STUDY ON SITE EFFECTS USING STRONG MOTIONDATA OF BYT-NET ARRAY IN TURKEY
Ozmen Ozgur Tuna ∗MEE08163

Supervisor: Hiroaki YAMANAKA∗∗ABSTRACT
Simultaneous separation of source, propagation path, and local site effects from observed strong
motion records is carried out by an inversion method in the frequency range from 0.5 to 20 Hz for thepurpose of empirical evaluation of the local site effects in different geological conditions. The
analyzed data are S-wave portions of 74 accelerograms from 18 events observed at 14 sites innorthwestern part of Turkey. These events are shallow earthquakes with magnitude from 3.0 to 5.2
and hypocentral distance from 10 to 90 km. A linear inversion method is applied to the logarithm of
the observed spectra, and solutions for source spectra, inelasticity factor of propagation path for
S-waves (Qs-value), and factor of site amplification at each site are obtained in least squares sense. In
the inversion, we assumed the factor of site amplification at the reference site 1101 is the same as thatof theoretical amplification of S-waves to the soil model whose bottom layer has an S-wave velocity(Vs) around 2100 m/s. The factors of site amplification Gj(f) obtained in the present study at each site
are verified by comparisons with the theoretical transfer functions. The estimated Qs-value of thepropagation path is modeled as Qs(f )=49.28f 0.86. Furthermore, we compare the amplifications withaverage Vs in top 30 meters of the S-wave profiles (Vs30) and proposed linear empirical formula
between them at each frequency.
Keywords: Accelerometer, Qs-value, Site amplification, Average S-wave velocity

1. INTRODUCTION
There has been a great progress in the last decade in the study of strong ground motions and its
engineering applications. Strong motion instrument arrays have been established for providing thebasis for the estimation of strong ground motion in future earthquakes. Many numerical techniques for
estimating the wave propagation in an irregular ground structure such as a sedimentary basin havebeen developed to explain the local amplification of seismic waves. The separation method using
inversion technique was first proposed by Andrews (1982) to separate the source and propagation
spectra from strong ground motion records. Iwata and Irikura (1986; 1988) extended the method inorder to consider both S-wave attenuation through the propagation path and local site effect at eachsite.
Since the cities of Bursa and Yalova are located at the southern zone of the westernextensions of the North Anatolian Fault (NAF) zone, importance of the seismic risk in the Bursa
region appeared to be progressively more crucial. The improvement of seismic hazard knowledge andthe last two large earthquakes near these cities, 17 August 1999, Golcuk-Kocaeli (Mw = 7.4) and 12November 1999, Duzce earthquakes (Mw = 7.2), led us to carry out this study.

∗General Directorate of Disaster Affairs, Ankara, Turkey.

∗∗Professor, Tokyo Institute of Technology, Tokyo, Japan.

12. DATA and DATA PROCESSING
The analyzed data are the S-wave portions of 74 accelerograms from 18 events observed at 14 sites.
The maximum values of observed acceleration are from 0.067 to 95 gal. Figure 1 shows theobservation sites and the locations of epicenters determined by Earthquake Research Department(ERD), These events are shallow earthquakes ranging in magnitude from 3.0 to 5.2 and in hypocentral
distance from 10 to 90 km. Table 1 shows the geological period, geology and average shear-wave
velocity Vs of the surface layer for 30 m. at each site.
Data processing is carriedout by the following procedures.
The S-wave portions of two
horizontal components (NS andEW) of seismic ground motions areanalyzed.
The data length for theanalysis is from 2.5 to 25 s after
onset of the S-waves, which isdetermined by considering theduration time of faulting at thesource
estimatedfromtheearthquake magnitude by anempirical relation.
Fourier acceleration amplitude
spectraof
twohorizontalcomponents are computed by themethod of the finite Fouriertransformation and are summed
Figure 1. Locations of observation sites (triangles) and epicenters vectorially. Before using the(circles) on the geology map.
inversion scheme, the spectra weresmoothed with a 91 point moving
average method.
Table 1. Surface geology of the sites.
S t a ti o n
p ro v in c e

S ta t io n c it ytow n

C odeG o lo g ic a l
P e r io dG e o lo g y

V s3 0( m /s e c )B ile c ik

M e r k ez

1101

L o w e r J u ra ss ic S an d s to n e

B u rs a

M e r k ez

1601

Q u a t er n ar y

S ilt y sa n d - C la ye y sa n d251

B u rs a

M e r k ez

1603

Q u a t er n ar y

T a lu s459

B u rs a

M e r k ez

1604

Q u a t er n ar y

T a lu s459

B u rs a

D em ir ta s1605

T ria ssic

M e ta c la st ic ro c k497

B u rs a

G em lik1606

L u te t ia n

L im e s to ne c o v e re d b y fine filling303

B u rs a

G em lik1607

Q u a t er n ar y

A llu vial370

B u rs a

G em lik1608

P lio c en e

A llu vial366

B u rs a

G em lik1609

Q u a t er n ar yA llu vial

228

B u rs a
B u rs a
Is t an b u lK e le s
O rh a ng a z i

L o w e r P lio c en e C o n g lo m er a te - S a nd st o n e -M u d st o ne
P lio c en e
P lio c en e se d im en t sM e r k ez

