n a

ical Engineering, 06800 Beytepe, Ankara, Turkey

settlement. In addition, the comparison between fundamental site periods and fundamental building periods, which were measured

It is well known and widely accepted that the effects of particularly important because most of urban settlementshave occurred along river valleys over such young andsoft surface deposits. Ground-shaking site effect caused

Engineering Geology 87 (20surface geology on seismic motion exist and can be large.in a few buildings and estimated from an empirical expression, indicate that prime attention should be paid to resonancephenomena, particularly for the northern part of the settlement where high-rise buildings are still in construction. 2006 Elsevier B.V. All rights reserved.

Keywords: Microzonation; Microtremor measurement; Numerical modeling; Predominant site period; Shear wave velocity; Site amplification;Yenisehir (Bursa)

1. Introduction The earthquake damage is generally larger over softsediments than on firm bedrock outcrops. This isReceived 14 March 2006; received in revised form 23 May 2006; accepted 24 May 2006Available online 11 July 2006

Abstract

Seismic micro hazard zonation for urban areas is the first step towards a seismic risk analysis and mitigation strategy. Essentialhere is to obtain a proper understanding of the local subsurface conditions and to evaluate ground shaking effects. In this study,present and future settlement areas of Yenisehir, which is located in the earthquake-prone Marmara Region of Turkey, wereevaluated with respect to site amplification and site period. Borings in conjunction with in-situ penetration tests, seismic velocitymeasurements, resistivity surveys and microtremor studies were performed, and available data from previous investigations werecomplied to determine the variation of the soil profile as well as the characteristics of the soil layers within the study site. Inaddition, new empirical correlations between shear wave velocity (Vs) and number of blows from standard penetration test (SPT-N)were also developed to be used for the estimation of amplification factors. Site amplification was assessed using empirical methodsbased on estimated values of Vs, 1-D site response numerical modeling program and microtremor measurements. Among the threemethods employed, the numerical technique and microtremor method yielded considerably higher amplification factors whencompared to those obtained from the empirical method. This situation is considered as a limitation of the empirical methods. Thesurvey of site response suggests ground amplification. The microzonation map based on soil site amplification suggestsamplification factors between 1.6 and 5 in the present settlement, while the areas at the north and south of the settlement generallyamplify the motion 5 to 9 times. The site periods obtained from microtremor studies vary from 0.51 to 0.8 s throughout theHacettepe University, Department of Geologmethods for an earthquake-prone settlement in Western Turkey

Nilsun Hasancebi, Resat Ulusay Evaluation of site amplificatio Corresponding author. Fax: +90 312 299 20 34.E-mail address: resat@hacettepe.edu.tr (R. Ulusay).

0013-7952/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2006.05.004nd site period using different

06) 85104www.elsevier.com/locate/enggeoby an earthquake can vary significantly within a smalldistance. This is because at sites having soft soil and/or

ineeritopographic and basement undulations, seismic energygets trapped, leading to amplification of vibration to man-made structures. Man-made structures with resonancefrequency matching that of the site have the maximumlikelihood of getting damaged. Therefore, informationabout the site response is an integral part of theconstruction of seismically-safe structures and urbanplanning. One of the well known examples of such effectsis Mexico City. In Mexico City, there exist very soft claydeposits underneath the downtown area of the city. Theseled to very large amplifications which caused loss of lifeand structural damages during the distant GurreroMichoacan earthquake of 1985 (Kramer, 1996).

Turkey is one of the most seismically active countriesin the World. In particular, the August 17, 1999 Kocaeliearthquake, which resulted in more than 20,000 fatalitiesand extensive structural damage, was a major disaster forthe most industrial and urbanized region of Turkey calledMarmara Region. Therefore, this earthquake focused theattention on densely urbanized and industrialized settle-ments. In addition to extensive liquefaction and associatedground failures, and submarine landslides at differentparts of the earthquake region, site amplification andrelated damages were also reported. The most typicalamplification during this earthquake was experience atAvcilar district of Istanbul (Tezcan et al., 2002; Ergin etal., 2004).

