method of gaining
information on underground structures beneath a site where a seismometer
array is deployed. The method is basically to extract surface waves from
microtremors in a form of dispersion, and then to invert the dispersion
to an underground structure. Practically, the vertical component of
fundamental-mode Rayleigh waves is a direct object of extraction.

To extract surface waves from microtremors, two methods are available;
they are the frequency (f)-wavenumber (k) power spectral density method
(f-k method) (e.g. Asten and Henstridge, 1984; Capon, 1969; Lacoss et
al., 1969) and the spatial autocorrelation coefficient method (SPAC
method) (Aki, 1957; Okada, 1998). Both f-k and SPAC methods are based on
the assumptions that microtremors are spatio-temporally stationary
stochastic process. SPAC method, furthermore, needs such an assumption
that microtremors are predominated by a single mode of surface waves.

We employed the SPAC method, because it has such an advantage over the
f-k method; the SPAC method only requires fewer stations and smaller
array than the f-k method to arrive at the similar result. The method
employs a special seismic array that consists of vertical component
seismometers placed on a circle with a seismometer placed at the center.
Practically, 3 seismometers at least should be equally spaced on a
circle so that these form an equilateral triangle array.

The microtremors as a stationary stochastic process are mathematically
expressed in the polar coordinate system. Here we define the spatial
autocorrelation function $S(r,\theta\ )$ between microtremors $X(t,0,0)$
at the center $(0,0)$ of a circular array with a radius $r$ and
$X(t,r,\theta\ )$ at an observation station $(r,\theta\ )$ on the
circumference of the circular array.

$\begin{array}{r}
S(r,\theta\ ) = E\ \lbrack X^{}(t,0,0)\ X(t,r,0)\rbrack \\
\underset{T \rightarrow \infty}{\text{lim}}\frac{1}{2T}\int_{}^{}{X^{}(t,0,0)\ X(t,r,\theta\ )\ \text{dt}}
\end{array}$ A1

where ${}^{}$ designates complex conjugate. Microtremors
$X(t,r,\theta\ )$ as a stationary stochastic process are expressed as
(Priestly \[1981\]);

$X(t,r,\theta\ ) = \int_{}^{}{\int_{}^{}{\int_{}^{}{\text{exp}\left\{ i\ \omega\ t + i\ r\ k(\omega\ )\text{cos}(\theta - \varphi\ ) \right\}}}}\ d\zeta(\omega,k,\varphi\ )$
A2

where $\varphi$ is the direction from which the waves concerned arrive
in array. The stochastic process $\zeta$ satisfies the following
orthogonal properties:

i)  $E\lbrack d\zeta(\omega,k,\varphi\ )\rbrack = 0$, for all
    $\omega,k,\varphi$ A3

ii) $E\ \lbrack|d\zeta(\omega,k,\varphi\ )|^{2}\rbrack = \text{dH}(\omega,k,\varphi)$,
    for all $\omega,k,\varphi$ A4

iii) for two different values of $\omega$ and
     $\omega^{'}\ (\omega \neq \omega^{'})$ and two different values of
     $(k,\varphi\ )$ and
     $(k^{'},\varphi\ '\ )\ (k \neq k^{'},\varphi = \varphi\ '\ )$,

> $E\ \lbrack{d\zeta}^{}(\omega,k,\varphi\ )d\zeta(\omega^{'},k^{'},\varphi\ '\ )\rbrack = 0$
> A5

where $H$ is the integrated spectra of microtremors. As the spectra of
microtremors are generally continuous and able to differentiate, the
stochastic process $\zeta\ (\omega,\varphi\ )$ in equation A4 satisfy
the following property:

$E\ \lbrack|d\zeta\ (\omega,\varphi\ )|\ ^{2}\rbrack = \text{dH}(\omega,\varphi\ ) = h(\omega,\varphi\ )\ d\omega\ d\varphi$
A6

where $h(\omega,\varphi\ )$ may be called "frequency-direction spectrum
density function". When the average of the total power is integrated
with respect to all the directions, the power spectral density function
(or power spectrum) of microtremors at one station $h_{0}(\omega\ )$ is
obtained as:

$h_{0}(\omega\ ) = \int_{}^{}{h(\omega,\varphi\ )}\ d\varphi$ A7

Substituting equation A2 for equation A1 and using equations A3\~A6, the
spatial autocorrelation function is rewritten as;

$S\ (r,\theta\ ) = \int_{}^{}{\int_{}^{}{\text{exp}\left\{ \text{irk}\text{cos}(\theta - \varphi\ ) \right\}\ h(\omega,\varphi\ )\ d\varphi}\ }\ d\omega$
A8

Next, we define the "spatial autocorrelation coefficient at the angular
frequency $\omega$" $\rho\ (\omega,r)$, as the averaged spatial
covariance function normalized with the power spectrum of microtremors
at one station within the array space (for instance, the center of the
circular array), $h_{0}(\omega\ )$;

