Physics and Chemistry of the Earth 63 (2013) 92–101

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Physics and Chemistry of the Earth

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VS30, site amplifications and some comparisons: The Adapazari(Turkey) case

1474-7065/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.pce.2013.05.003

⇑ Corresponding author.E-mail addresses: tazegul_ozcep@hotmail.com (T. Ozcep), ferozcep@istanbul.

edu.tr (F. Ozcep), oguzozel@istanbul.edu.tr (O. Ozel).

Tazegul Ozcep a, Ferhat Ozcep b,⇑, Oguz Ozel b

a Istanbul University, Institute of Science, Department of Geophysics, Istanbul, Turkeyb Istanbul University, Engineering Faculty, Department of Geophysical Engineering, Istanbul, Turkey

a r t i c l e i n f o

Article history:Available online 31 May 2013

Keywords:VS30Site effectsSeismic design codesAdapazari

a b s t r a c t

The aim of this study was to investigate the role of VS30 in site amplifications in the Adapazari region,Turkey. To fulfil this aim, amplifications from VS30 measurements were compared with earthquake datafor different soil types in the seismic design codes. The Adapazari area was selected as the study area, andshear-wave velocity distribution was obtained by the multichannel analysis of surface waves (MASWs)method at 100 sites for the top 50 m of soil. Aftershock data following the Mw 7.4 Izmit earthquake of17 August 1999 gave magnitudes between 4.0 and 5.6 at six stations installed in and around the Adapaz-ari Basin, at Babalı, S�eker, Genç, Hastane, Toyota and Imar. This data was used to estimate site amplifica-tions by the reference-station method. In addition, the fundamental periods of the station sites wereestimated by the single station method. Site classifications based on VS30 in the seismic design codeswere compared with the fundamental periods and amplification values. It was found that site amplifica-tions (from earthquake data) and relevant spectra (from VS30) are not in good agreement for soils in Ada-pazari (Turkey).

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Earthquake design codes are periodically revised and updateddepending on improvements in the representation of ground mo-tions, soils and structures. These have been frequently revised in re-cent years (Dogangün and Livaoglu, 2004). Local site effects play animportant role in the development of design spectra for earthquake-resistant structural design codes. This is because strong-motionparameters, such as amplitude, frequency and duration, aresignificantly affected by the site conditions.

Site conditions are divided into different categories in theearthquake design codes. The site classification system is basedon definitions of site classes in terms of a representative averageshear-wave velocity (VS30), standard penetration test (SPT)blow-count, unconfined compression strength, etc.

Borcherdt (1994), based on empirical studies, recommendedshear-wave velocity as a means of classifying sites for buildingcodes. Similar site categories were selected for the FEMA and NEH-RP seismic design provisions (NEHRP, 2003) for new buildings(Dobry et al., 2000). Boore et al. (1994) indicated that the idealparameter would be the average shear-wave velocity to a depthof one-quarter of the wavelength of the period at the location of

interest, as had been proposed by Joyner and Fumal (1984). Bythe quarter-wavelength rule, 30 m is the appropriate depth for aperiod of 0.16 s for stiff soil; the period values tend to increase insofter soils (Boore et al., 1994). The average shear-wave velocityin the upper 30 m of soil (VS30) is also considered in Eurocode 8(2001), and has been internationally accepted since the NEHRPclassification was introduced in the USA (Mucciarelli and Gallipoli,2006). The original work by Borcherdt (1994) was based on datafrom the western USA, as were the earliest papers discussing theadvantages (Anderson et al., 1996) and disadvantages (Wald andMori, 2000) of the method. Outside the region where the methodwas developed, some doubts were raised as to the capability ofVS30 to predict amplification in deep basins (Park and Hashash,2004) and in other tectonically active regions (Stewart et al.,2003), or in velocity-inversion situations (Di Giacomo et al., 2005).