On the other hand, a large earthquake, which isexpected to occur in the Marmara Sea within the next30 years (Parsons et al., 2000), also pose a threatparticularly to the settlements located in the MarmaraRegion. In addition to Istanbul and Kocaeli provinces,Bursa is also one of the three most industrialized andpopulated cities of the Marmara Region. There are 17towns officially belonging to Bursa. One of them isYenisehir which is found 50 km east of Bursa (Fig. 1).Increase in its population resulted in urbanization andconstruction of new buildings. The 1999 Kocaeliearthquake was also felt in Yenisehir, but did not causeserious structural damage in this settlement. After thisdevastating earthquake, prime consideration began to bepaid to geological and geotechnical investigations bymunicipalities particularly by those in the affected regionincluding the municipality of Yenisehir.

The first geotechnical study in Yenisehir wasperformed by Doyuran et al. (2000) for the evaluationof the foundation conditions of the present and futuresettlement areas of the town. The study involved drillingat 17 locations, standard penetration testing (SPT), trialpitting and laboratory testing. Doyuran et al. (2000)established a microzonation map of the town based on

86 N. Hasancebi, R. Ulusay / Engthe earthquake risks and geotechnical characteristics ofthe foundation material and identified two zones interms of suitability of settlement. However, ground-shaking site effects such as site amplification andfundamental site periods were not included in thisprevious study.

In this most recent study, site amplifications andfundamental site periods in the settlement area ofYenisehir and its close vicinity were investigated usingGrade-2 and Grade-3 methods recommended by theTechnical Committee for Earthquake Geotechnical Engi-neering (TCEGE, 1999). For the purpose, availablegeotechnical data from Yenisehir were compiled, geo-technical studies involving borings with SPT tests andgroundwater level measurements, and laboratory testing.In addition, the data of seismic and resistivity surveys andmicrotremor measurements (MTM) collected by theGeneral Directorate of Disaster Affairs (GDAA) (Dikmenet al., 2004) for this study were also evaluated. The soilamplification was assessed using three methods, such asshear wave velocity-based empirical relationships, 1-Dsite response program SHAKE andmicrotremor data. Siteperiods obtained by SHAKE modeling were presentedand compared with those obtained from microtremorstudy. Finally, an attempt was made to establish micro-zonation maps derived from amplification factors and siteperiods for Yenisehir town.

2. Description of the site

The town of Yenisehir is situated within an ellipticalbasin called the Yenisehir Plain (Fig. 2). This basin isseparated from Iznik Plain and Inegol Plain by ridges atthe north and south, respectively. The Kocasu stream,flowing from southwest to northeast (Fig. 2), is the mainstream of the basin. Yenisehir is founded on a flat areamainly consisting of alluvial deposits (Fig. 3a). Howev-er, towards the south and the north, where urbanizationhas not extended yet, elevations gradually increase. Theaverage slope of the site is generally less than 5. Itreaches to 10 in the north, while is between 10 and 30along the ridges formed by metamorphic rocks in thesouth.

The population of Yenisehir is 26,000. Increase in itspopulation resulted in urbanization. Therefore, newbuildings particularly high-rise buildings are underconstruction in the northern part of the settlement(Fig. 3b). The present and future settlement areas ofYenisehir cover about 18 km2. An organized industrialdistrict on gentle slopes at the southern part of thesettlement is being planned. In addition, a civil airportlocated 4 km west of the town was opened to domestic

ng Geology 87 (2006) 85104flights one year ago (Fig. 2).

Fig. 1. Location map of the study area.

87N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104

3. General settings

3.1. Geology

The geology of the Yenisehir basin was studied byGenc (1986) in detail. In this study, therefore, formationnames assigned by this investigator were used. Yenise-hir settlement and its vicinity are comprised of Pre-Neogene basement rocks, Neogene deposits and

Quaternary deposits (Fig. 4). The basement rocks,belonging to Dereyoruk formation, are mainly repre-sented by the foliated metamorphic rocks such asmicaschists, talcschists and fillates with occasionalmarble bands. These rocks crop out only on the slopesat the south of the site (Fig. 4). The schistosity planesstriking in NESW direction generally dip towards thenorth. These units are unconformably overlain by theNeogene deposits.

88 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 2. Digital elevation model of the Yenisehir Plain.