1613
1615
3401

V is e an

C la ys to n e- S ilt st o n e

600

Y a lo va

M e r k ez

7701

M io ce n e

C la ys to n e se d im e n ts

375

Y a lo va

M e r k ez

7702

P lio c en e

S ilt y- c la y st o n e a n d sa n d st o n e

359

901

412
348

Subsurface structures were investigated by MASW (Multi Channel Analysis of SurfaceWaves), and provide us with opportunities to calculate theoretical transfer functions. The transfer
functions are calculated by one-dimensional multiple reflection theory of SH-waves (Haskell, 1960)
with incident angle of 0 degree, and superposed in Figure 2.

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3. METHOD OF ANALYSIS
3.1. Spectral Inversion
Several authors have tried to analyze observed seismograms to separate source, and path effects. Iwataand Irikura (1986; 1988) extend Andrew’s equation to consider both S-wave attenuation throughout
the propagation path and the local site effect at each site. They assume the quality factor of the wave
attenuation is independent from the path and only depends on frequency (i.e., Qs = Qs( f )). Then theobserved value of the S-wave Fourier amplitude spectra is expressed by⎛ − πRij f⎞,
Oij ( f ) = S i ( f )G j ( f ) Rij−1 exp⎜
⎟Q(f)
Vss
⎝
⎠

(1)

where, Oij ( f ) , observed S-wave Fourier amplitude spectrum of i-th event at j-th site; S i ( f ) ,source amplitude spectrum of i-th event; G j ( f ) , factor of site amplification at j-th site; Rij ,hypocentral distance between i-th event and j-th site; Qs ( f ) , average Qs -value along the wave
propagation path;Vs, average S-wave velocity along the wave propagation path (=3.7 km/s).
By taking logarithm, Eq. (1) is modified as follows,−log O ij ( f ) = − log Rref + log S i ( f ) + log Gi ( f ) − log e(πRij f / Qs ( f )Vs ) ,(2)_where e is Napier number, O ij ( f ) = ( Rij / Rref )Oij ( f ) , and Rref is the arbitrary normalized

distance. With regard to S i ( f ) , the effect of directivity due to the radiation pattern coefficient isneglected, since the observed waves in high frequency range are usually less influenced by theradiation pattern coefficient (e.g., Iwata and Irikura, 1986; 1988). For each frequency, I (source
amplitude spectrum) + J (factor of site amplification) + 1 (Qs-value) parameters from I x J data setsare determined in least squares sense.
The above inversion procedure allows us to separate the source spectrum, the Qs values ofpropagation media, and the local site effect.
3.2. Choice of Reference Site
The site 1101 is located on the rock with Vs around 2100 m/s. For inversion, we assumed the factor ofsite amplification at a reference site 1101 is the same as that of theoretical amplification of S-waves tothe soil model whose bottom layer has Vs around 2100 m/s.

4. RESULTS AND DISCUSSION
4.1. Local Site Effect
In the present study, we adopt the inversion scheme by Iwata and Irikura (1986; 1988). The inversion
is executed by the least squares method with linear equality constraint by the singular value
decomposition method.
Figure 2 shows the characteristics of site responses at the sites 1603, 1604 and 1606. Gj(f) at
the site 1101 was calculated by one-dimensional multiple reflection theory of SH-waves (Haskell,1960) as the constrained condition for solving the inversion problem.

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Figure 2. Example of the characteristics of site responses for the sites 1603, 1604 and 1606.
4.2. Source Spectra’s and Qs value

Figure 3. Examples of seismic
moment density functions.

Figure 4. Qs-values as a function of frequency.
Circles indicates the Qs-value evaluated in thisstudy. Blue dash line show the results by Eyidogan
and Akinci (1997) and red dash line by Akyol,Akinci and Eyidogan (2002) for the same region.

Source spectra Mi (f) of 18 events are obtained by inversion analysis. Figure 3 shows examples of thesource spectra for events with magnitude from 3 to 5.2. The source spectrum Mi(f) is redefined fromSi(f) so that the dimension is identical with that of seismic moment. Mi(f) is called effective seismicmoment. When Mi(f) is converted from Si(f), we assume the shear-wave velocity of 4.0 km/s, densityof 3.0 g/cm3, and the average point-source radiation coefficient of 0.6, respectively. Qs-value obtainedby the inversion as a function of frequency and a formula of Qs(f)= 49.28f 0.86 fits the result. The valueof Qs obtained in this study is very similar to the value of coda Qc function Qc= 40f 0.84 obtained by
Eyidogan and Akinci (1997) using the single-scattering model and Qs= 46.59f 0.67 obtained by Akyolet al. (2002) using nonparametric inversion method for the same region (Figure 4).
4.3 Average s-wave velocity and site amplification
In this study, we compared the site amplifications with average S-wave velocity in top 30 meters of theS-wave profiles (Vs30) and proposed linear relationships between them at each frequency (Figure 5).
These relationships illustrate that Vs30 can be considered as a good indicator to estimate site4