Iwahashi et al. (2010) performed a multiple linear regressionanalysis of the logarithm of observed VS30 at 1646 locations in Ja-pan with three topographic variables (slope gradient, surface tex-ture, and the logarithm of elevation). Kuo et al. (2009) focusedon the top 100 m of sediment layers to compare the shear-wavevelocity profiles from three different methods. Lee and Trifunac(2010) examined the relative significance of two soil-site variablesused in empirical equations aimed at predicting the amplificationof strong earthquake motion: (1) soil site classification SL, and (2)average soil velocity, VL, in the top 30 m of soil. Allen and Wald(2009) presented a study on the use of high-resolution topographic

T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101 93

data as a proxy for seismic site conditions (VS30). Motazedian et al.(2011) developed a VS30 (NEHRP) map for the city of Ottawa. Yonget al. (2009) studied the terrain-based classification of VS30 forCalifornia. Castellaro et al. (2008) disscussed the VS30 value as aproxy for seismic amplification. Luzi et al. (2011) proposedestablishing a soil classification scheme based on parametersthat can be easily calculated from seismic records, to eithercomplement or replace VS30.

To study site classification, several authors have used earth-quake horizontal:vertical (H/V) spectral ratios (HVSRs) (Gallipoliand Mucciarelli, 2009; Zhao et al., 2006; Fukushima et al., 2007;Sokolov et al., 2007; Rodriguez-Marek et al., 2001; Pitilakis et al.,2006; Lang and Schwartz, 2006). Di Alessandro et al. (2012) pro-posed a site classification scheme based on the dominant periodof the site determined from the average HVSRs of ground motion.Using both experimental and numerical results, Panzera et al.(2011) confirmed the role of the geological and morphological set-ting of Catania. Their results showed that seismic motion amplifi-cation mainly occurs in three different stratigraphic conditions:(a) sedimentary deposits mainly diffused in the south of the studyarea; (b) areas of soft sediments surrounded by lava flows; and (c)intensely fractured and scoriaceous basaltic lavas. Riepl et al.(1998) compared the results of the traditional spectral ratio tech-nique with those of a generalised inversion technique, the HVSRtechnique, a coda wave technique, and Nakamura’s technique.

Raptakis et al. (1998) studied the effectiveness of the standardspectral ratio and HVSR techniques in investigating andquantifying the influence of geological conditions on strong groundmotion. Raptakis et al. (2000) described the process of constructionof the 2-D model of the geological structure of the Volvi Basin innorthern Greece, and the results of empirical and theoreticalapproaches to the evaluation of site responses at Euroseistest(established in 1993, the longest-running continuouslyinstrumented seismic test site in the world, originally to determinea 2-D (north–south) cross-section of the Mygdonian Basin, 30 kmNNE of Thessaloniki, Greece). Triantafyllidis et al. (1999)investigated the site effects of seismic ground motion in Thessalo-niki, estimated from experiments applied to acceleration datatogether with theoretical modeling. Bindi et al. (2009) examinedthe site amplification effects observed in the Gubbio plain, centralItaly, based on the records of 140 local earthquakes (1.2 6ML

6 4.7) obtained from two linear arrays along the longitudinal andlateral axes of the basin. This information was analysed in boththe time and frequency domains to determine the spatial variationin local site effects.

The aim of the present study was to provide and discuss somecomparisons of VS30 and site amplifications in a region with com-plex geologic and geophysical characteristics (the Adapazari re-gion, Turkey).

2. Tectonic features and geological setting

Fig. 1 is a geological map of the area in and around Adapazari(Komazawa et al., 2002). Adapazari is located in a basin coveringan area of about 25 � 40 km2. The alluvial plain is very flat. Down-town Adapazari is in the north-eastern foothills, which extendeastward like a peninsula into the basin. The Sakarya River in thebasin runs northwards to the Black Sea. South of the faults aresteep mountain ranges rising to about 1000 m. The age of the base-ment rocks varies from Devonian and Silurian in the north to Cre-taceous in the south. Metamorphic, intrusive and volcanic rocksare observed along the faults. The basement rocks are overlain byEocene volcanic ash–soil (Komazawa et al., 2002). During the1999 earthquake, surface ruptures with displacement up to 5 mappeared along the North Anatolian fault (Komazawa et al., 2002).

The investigation of the geological and tectonic features of thestudy region by Kocyigit et al. (1999) showed that Adapazarı cityand its environs overlie two main rock units: Quaternary alluvialdeposits, and main rocky units marking the boundary betweenthe Sakarya Basin and regions to its west and north Quaternarydeposits are widespread, and have a minimum thickness of300 m. Quaternary sedimentary soils are categorised as eithercoarse-grained (at the basin margins) or fine-grained units. Exceptfor a very small part of Sakarya (Arabacı, Alanı, Baglar, Hızırtepe,Baçelievler), the city is entirely located on saturated weak soils.Sediment thicknesses vary up to a maximum of 900 m.