The Kopruhisar formation consists of the Neogenedeposits (Genc, 1986). These deposits are mainlycomposed of loosely cemented conglomerates andsandstones, and claystonesiltstone alternations croppingout along the ridges in the north and marls in the south(Fig. 4). The rounded and semi-rounded particles formingthe conglomerates are of sedimentary and metamorphicorigin. Bedding planes in this sequence dip towards southand southeast with inclinations of 34. The yellowish-greenmarls observed in the south areweak rocks and havetransformed into clay at shallow depths.

The Quaternary deposits cover the middle of the basin.Yenisehir town is located within these deposits which arecharacterized by alluvium and detritic materials. Based onthe information from the previous boreholes drilled in theYenisehir Plain by the State HydraulicWorks (DSI), at thenorthern part of the plain by the State Harbors andAirports Directority (DLH, 2002) and in the settlementarea by Doyuran et al. (2000), and from those drilledduring this study, thick deposits of sand and gravel withclay interlayers dominate at the south, particularly alongthe Kocasu stream. While the sequence is mainlyrepresented by thick clay and silt deposits with sand andgravel interbeds and/or lenses at the northern part of the

basin. Hydrogeological boreholes opened byDSI indicatethat the thickness of these deposits reaches up to 115 m.

3.2. Seismotectonics

The Yenisehir Basin is a pull-apart basin bounded bythe Yenisehir fault extending in NESW direction andsmall faults (Barka and Kadinsky-Cade, 1988) as seen inFig. 5. The field studies performed by Doyuran et al.(2000) indicated that there is no field evidencesuggesting that these faults are still active. In addition,a fault, which was described as a probable fault betweenthe pre-Neogene and Neogene units by Doyuran et al.(2000) (Fig. 5) was clarified by electrical soundingstudies during this study. This fault dips towards northand is a normal fault.

The main active faults controlling the seismicity ofthe Yenisehir Plain are GeyveIznik fault zone (GIFZ),which is the southwestern strand of the North AnatolianFault Zone (NAFZ) and Bursa Fault (BF) (Doyuranet al., 2000) (Fig. 5). The GIFZ includes right lateralstrike-slip faults and its distance to Yenisehir is 25 km. Ithas a potential to generate an earthquake with amagnitude of 7.5 (Gulkan et al., 1993). The BF is a

89N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 3. Views from (a) Yenisehir settlement and (b) high-rise buildings at the northern part of the town.

right-lateral strike-slip fault with some normal compo-nent. The most recent earthquake on this fault, which iscalled BursaMustafa Kemal Pasa earthquake, occurredin 1855 with maximum modified Mercalli intensity IX(Coburn and Kuran, 1985).

The recent destructive 1999 Kocaeli earthquake wasalso felt in Yenisehir, but it did not caused any loss of lifeand structural damage in this settlement. Based on theevaluations by Doyuran et al. (2000) on previous earth-quakes occurred in the region, these investigators indicatethat theGIFZ andBFmay cause destructive earthquakes in

the region due to the accumulation of strain energy alongthese fault zones since 145 and 500 (?) years, respectively.

4. Geotechnical investigations and subsurfaceconditions

In the present and future settlement areas of Yenisehir,geotechnical studies for the assessment of foundationconditions (Topal et al., 2003; Doyuran et al., 2000) andrailway route conditions by DLH (2002) were con-ducted. These studies included a total of 37 boreholes

90 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 4. Geological map of the study area (modified from Doyuran et al., 2000).

accompanied by SPT. The maximum depth of theboreholes drilled during the study by Doyuran et al.(2000) was 20 m. By considering the locations of thesepreviously drilled boreholes, 12 boreholes were alsodrilled during this most recent study. It is well known thataverage shear wave velocity of the uppermost 30 m of theground is an important factor (Borcherdt, 1994; Dobryet al., 2000) for ground characterization. Therefore, theboreholes were planned to reach up to 30 m as possible as.Depth of 9 boreholes was 30 m and others ranged between4.5 and 17 m. SPT tests were conducted at 1 m intervalsand the samples from SPT were employed for laboratorytesting. Depth of groundwater level in each borehole wasalso measured. Locations of the boreholes and simplifiedlogs of three selected boreholes are shown in Figs. 6 and 7,respectively.