amplification for lower frequencies. On the other hand, Vs30 is not good for estimating site effect forhigher frequencies.
Furthermore, we also compared the amplifications with average S-wave velocity for different
top meters of the S-wave profiles (Vs2, Vs5 and Vs10) and proposed linear relationships between themat 12 Hz (Figure 6). These relationships show that for higher frequencies Vs2, Vs5, Vs10 can beconsidered as a good indicator to estimate site amplification.

Figure 5. Relationship between site amplifications obtained from spectral
inversion and Vs30 calculated for S-wave profiles at earthquake observationsites for different frequencies. Solid lines are linear regression lines ofobserved data shown by crosses and R is correlation coefficients.

Figure 6. Relationship between site amplifications obtained from spectral
inversion and different average S-wave velocities (Vs2, Vs5 and Vs30)
calculated for S-wave profiles at earthquake observation sites for highfrequency (12 Hz). Solid lines are linear regression lines of observed data
shown by crosses and R is correlation coefficients.

5. CONCLUSIONSThe site amplification factors for S-waves were evaluated from strong motion records at 14 acceleration
meter sites, one of which is located in Istanbul, 1 in Bilecik, and 12 in Bursa-Yalova Provinces, in theNorthwestern region of Turkey and operated by Earthquake Research Department (ERD). The relation
between local site effects and surface geology and also Vs30 are obtained from the site amplification
factor for these sites. These results show that:
The value of Qs obtained in this study is very similar to the value of coda Qc function
obtained by Eyidogan and Akinci (1997) and Qs obtained by Akyol et al. (2002) for the same region.

5Almost we obtained same fundamental frequencies ( 2 – 5 Hz) but different amplification
values due to different surface geology effect.
Different Gj(f) values in sites 1603 and 1604 are observed which means assumption of Vs
profile for 1604 is not appropriate so more investigations for this site are recommended.
We obtained high amplification value for the site 1606. This high amplification can be
explained by the fine filling cover around 9 – 13 m over the bedrock.
By comparing amplification functions with theoretical transfer functions, it is possible to saythat they are generally in harmony, some differences are investigated, it is possible to foresee thatsufficient depth is not obtained in the theoretical transfer functions calculated for these sites. Inaddition, difference in amplification values obtained with both methods may be related withinsufficiency of selected constraint conditions, as mentioned before.
Considering the NHERP classification, the value of Gj(f) is increased when Vs30 decreasedin the frequency range 1-10 Hz.
Considering surface geology, our sites classified into three groups which are consistent with
our results except site 1604 as mentioned before.
Relationship between site amplifications obtained from spectral inversion and Vs30
demonstrate that Vs30 a good tool to estimate site effect for lower frequencies. While for higher
frequencies Vs2, Vs5, Vs10 are more recommended to estimate site effect.
These results show that earthquake waves approaching from different directions scatter under
the local ground conditions and reach the site from more than one direction and also these results
shown in this study may strongly influence the definition of seismic hazard around Bursa and Yalova
cities. New studies are necessary for the understanding of seismic wave attenuation and amplificationas consequences of travelling through poorly consolidated soils in Northwestern Anatolia.
AKNOWLEDGEMENT
I am really indebted to Dr. Toshiaki YOKOI - Building Research Institute - who sincerely
advised and guided me during this work and I would like to express my deepest gratitude to mycolleagues in my institute –Earthquake Research Department- and Mr. Hussam Eldein
ZANIEH -Tokyo Institute of Technology- for their highly intensive efforts during thisresearch.
REFERENCES
Akinci, A. and Eyidogan, H. 1996, Phys. Earth Planet Inter. 97, 109-119.
Akyol, N., Akinci, A., and Eyidogan, H. 2002, Pure Appl. Geophys., 159, 2002, 1253-1269.
Andrews, D. J., Proceedings of Workshop 16, Dynamic Characteristics of Faulting, 1981, U.S.
Geological Survey Open File Rep. 82-591, 437, 1982.
Eyidogan, H. and Akinci, A. 1997, 4th National Earthquake Engineering Conference, 63-71 (inTurkish).
Haskell, N. A. 1960, J. Geophys. Res., 65, 4147-4150.
Iwata, T. and K. Irikura 1986, Zisin, Ser.2, 39, 579-593, (in Japanese with English Abstract).
NEHRP, 2001, Recommended Provisions; National Earthquake Hazard Reduction Program. Part 1.
Provisions (FEMA 368) prepared for the Federal Emergency Management Agency, Washington
DC. Building Seismic Safety Council.
Iwata, T. and K. Irikura 1988, J. Phys. Earth. 36, 155-184.

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