The region is bounded by the main North Anatolian fault (strikeE–W, forming the southern margin), and the Duzce fault (strikeNE–SW, forming the south-eastern margin). According to Kocyigitet al. (1999), there are four E–W-striking sub-fault zones in Sak-arya city and vicinity. Two of these, which are located in Sakaryacity itself, are inactive. The other two are active faults, one located4 km south of the city, and the other 5 km to its north. The south-ern fault (the Arifiye Fault) was activated in the 17 August 1999earthquake. The northern fault (the Kazimiye Fault) was activatedin 1943, causing the Adapazarı–Hendek earthquake, and part of its6 km length was also activated in the 17 August 1999 earthquake.

3. Methodology

3.1. Experimental arrangement for MASW and MAM

In the early 1980s, Nazarian (1984), and Nazarian and Stokoe(1984) introduced spectral analysis of surface waves (SASWs), awave-propagation method for generating the near-surface shear-wave velocity (VS) profile, using the spectral analysis of ground rollgenerated by an impulsive source and recorded by a pair of receiv-ers. The method has since been widely and effectively used inmany geotechnical engineering projects (Stokoe et al., 1994). Dueto the need to record repeated shots during multiple field deploy-ments at a given site, which increases the time and labour require-ments in a multichannel procedure, the multichannel analysis ofsurface waves (MASWs) method (Park et al., 1999) was designedto overcome the few weaknesses of the SASW method. It takeslower-frequency surface waves into account (e.g., 1–30 Hz) andoperates within the much shallower depth range of a few to afew tens of metres (Park et al., 2007).

The research project was connected to Istanbul University in2007 by a Geometrics ‘Geode’ seismic recorder to collect field data,which was then processed. The phase velocities for the differentfrequencies were obtained by using the program (The SeisImag-er/SW™) and a dispersion curve is obtained.

A MASW and microtremor array measurement (MAM) Geodeseismograph system consisting of 12 channels with 12 verticalgeophones of 4.5 Hz capacity were used to measure VS. In this sys-tem, active seismic waves are created as an impulsive source froma sledgehammer blow to a 300 mm � 300 mm hammer plate, re-peated 10 or more times. The captured Rayleigh wave is furtheranalysed using SeisImager/SW™ (2005) software to obtain VS pro-files. The SeisImager/SW™ procedure for generating VS data ineither 1-D or 2-D format is in three steps: (1) preparation of a mul-tichannel record; (2) dispersion-curve analysis; and (3) inversion.The optimum field parameters (e.g., distance of source to firstand last receiver, receiver spacing, spread length of survey lines)are selected to ensure that information is obtained for the requiredminimum 50 m depth.

A dispersion curve is then generated from the seismic wave re-cords. Then, using an iterative inversion process that requires thedispersion curve as input, a VS profile is calculated. This is updatedafter each iteration, with the soil property parameters such as

EOCENE PLIOCENE CRETACEOUS PALAEOZOIC DEVONIAN METAMORPHIC SILURIAN VOLCANIC QUATERNARY

Sapanca Lake

Sakary

a River

1999 Main Shock Rupture

Akyazı

ADAPAZARI

İMARBABALI

SEKERGENC HASTANE

30 30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 30.9 31

40.5

40.55

40.6

40.65

40.7

40.75

40.8

40.85

40.9

40.95

41

Fig. 1. Simplified surface geology map showing earthquake stations (triangles) in the study region (redrawn from Özel and Sasatani, 2004).

94 T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101

Poisson’s ratio, density and thickness remaining unchanged. An ini-tial earth model is specified to begin the iterative inversion process.

The VS values were obtained in this way by multichannel anal-ysis of surface waves from active and passive sources (i.e., theMASW and MAM measurements) for 100 points in the study area.MAM and MASW data were combined to obtain the VS–depthstructure, giving a VS and phase velocity–dispersion curve at eachmeasurement point for the upper 50 m of soil.

The fundamental assumption of MAM analysis using the spatialautocorrelation (SPAC) method (SeisImager/SW™, 2005) is that thesignal wavefront is planar, stable, and directionally isotropic. Thepreferred noise sources are steady, at a constant level. A high levelof intermittent noise (such as passing cars) is tolerable if they arerelatively distant (greater than one array length). The effect ofnearby intermittent noise sources is usually countered by takingnot less than 10 min in length to provide a statistically steady rep-resentation of the preferred noise.