An accurate evaluation of seismic site dependingparameters needs a proper shear wave velocity profile.Seismic refraction is one of the techniques which arelargely used in determining dynamic properties of theunderlying layers. Shear wave velocities were measured at

the locations of 9 boreholes drilled during this study. Dueto some restrictions encountered at the locations ofboreholes H6 and H7, and shallow depth of boreholeH12, seismic refraction measurements could not beperformed at these locations. In order to obtain someinformation about stratigraphical knowledge of the basin,three electrical sounding profiles (resistivity surveys) werealso taken at a total of 11 points (Fig. 6). In addition, asseen from Fig. 6, microtremor measurements were alsotaken at different points in the study area for microzona-tion. Assessments on microtremor records are discussed inthe next section. All these geoseismic investigations wereperformed by the team of the General Directorate ofDisaster Affairs (Dikmen et al., 2004) for this study.Besides, during the seismic refraction studies, somecontributions were also provided from the GeophysicalEngineering Department of Ankara University.

Both previous and recent geotechnical borehole logssuggest that the Quaternary deposits generally start withlight brown silty clay with high SPT blow-countsindicating a stiff soil. Below this, there exists medium-

91N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 5. Seismotectonic map of the eastern Marmara Region (after Doyuran et al., 2000).

dense-to-loose silty sand (Fig. 8a and b). At the south,particularly in the vicinity of boreholes H7, H8 and H11,which are located in the old flood plain of the Kocasustream, sand layers form the top layer of the sequence.However, at some places the silty clay may also appearbelow the sandy zone. Occasional gravel layers ofvariable thickness are also observed in the Quaternarysequence at shallow depths (Fig. 8a and b). During theelectrical soundings, marls were penetrated at a depth ofabout 10 m below the Quaternary deposits at the southernpart of the site. The higher resistivity values (Fig. 8c)

obtained in the north when compared to those in the southsuggest that fine grained soils and saturated sandy layersdominate in the north and south, respectively.

In laboratory, sieve and hydrometer analyses andAtterberg limit determinations on 149 SPT samples werecarried out in accordance with the standards of ASTM(1994). Then, based on the test results, the samples wereclassified according toUnified Soil Classification System.

The laboratory studies suggest that the Quaternarydeposits in the southern part of the site are mainlyconsisted of poorly- and well-graded sandy soils falling

92 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 6. Location of geotechnical boreholes, geoseismic investigations and microtremor recording points.

into SP and SW soil classes, and silty and clayey sandsof SC and SM groups. While CL and CH groups fine-grained soils with low and high plasticity dominatetowards the north. Range of grain size distribution of thesoils and plasticity chart are given in Fig. 9a and b,respectively. These findings show a good agreement tothose obtained by Doyuran et al. (2000).

Depths of groundwater table measured in thepreviously drilled boreholes (Doyuran et al., 2000) andin those drilled in this study are shallow and rangebetween 2.2 and 10 m. However, Doyuran et al. (2000)indicate that the groundwater levels are deeper (14 m)in the north and shallower in the south. Sand and gravellenses and/or interbeds in the alluvial sequence form thewater bearing zones.

5. Assessments on site amplification and site period,and microzonation

5.1. Amplifications estimated from shear wave velocity

Prediction of ground shaking response at soil sitesrequires knowledge of stiffness of the soil, expressed interms of shear wave velocity (Vs). This property is usefulfor evaluating site amplification (Borcherdt, 1994).While it is preferable to determine Vs directly from fieldtests, it is not often economically feasible to make Vsmeasurements at all locations. When the direct measure-ments of Vs for soil layers are not available, the existingcorrelations between SPT blow-counts (SPT-N) and Vscould be used. For the purpose, a number of correlations

93N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 7. Some typical engineering logs illustrating the subsurface ground conditions and depth of groundwater table in the study area.

were developed (Table 1). The correlations given inTable 1 are all based on uncorrected SPT-N values.

Since the aerial extend of the study site is large and Vsmeasurements are limited only for the locations of nineboreholes drilled during this study, derivation ofempirical correlations between Vs and SPT-N values ofthe soils of the study site was considered as a useful toolto be used in amplification evaluations.