$\begin{array}{r}
\rho\ (\omega,r\ ) \equiv \frac{1}{2\ \pi}\int_{}^{}{S(\omega:r,\theta\ )d\theta}/h_{0}(\omega\ ) \\
\frac{1}{2\ \pi}\int_{}^{}{\text{exp}\left\{ i\ r\ k\ \text{cos}(\theta - \varphi\ ) \right\}\ d\theta}
\end{array}$ A9

Finally, we obtain;

$\rho\ (\omega,r\ ) = J_{0}(\text{rk}) = J_{0}(2\pi\ f\ r/c(f\ ))$ A10

since, $J_{0}$ is the Bessel function of the first kind of zero order
and $k = \omega/c(\omega\ )$ where $c(\omega\ )$ is the phase velocity.
Therefore, the spatial autocorrelation coefficient at the frequency $f$
is related to the phase velocity $c(f\ )$ via the Bessel function of the
first kind of zero order.

As it is clear from the above theoretical consideration, the phase
velocity at a certain frequency can be calculated by the spatial
autocorrelation coefficient of the component wave of $\omega$, from the
record of microtremors by a circular array with a radius $r$, by fitting
the Bessel function using least square method.

Table 1. Sites where the array observations of microtremors were carried
out and the array sizes (L: length of one side of a triangle).

![](media/image1.wmf)

Table 1.

Table 2. Estimated S-wave velocity structures by array observations of
microtremors.

  ------- ----------- ------- ----------- ------- ----------- ------- -----------
  ATS                 ISU                 AVC                 CNA     

  Vs      Thickness   Vs      Thickness   Vs      Thickness   Vs      Thickness
  (m/s)   (m)         (m/s)   (m)         (m/s)   (m)         (m/s)   (m)

  204     32          225     18          291     46          311     15

  364     57          370     75          434     91          613     32

  608     164         584     197         584     267         778     42

  919     295         919     188         919     inf.        1018    66

  1018    100         1018    164                             1230    141

  1230    inf.        1230    inf.                            1500    inf.

  YPT                 DMD                 GLH                 GLF     

  Vs      Thickness   Vs      Thickness   Vs      Thickness   Vs      Thickness
  (m/s)   (m)         (m/s)   (m)         (m/s)   (m)         (m/s)   (m)

  344     42          283     19          303     22          150     14

  445     94          512     52          455     60          259     70

  591     90          694     97          594     295         531     270

  727     290         752     61          950     inf.        950     300

  950     200         950     203                             1120    inf.

  1120    inf.        1120    inf.                                    

  SKR                 ADC                 ADU                 DZC     

  Vs      Thickness   Vs      Thickness   Vs      Thickness   Vs      Thickness
  (m/s)   (m)         (m/s)   (m)         (m/s)   (m)         (m/s)   (m)

  1050    72          234     38          166     44          257     36

  1500    56          441     97          331     88          459     44

  2000    inf.        728     242         500     281         514     48

                      1500    70          878     63          769     236

                      2000    inf.        1050    100         1020    273

                                          1500    inf.        1268    309

                                                              1500    inf.
  ------- ----------- ------- ----------- ------- ----------- ------- -----------

Table 3. Temporal aftershock observation sites and the observed
aftershocks.

![](media/image2.wmf)

Table 4. S-wave velocity structure model used for synthetic ground
motion.

  ------- ----------- ----- ------- ----------- ----- ------- ----------- -----
  ATS                       ISU                       AVC                 

  Vs      Thickness   Q     Vs      Thickness   Q     Vs      Thickness   Q
  (m/s)   (m)               (m/s)   (m)               (m/s)   (m)         

  204     32          15    225     18          20    291     46          25

  364     57          30    370     75          30    434     91          40

  608     164         60    584     197         50    584     267         50

  919     295         200   919     188         200   919     188         200

  1018    100         200   1018    164         200   1018    164         200

  1230    inf.        200   1230    inf.        200   1230    inf.        200

  SKR                       ADC                       ADU                 

  Vs      Thickness   Q     Vs      Thickness   Q     Vs      Thickness   Q
  (m/s)   (m)               (m/s)   (m)               (m/s)   (m)         

  1050    72          200   234     38          20    166     44          15

  1500    inf.        200   441     97          40    331     88          30

                            728     242         70    500     281         50

                            1500    inf.        200   878     63          80

                                                      1050    100         200

                                                      1500    inf.        200
  ------- ----------- ----- ------- ----------- ----- ------- ----------- -----

![](media/image3.wmf){width="1.3251968503937008in"
height="1.255511811023622in"}![](media/image4.wmf){width="1.0062992125984251in"
height="1.2326771653543307in"}![](media/image5.wmf){width="1.7070866141732284in"
height="0.8736220472440945in"}![](media/image6.png){width="0.797244094488189in"
height="0.32559055118110236in"}![](media/image7.wmf){width="0.9929133858267717in"
height="0.7429133858267717in"}

Figure 1. A flow of observation and analysis in the SPAC method for
estimating S-wave velocity structures using array observations of
microtremors.