Coccia et al. (2012) and Panzera and Lombardo (2013) havepointed out that the main empirical parameters controlling theinversion process are VS, VP, layer density and layer thickness. Inthe present study, five parameters were considered for a set ofone to five uniform layers with homogeneous properties: VS, VP,layer density, layer thickness, and Poisson’s ratio for inversion pro-cess obtained by seismic refraction (minimum offset 5 m, geo-phone spacing 10 m). The VS value for all layers was consideredto lie in the range 100–1000 m/s. The influence of soil density onthe dispersion curves was obtained by the empirical formula inTezcan et al. (2006). VP and VS are linked by Poisson’s ratio, which

constrains the possible range of variation. As Coccia et al.(2012)also pointed out, Rayleigh wave dispersion curve is most sensitiveto VS variation with depth; thus the influence of VS on the Rayleighwave data inversion is critical, whereas the influence of the otherstructural parameters is relatively minor.

Special care was taken in processing the MAM data since it wasacquired through a linear array (Panzera and Lombardo, 2013). Inthe presence of a single source and a single plane wave, the appar-ent velocity detected by a linear array depends on the angle be-tween the array direction and the source azimuth (Strobbia andCassiani, 2011). When a linear array is used, as in the MAM tech-nique, resolution of this problem requires the assumption thatthe ambient noise is omnidirectional and that an isotropic diffusewave field exists with sources present at all azimuths. When theseconditions are met, the minimum apparent velocity corresponds tothe true phase velocity (Strobbia and Cassiani, 2011) and conse-quently a reliable estimation of VS may be obtained (Louie, 2001;Park and Miller, 2008).

3.2. Details of earthquake data processing

Aftershock data following the Mw 7.4 Izmit earthquake of 17August 1999 gave magnitudes between 4.0 and 5.6 at six stationsinstalled in and around the Adapazari Basin at Babalı, S�eker,Genç, Hastane, Toyota and Imar. The six-station tempory arraywas installed by the Kandilli Observatory and Earthquake ResearchInstitute of Bogazici University (KOERI) from August 30, 1999 toSeptember 30, 1999. Four of these stations (Genç, S�eker, Babalı

Fig. 2. Eurocode 8 soil types in the study area.

Table 1Deep shear-velocity profile obtained by the SPAC method (Özel and Sasatani, 2004).

SKR ADC ADU

Latitude Longitude Latitude Longitude Latitude Longitude40.737 30.381 40.753 30.411 40.787 30.419VS (m/s) thickness

(m)VS (m/s) thickness

(m)VS (m/s) thickness

(m)

1050 72 234 38 166 441500 56 441 97 331 882000 1 728 242 500 281

1500 70 878 632000 1 1050 100

1500 1

Fig. 3. Combination of locations for deep shear-velocity data (Özel and Sasatani,2004) and VS30 values from the present study.

T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101 95

and Hastane) were installed in downtown Adapazari, which hadincurred heavy damage during the main earthquake; hereafter,these are referred to as the ‘downtown stations’. The other stations(Toyota and Imar) were installed around the Adapazari Basin. Eachtemporal station consisted of a GSR-12 or SSA-12-type digital re-corder, each with sampling frequency 200 Hz and resolution12 bits. The S�eker station consisted of a GSR-16 (16 bits) digital re-corder with a sampling frequency of 100 Hz. The instruments wereinstalled on the foundation or basement of small buildings or on anopen field. The soil-type information based on VS30 showed thatthe Genç, S�eker, Babalı and Hastane stations were located on softsoil, while Imar and Toyota were stiff-soil or soft-rock stations(Özel and Sasatani, 2004).

The wave spectra were calculated for the component recordsusing a 5 s time window with a 10% cosine-shaped taper. The wavespectra were smoothed using a moving-average technique. Thelogarithmic average and the standard deviation were then ob-tained at each site for four spectral ratios in the frequency range

Fig. 4. Phase velocity–dispersion curve

0.2–20 Hz, estimated from examination of the signal-to-noise ratioof the wave spectrum.

The acceleration data of earthquakes with different magnitudesobtained in the region was used for the following:

s at the SKR, ADC and ADU sites.