The correlation equations were developed using asimple regression analysis for the existing database. Thedatabase consists of 97 data pairs (Vs and SPT-N) obtainedboth from boreholes and shear wave velocity profiles at

each borehole location. The relationships between Vs andSPT-N were proposed in three categories considering soiltypes, such as sandy soils, clayey soils and all soils.Because only a few data from silty layers and no data fromgravelly layers were available, these categories could notbe evaluated. These relationships are shown in Fig. 10.Comparisons between measured and predicted values ofVs using the equations given in Fig. 10 are presented inFig. 11. The plotted data are scattered between the lineswith 1:0.5 and 1:2 slopes confirming that the regressionequations generally show a reasonable fit of the complieddata for the investigated soils.

94 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 8. (a) and (b) typical cross-sections showing the subsurface conditions oFig. 6 for section lines).f the study area and (c) resistivity cross-section of profile A1A5 (see

In addition to comparison shown in Fig. 13a, in orderto compare the performance of the relationships, a graphbetween the scaled percent error given in Eq. (1) and

Table 1Some existing correlations between Vs and SPT-N for all soils

Author (s) Vs (m/s)

Ohba and Toriumi (1970) Vs=84N0.31

Imai and Yoshimura (1970) Vs=76N0.33

Fujiwara (1972) Vs=92.1N0.337

Imai (1977) Vs=91N0.337

Ohta and Goto (1978) Vs=85.35N0.348

Imai and Tonouchi (1982) Vs=0.97N0.314

Jinan (1987) Vs=116.1(N+0.3185)0.202

Sisman (1995) Vs=32.8N0.51

Iyisan (1996) Vs=51.5N0.516

Kiku et al. (2001) Vs=68.3N0.292

95ineering Geology 87 (2006) 85104Fig. 9. (a) Range of grain size distribution of 149 soil samples from SPT

N. Hasancebi, R. Ulusay / EngIn order to assess the effect of soil type on thesecorrelations, the correlation curves for different soilsshown in Fig. 10 are transferred onto the same plot.Fig. 12 suggests that types of soil investigated in this studyhas no influence on the values of Vs. This result shows agood agreement with that obtained by Iyisan (1996), whostudied on the soils collected from an earthquake-affectedarea in the eastern part of Turkey. He indicated that expectthat for gravels, the relationship equations developed forall soils, sands and clays yield approximately similar Vsvalues. Therefore, in thismost recent study by the authors,the relationship for all soils given in Fig. 10a wasemployed in further evaluations. This equation of thepresent study is plotted in Fig. 13a together with otherexisting comparable correlations given in Table 1. Fig.13a indicates that the equations developed by Sisman(1995) and Kiku et al. (2001) underpredict the Vs values.Except these equations, all the equations comparedestimate Vs values close to each other in the case ofSPT-N values less than 20 as seen from Fig. 13a. For SPT-N values greater than 20, the equation developed in thisstudy compares well with the regression equations byOhba and Toriumi (1970), and Imai and Yoshimura(1970). The differences seen between the relationshipsmay be due to the quantity of the processed data anddifferent methods employed during Vs measurements.

tubes and (b) distribution of the fine-grained soils on plasticity chart.Fig. 10. Correlations between Vs and SPT-N for (a) all soils, (b) sandyand (c) clayey soils.

cumulative frequency was drawn (Fig. 13b) consideringthe data employed in this study.

Scaled percent error VscVsm=Vsm100 1Where, Vsc and Vsm are the predicted and measured

shear wave velocities, respectively. As seen in Fig. 13b,

A 68V0:61 V1 < 1100 m=s 2:1

A 1:0 V1 > 1100 m=s 2:2

AHSA 700=V1 for weak motion 3:1

AHSA 700=V1 for strong motion 3:2

Where; A is the relative amplification factor for peakground velocity, AHSA is the average horizontalspectral amplification in period range of 0.4 to 0.2 s,and V1 is the average shear wave velocity over a depth of30 (in m/s). The Grade-2 methods recommended by