![](media/image9.wmf){width="5.905555555555556in"
height="1.7930555555555556in"}

Figure 2. A location map of the sites where array observations of
microtremors were carried out

![](media/image10.wmf){width="5.905555555555556in"
height="4.450694444444444in"}

Figure 3. Velocity waveforms of microtremors integrated from
simultaneously recorded accelerations at one place (a), power spectra of
observed accelerations (b), and the coherence (c).

![](media/image11.wmf){width="6.509027777777778in"
height="1.7673611111111112in"}

Figure 4. Examples of the Bessel function fitted to the SPAC
coefficients at ATS for some representative frequencies. The solid lines
show fitted Bessel function and circles and the error bars indicate the
SPAC coefficients and their standard deviations, respectively.

![](media/image12.wmf){width="5.4875in" height="7.928472222222222in"}

Figure 5. Phase velocity dispersions of Rayleigh waves determined by
array observations of microtremors at the surveyed area (a) and the
corresponding estimated S-wave velocity structures (b). Circles as well
as triangles and continuous lines in (a) show the observations and the
computed dispersions obtained by using the genetic algorithm with minor
manual modifications.

![](media/image13.png){width="3.7402777777777776in"
height="2.7354166666666666in"}

Figure 6. A location map of aftershock observation sites in the Izmit
bay area. GLS is located on a hill that is geologically old formation
(MTA, Geology map) and the others are flat area of young sediments.

![](media/image14.wmf){width="6.136111111111111in"
height="3.7909722222222224in"}

Figure 7. Examples of waveforms from aftershocks. Record sections of
integrated velocity of NS components by filtering with pass-band between
1-10Hz from the events 13 (a), 20 (b), 31 (c), and 25 (d) in Table 3.
GLS predominate high frequency motion, while GLF is characterized by low
frequency and long duration.

![](media/image15.wmf){width="2.125in" height="2.0694444444444446in"}

Figure 8. Relations between S-P time and focal distance using the
aftershock data at

GLS and GLF. At GLF, S-P times were corrected for the time delays due to
the sedimentary layers.

![](media/image16.wmf){width="4.314583333333333in"
height="4.805555555555555in"}

Figure 9. (a): Spectral ratios of YPT, GLA, GLS, GLN, and GLJ to GLF
obtained by aftershock data. The spectra were obtained by vectorial
summation of two horizontal components Solid and thin gray lines show
the average and the individual ratio, respectively. (b): Recalculated
spectral ratios referring to YPT.

![](media/image17.wmf){width="5.905555555555556in"
height="2.917361111111111in"}

Figure 10. Comparisons of velocity seismograms and spectra at ATS and
CNA, west of Istanbul.

![](media/image18.wmf){width="3.2729166666666667in"
height="3.8055555555555554in"}

Figure 11. Acceleration time histories from the Marmara Sea earthquake
of October 20 (M4.4) observed at CNA, ATS, and ISU

![](media/image19.wmf){width="5.163194444444445in"
height="4.534722222222222in"}

Figure 12. (a): Spectral amplitudes of CNA and ATS during the mainshock
of August 17 are shown by thin broken and solid lines, respectively.
Thick solid, broken, and gray lines show those of CNA, ATS and ISU from
the October 20 event (M4.4), respectively. (b): spectral ratios of CAN
and ISU to ATS showing both for the mainshock and the M4.4 earthquake.

![](media/image20.wmf){width="5.125in" height="1.8895833333333334in"}

Figure 13. Synthetic transverse ground acceleration at ISU for the
Marmara Sea earthquake of October 20 (M4.4) is compared with the
observed ones. A good match was obtained at least initial part of S-wave
arrival.

![](media/image21.wmf){width="4.875in"
height="2.495138888888889in"}Figure 14. Synthetic transverse
accelerations at ISU and AVC are compared with the observed acceleration
at ATS

![](media/image22.wmf){width="4.875in" height="4.722222222222222in"}

Figure 15. Observed ground accelerations and integrated velocities
during the mainshock without any filtering processes at SKR and
synthetic ground motions at ADC and ADU, assuming 1D propagation of
S-wave without any corrections of distance and non-linearity.

![](media/image23.wmf){width="5.75in" height="3.1131944444444444in"}

Figure 16. Comparison of the pseudo velocity response spectra (h=5 %) of
the simulated and the observed strong motions during the mainshock with
those of strong motion records that have been used in earthquake
engineering. Estimated ground motion at Avcilar (AVC, ISU) during the
mainshock is similar to those at Hachinohe and El Centro as shown in the
left figure. The thick solid and broken lines in the right figure
indicate the response spectra at ADC without distance correction and
with the correction using the empirical relationship by Joyner and Boore
(1983), respectively. Similarly, the gray solid and broken lines also
indicate those at ADU with and without distance correction,
respectively.