Fig. 5. Location of acceleration recording stations used in this study superimposedon VS velocity map.

Table 2The parameters used in this study for aftershocks of the 1999 Izmit earthquake.

Stations (XX: there is no record) IMAR IMAR IMAR IMAR IMARTOYOTA XX TOYOTA TOYOTA XXBABALI BABALI BABALI BABALI BABALIXX XX S�EKER XXX S�EKERXX XX GENÇ GENÇ XXHASTANE XX HASTANE XX HASTANE

Events Date 31.08.1999 31.08.1999 13.09.1999 29.09.1999 24.09.1999Time 08:10:51 08:33:00 11:55:29 00:13:06 13.44:49Lat. 40.75 40.78 40.77 40.7 40.79Long. 29.92 29.96 30.1 29.34 30.29Magnitude (M) 5.2 4.6 5.8 4.8 3.8

96 T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101

Fig. 6a. Fundamental period values for short periods obtained by the H/V spectral ratstations.

(1)Todetermine a criterion for selecting reference stations by usingthe single-station method (i.e. the ‘Nogishi-Nakamura tech-nique’ set out in Nogoshi and Igarashi, 1971; Nakamura, 1989).(2) Station Imar was chosen as the reference station.(3) Determining the amplifications and fundamental periods of

soils using the spectral ratio method according to the refer-ence station for several earthquakes.

(4) Comparisons with VS30.

io method from earthquakes recorded at Imar, Toyota, S�eker, Hastane, Genç and Babalı

Table 3Fundamental periods obtained from the H/V spectral ratio of earthquakes.

Event Soil type Main spectral ratio peaks and their corresponding periods

Imar station 13.09.1999, 11.55 B Peaks between 0.05s and 0.1 s29.09.1999, 00.13 B Peak range 0.25–0.35 s and peak at 0.55 s31.08.1999, 08:10 B Peak range 0.10–0.20 s31.08.1999, 08:33 B Peak range 0.10–0.15 s

Toyota 13.09.1999, 11.55 C Peak range 0.15–0.20 s29.09.1999, 00.13 C Peak range 0.40–0.45 s31.08.1999, 08.10 C No clear peak observed

S�eker 13.09.1999, 11.55 D Peak range 0.6–0.95 s29.09.1999, 00.13 D Peak range 0.65–0.70 s24.09.1999, 13.44 D Peak at 0.1 s

Peak range 0.35–0.40 s

Hastane 13.09.1999, 11.55 D Peak range 0.15–0.20 s, with peaks at 0.55 s and 0.65 s31.08.1999, 08.10 D Peak range 0.25–0.85 s24.09.1999, 13.44 D Peaks at 0.45 s and at 0.95 s

Genç 13.09.1999, 11.55 D Peaks at 0.6 s and at 0.95 s29.09.1999, 00.13 D Peak at 0.9 s

Babali 13.09.1999, 11.55 D Peak at 0.3 s29.09.1999, 00.13 D Peak range 0.90–0.95 s31.08.1999, 8.10 D Peak range 0.75–0.85 s31.08.1999, 8.33 D Peak range 0.6–0.65 s

Babalı (H/V)

0

0,5

1

1,5

2

2,5

100101

Frequency (s)

Rel

ativ

e A

mpl

ifica

tion

130999 11.55

290999 00.13310899 08.33

240999 13.44310899 08.10

Fig. 6b. Fundamental frequency values for short periods obtained by the H/Vspectral ratio at Babalı station.

Table 4Comparison of earthquake events related to dominant frequency peaks.

Earthquake event Magnitude Peaks (Hz)

31.08.1999, 8.10 5.2 Between 1.33 and 1.1731.08.1999, 8.33 4.6 Between 1.66 and 1.53

T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101 97

4. Results and discussion

4.1. VS30 and shear-wave velocity distributions at different depths instudy area

All four types of soil (A, B, C and D) defined in Eurocode 8 (2001)for the design of earthquake-resistant structures, based on VS30criteria, are found in the study area (Fig. 2) although Type A soilis located only in a very small part of study area (pink1 colour inFig. 2).

In the study area, deep VS profiles were obtained by Kudo et al.(2002) and Özel and Sasatani (2004) using the SPAC method(Table 1). These, together with VS30 values from the present study,are mapped in Fig. 3. The phase velocity–dispersion curves for theSKR, ADC and ADU sites are shown in Fig. 4.