96 N. Hasancebi, R. Ulusay / EngineeriFig. 11. Comparison of the measured and predicted Vs for (a) all soils,(b) sand and (c) clayey soils.85%of the values ofVs predicted from the equation of thisstudy (Fig. 10a) fall into 20% of the scaled percent errorindicating a better estimate for the studied soils whencompared to those from the existing equations. Based onadditional regression analyses performed by Hasancebi(Okan) (2005) using corrected SPT-N and Vs values fromthe study site revealed that the relationships with highestcorrelation coefficients between Vs and SPT-N areobtained when uncorrected SPT-N values are used forestimation of Vs. Thus, in this study, the equation in Fig.10a was preferred to estimate the values of Vs which areemployed to calculate soil amplification factors.

Shear wave velocity of surface layers is a usefulindex property for evaluating site amplification. Shima(1978) found that the analytically calculated amplifica-tion factor is linearly related with the ratio of Vs of thesurface layer to that of bedrock. Investigations based onthe observation and analyses of ground motion haverevealed that the average Vs of surface soils to a certaindepth shows strong correlation with the relativeamplification (Midorikawa, 1987; Borcherdt et al.,1991). The available correlation equations by Midor-ikawa (1987) and Borcherdt et al. (1991) are given inEqs. (2) and (3.1) (3.2) , respectively.

Fig. 12. Effect of soil type on VsSPT(N) relationships.

ng Geology 87 (2006) 85104TCEGE (1999) include the use of above-given empirical

equations for the estimation of amplification factor. Byconsidering this, in this study, Eqs. (2.1) and (3.1) wereemployed. The amplification factors computed from Eq.(2.1) varied in a relatively narrow range of 2.3 to 2.8.But the amplification factors obtained from Eq. (3.1)were slightly greater and range between 2.4 and 3.4.

5.2. Site modeling

The Grade-3 approach recommended by TCEGE(1999) requires an in-depth understanding of the necessaryanalytical models and numerical procedures, when thegeotechnical characteristics of the site are known; siteeffects can be, in principle, estimated through numericalanalysis. These analyses may be performed considering

either linear or a non-linear behavior for the soil. The non-linearity is very often approximated by a linear equivalentmethod that uses an interactive procedure to adapt the soilparameters, such as rigidity and damping to actual strain itundergoes. The soil column is modeled as a series ofhorizontal layers. These layers are subjected to basemotions that are considered representative of those likelyoccur in the region of interest. The SHAKE program is oneof the most widely used for such calculations (Schnabelet al., 1972). In this study, preliminary one-dimensionalshear wave prorogation analysis was conducted, frombedrock to surface, for nine soil profiles at the locations ofboreholes H1 toH5 andH8 toH11 using the SHAKE2000computer program (Ordonez, 2004). The thickness andequivalent shear wave velocity used for each layer were

97N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 13. (a) Comparison of VsSPT(N) regression equation (Fig. 10a) developof Vs predicted from different correlation relationships.ed in this study with those given in Table 1, and (b) scaled percent error

obtained from boreholes and seismic refraction surveys.The unit weights of sands and clays used for modeling are18.6 kN/m3 and 18 kN/m3, which were taken from thetable of unit weights recommended by Tatsuoka et al.(1980), and unit weight determinations in laboratory,respectively. The time history motion assumed at bedrocklevel is the EW-component of the 1999Kocaeli earthquakerecorded at Sakarya strong ground motion station which isfounded on rock ground and the closest station to the studysite (see Fig. 5). The other parameters required for theanalysis, such as G/Gmax ratio, damping ratio and shearmodulus were estimated from the graphs recommended bySeed and Idriss (1970) and Sun et al. (1988) for sands andclays, respectively. The amplification spectra and responsespectra for 0.5% and 10% damping values obtained for thesoil profiles representing the locations of boreholes H3 andH8 are depicted in Fig. 14 as typical examples, and siteperiods for the locations investigated were also obtained(Table 2). Table 2 suggests that except borehole locationsH2 and H10, amplification factors are between 3.5 and

9.03. Boreholes H2 and H10, where amplification factorsare greater than 10 and generally range between 11 and 12were obtained, are located near the basin margins in thenorth and south, respectively, above an inclined bedrocktopography as seen in Fig. 8. Although the amplificationfactors at the locations of boreholes H5 and H8 are nearly

Table 2Values of site amplification and natural site periods estimated fromnumerical analysis

Borehole no. Amplification Period (s)

H-1 6.16 0.15H-2 12.06 0.19H-3 9.03 0.47H-4 7.08 0.57H-5 3.58 0.57H-8 3.59 0.80H-9 6.00 0.32H-10 11.05 0.17H-11 5.22 0.44

98 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 14. Amplification spectra and response spectra for the location of boreholes H3 and H8.

same, site period at location H8, where sand layersdominate, is considerably higher. Site periods and amplifi-cation factors obtained by SHAKEmodeling are comparedwith those obtained from MTM in the followingparagraphs.

5.3. Microtremor measurements

The use ofMTM in estimation of site response has beeninvestigated since it was proposed in the 1950s. Althoughthere is ongoing discussion about the applicability of it invarious site conditions and ground shaking levels, it hasbeen widely used to estimate the fundamental period ofsoil deposits (Lermo and Chavez-Garcia, 1994) andrecommended as one the approaches in Grade-2 methodsin zoning for ground motions (TCEGE, 1999). Micro-tremormeasurements are relatively easy and economicallyfeasible method to estimate site response under earthquakeexcitations. For this study, microtremor measurementswere conducted by the team of GDDA (Dikmen et al.,2004) at 131 points within the settlement area of Yenisehirand its close vicinity to estimate site amplification andpredominant soil periods. The locations of measurement

sampled at 100 Hz was recorded. Nakamura's (1989)methods was employed for the determination of the fun-damental site period and estimation of seismic amplifica-tion at each point. The microtremor measurementsindicated amplification factors ranging between 1.64 and8.5, and predominant site periods varying from 0.15 to 1 s.The site amplifications and predominant site periodsobtained by this method were compared to those obtainedfromothermethods employed in the following paragraphs.

5.4. Comparison of the results and microzonation

The amplification factors obtained from Vs-basedempirical equations, SHAKE modeling and MTM arecompared in Fig. 15a. Because SHAKE analyses werecarried out only for soil columns at the locations of nineboreholes, the comparisons among three methods couldbe made only for these locations. It is clear from Fig. 15athat the amplifications obtained from the empiricalequations are considerably lower than those obtainedfrom other two methods. Although the coefficientcorrelations of the existing empirical equations betweenVs and SPT-N are high, it should be kept in mind that Vs

99N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104points are shown in Fig. 6. A Datamark LS-8000wd typerecorder and Akashi-Jep6a3 type three-component accel-erometer were used through the measurements. At eachobservation point, a minimum of 3 min ambient noiseFig. 15. Comparison of the amplification factors (and SPT-N values are affected from various factors, andtherefore, values of Vs obtained from the empiricalequations should not be evaluated as measured values.Fig. 15a also suggests that the amplification ratios froma) and site period (b) for the Yenisehir area.

SHAKE are generally larger than those obtained fromMTM, but at few locations (H5, H8 and H11) thesefactors from both methods are close to each others. Onthe other hand, the periods computed by MTM are largerthan the SHAKE periods (Fig. 15b). As mentioned byVentura et al. (2004), while the periods from SHAKEgenerally reflect the stratigraphy, the periods determinedfrom microtremor measurements may be affected bylocal variations in geology and may be also reflective oftopographical (e.g. basin edge) effects and 3-D wavereflection/refraction effects due to the geometry andrapid changes in thickness of the layers.

Based on MTM, maps showing the distribution ofamplifications (Fig. 16) and fundamental site periods(Fig. 17) over the investigated area were established.Fig. 16 suggests that the amplification ranges between1.6 and 5 in the present settlement area of Yenisehir.While the northern and southern parts of the basin gener-ally amplifies the motion 5 to 7 times, and locally 8 to 9times. Particularly greater amplifications found for thesouthern part of the investigated area are probably asso-ciated with the presence of thick and loose sand layersand very shallow-seated groundwater table. As seen fromFig. 17 the fundamental periods generally increase from

100 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 16. Spatial distribution of amplification factors.

the south to north which is interpreted as a thickening ofthe sediments of the alluvial deposits towards north.Similar trends were also observed by Bour et al. (1998)and Ventura et al. (2004) who studied on amplificationand natural period of soils on a plain near Rhone Delta(France) and Fraser River Delta (Canada), respectively.The periods in the current settlement area of Yenisehirgenerally range between 0.51 and 0.8 s, while some spotareas with periods between 0.91 and 1 s exist (Fig. 17).