As shown in Fig. 3, the SKR site is located on Type B soils(400–700 m/s), and the ADC and ADU sites are located on Type C(200–400 m/s) and Type D soils (100–200 m/s). The present studyresults related to VS30 have high resolution at shallow depths; thelower-resolution data from Kudo et al. (2002) and Özel and Sasa-tani (2004) were used to obtain comparisons with shear velocitiesat greater depths.

4.2. Fundamental periods obtained by H/V spectral ratio fromearthquake records

Fig. 5 shows the locations of the stations in this study, togetherwith the distribution of VS, using data from the Mw 7.4 Izmit earth-quake of 17 August 1999.

Fig. 5 shows also that the Hastane, Genç, S�eker and Babalı sta-tions are located on Type D sites, Toyota on a Type C site and Imaron a Type B site. Details of the earthquakes and their parametersrecorded at these stations are given in Table 2. The site amplifica-tion of the Adapazari Basin was evaluated by the traditional spec-tral ratio method for S-waves; the Imar station on a stiff-soil (TypeB) site was adopted as the reference site. The results were thencompared with the seismic design code recommendations.

Only short-period data was used in the comparisons with theseismic design codes. The results of the H/V spectral ratio for short

1 For interpretation of color in Fig. 2, the reader is referred to the web version othis article.

f

periods are given in Fig. 6a for the Imar, Toyota, S�eker, Hastane,Genç and Babalı stations. Ground shaking levels varied from0.03 g to 0.07 g (Table 3). The selection of Imar station (Type B soil)as the reference site was based on both the H/V method andMASW–MAM data from Kudo et al. (2002) and, Özel and Sasatani(2004). All data were combined.

Inspection of the results of the H/V spectral ratio show that thefundamental period values lay in the range 0.15–0.45 s for the Toy-ota station, 0.1–0.95 s (S�eker), 0.15–0.85 s (Hastane), 0.6–1.0 s(Genç), and 0.3–0.95 s (Babalı).

Fig. 7. Relative amplifications for short periods by the reference station method for X-, Y- and Z-components at Babalı, Genç, S�eker and Hastane stations.

98 T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101

T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101 99

The spectral ratios are plotted as a function of frequency–ampli-tude on a log-normal scale (Fig. 6b). Comparison of these results(Table 4) may indicate a nonlinear effect.

4.3. Determination of amplification and fundamental periods with thespectral ratio method for the reference station

The H/V ratio at the Imar station gave fundamental periods of0.05–0.25 s, reinforcing its choice as the reference station. As notedabove, Table 2 gives the features of the earthquake data used inthis study, and the stations at which they were recorded.

Relative amplifications and periods for the X-, Y- and Z-compo-nents at the Babalı, Genç, S�eker and Hastane stations are shown inFig. 7. Fundamental period values for the X-, Y- and Z-componentsat these stations are given in Table 5.

Table 5Amplifications and fundamental periods for each reference station, obtained from earthqu

Component (X, Y or Z) Soil ty

Babali X DY DZ D

Genç X DY DZ D

Hastane X DY DZ D

S�eker X DY DZ D

Fig. 8. Average X- and Y-components of relative amplifications for short periods

Inspection of the values of the fundamental periods (Fig. 7)shows the following changes:

� Babalı station: 0.25–0.80 s (X-component); 0.30–0.80 s (Y-com-ponent); 0.50–0.85 s (Z-component).� Genç station: 0.85–0.95 s (X-component); 0.85–0.95 s (Y-com-

ponent); 0.45–0.80 s (Z-component).� Hastane station: 0.6–1.0 s (X-component); 0.45–0.90 s (Y-com-

ponent); 0.30–0.65 s (Z-component).� S�eker station: 0.7–1.0 s (X-component); 0.35–0.8 s (Y-compo-

nent); 0.35–0.80 s (Z-component).

The amplification and fundamental period values obtained fromthe Toyota station were uncertain, and have not been used. Thedata quality for both the Toyota and S�eker stations was poor, whichmay account for this result.

akes using the spectral ratio method.

pe Main spectral ratio peaks and corresponding periods

0.25–0.80 s0.30–0.80 s0.50–0.85 s

0.85–0.95 s0.85–0.95 s0.45–0.80 s

0.6–1.0 s0.45–0.90 s0.30–0.65 s

0.7–1.0 s0.35–0.8 s0.35–0.80 s

by the reference station method at Babalı, Genç, Hastane and S�eker stations.

Fig. 9. All amplifications and fundamental periods in the study area.

100 T. Ozcep et al. / Physics and Chemistry of the Earth 63 (2013) 92–101

Fig. 8 shows the average X- and Y-component values of the rel-ative amplifications for short periods by the reference stationmethod for all of these stations.

4.4. Relationships between site class (Eurocode 8), amplification andfundamental period

Based on the average VS values in the upper 30 m of soil (VS30)obtained by MASW–MAM procedures, the soils were classifiedaccording to Eurocode 8. All amplifications and fundamental peri-ods are shown in Fig. 9: the fundamental period values for TypeD soils fall between 0.3 and 1.0 and the amplification values varybetween 3 and 9. The arithmetical mean amplification is 4.5.

If the earthquakes that contribute most to the seismic hazarddefined for the site for the purpose of probabilistic hazard assess-ment have a surface-wave magnitude, Ms, not greater than 5.5, itis recommended that the Type 2 spectrum be adopted. For periodvalues in Eurocode 8 (Type 2), the amplification values are givenfor soil types A and D.

5. Conclusions

In the first step of this study, shear-wave velocities for theAdapazarı region were determined for the uppermost 50 m of soil,using multichannel surface wave analysis. The average velocities ofthe shear waves were then calculated and mapped at 10 m inter-vals. Then the average shear-wave velocities for the top 30 m(VS30) were classified and mapped in accordance with the Euro-code 8 code.

In the second step, the relative amplification and fundamentalperiod were obtained from weak- and strong-motion earthquakedata records. In this context, to obtain the fundamental periods,the H/V spectral ratio method is applicable to earthquake data.The features of the soils lying beneath the acceleration stationswere found to be Types B, C and D according to Eurocode 8. TypeB type soil (beneath Imar station) was considered to be equivalentto engineering bedrock (i.e., reference bedrock).

It is known that fundamental periods can be reliably deter-mined by the single station method (H/V ratio) (Lachet and Bard,1994). For this reason, to determine fundamental periods at thesites of the earthquake record stations, the H/V method was usedat the Imar site, which was selected as engineering bedrock onthe basis of the results obtained at that site. The fundamental peri-ods at Imar station were between 0.05 and 0.15 s, and between 0.6

and 1.0 s at both Genç and Babalı stations. The shear-wave veloci-ties at the Imar site obtained by MASW–MAM suggest stiff soil be-neath the site.

Wald and Mori (2000) discussed the reasons why soil amplifica-tions are not determined correctly by VS30, suggesting that the dif-ferences between the amplifications obtained from earthquakesand from VS30 data were due to:

� the smoothing effect of using an average velocity� considering only the properties of the top 30 m of site material;

and� complexities in the wave propagation that are not addressed by

these methods.

The reasons for the inadequacy of VS30 amplification data havebeen discussed by Park and Hashash (2004) relating to deep ba-sins; by Stewart et al. (2003) for active tectonic regions; and byDi Giacomo et al. (2005) in situations where a velocity inversionexists.

In conclusion, we found that the amplifications obtained byVS30 did not agree well with data from observed earthquakes inthe study area. The reason for this result is thought to be the com-plex basin structure of the region. As Özel and Sasatani (2004)pointed out, the Adapazari Basin is characterised not only by con-siderable amplification of the S-waves but also by long-period ba-sin surface waves of long duration. In basin structures of this type,VS30 does not adequately represent amplifications that occur in ac-tual earthquake conditions. For this reason, these types of com-plexity must be considered as specific conditions in the seismicdesign codes.

We realise that we have very little knowledge of the basin andlimited earthquake data to demonstrate that statement. In this pa-per we have presented what comparisons are possible, given thelittle earthquake data available. Our case study was restricted tothe soil conditions of the Adapazari Basin as represented by VS30,and data from the 1999 Izmit earthquake. Detailed studies on deepbasin structures in the region must be carried out through deepgeophysical investigations.

Acknowledgments

This study was supported by the Istanbul University ResearchFund (Project Number: T-1355). We wish also to thank Dr DonatFah from ETH Zurich Geophysical Institute, Kerim Avci, and A. Si-nan Gursoy, Geophysical Engineers from Çoruh Engineering Com-pany, for their valuable support.

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