Because the settlement area of Yenisehir extendsparticularly to the north, where high-rise buildings arebeing constructed, it was considered that a simple com-

parison between the fundamental periods of some buildingsand site would be useful for the sake of providingpreliminary information to future comprehensive micro-zonation of Yenisehir. For the purpose, an additional MTMstudy was also performed by the team of GDDA (Dikmenet al., 2004) at four buildings for residential use (Fig. 17).Buildings numbered from 1 to 3 are four-storied andbuilding 4 is six-storied. All buildings have reinforcedconcrete framed structures and height of each floor isapproximately 3 m. At the top and ground floors of threebuildings microtremor monitors were observed. In thelatest Turkish building code (GDDA, 1999), approximate

101N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104Fig. 17. Fundamental site period distribution zoning map.

b2 0.660.67 0.45 0.510.60b3 0.71b 0.45 0.610.70

ineerifundamental period, T1, of moment-resisting framed con-crete buildingsmaybe estimated by the following empiricalexpression.

T1 CtH3=4N 4Where; HN is the total height of building and Ct is a

constant taken as 0.07 for buildings with HN25 m inthe first- and second-degree earthquake zones. Table 3tabulates the observed and approximately computed(Eq. (4)) fundamental periods of the buildings with theranges of fundamental soil periods at the location ofthese buildings obtained from the microzonation mapgiven in Fig. 17. In building 1, the observed andcomputed fundamental periods are close to each others,while in buildings 2 to 4 the observed fundamentalperiods are larger than the computed ones. However,except that of building 2, the ranges of measuredfundamental periods of three buildings and the enclosedsites show good agreement. In addition, Eq. (4) suggestsa fundamental period of 0.76 s for 8-storey buildingsunder construction at the northern part of the settlementarea. At the location of these apartment blocks, thefundamental site period ranges between 0.71 and 0.8 s(Fig. 17). Although this preliminary evaluation is basedon a very limited observations and estimations from anempirical equation, it emphasizes that the coincidence of

b4 0.750.76 0.60 0.510.60a Measured at ground floor and top floor.b Periods both in ground and top floors are same.Table 3Comparison of the selected building periods and site periods

BuildingNo.

Building period (s) Siteperiods (s)

Measureda Estimated

b1 0.500.51 0.45 0.510.60

102 N. Hasancebi, R. Ulusay / Engthe fundamental periods of the buildings particularlythose of high rise buildings, and site seems to be highlypossible. Therefore, resonance phenomena should betaken into consideration to select appropriate and safestructural configurations.

6. Conclusions

In this study, ground amplification was evaluated usingempirical relationships, 1-D numerical modeling andMTM, and an empirical relationship between VsSPT(N) values was established. In addition, natural site periodswere also determined and compared to those of someselected buildings. The following conclusions are drawnfrom this study.

Among the three methods employed in the study, thenumerical modeling and microtremor measurementsyielded higher soil amplifications. This situation isprobably due to different methods of shear wave velocitymeasurement, the quantity of processed data and procedureof SPT which are considered in derivation of empiricalequations for VsSPT(N) and amplification factor. Thesurvey of site response, using both numerical method andMTM, has shown that ground amplification exists. Map ofamplification obtained from MTM indicates amplificationfactors ranging between 1.6 and 5 in the present settlementarea.While northern and southern parts of the basin, wherethe settlement extends, generally amplifies the motion 5 to9 times. The presence of loose sand layers and shallow-seated groundwater table at the southern part of the site areprobably the main factors contributing high amplifications.

The fundamental site periods computed from MTMwere larger than those computed from the numericalmethod. The site periods obtained from MTM vary from0.15 to 1 s, and 0.51 to 0.8 s throughout the whole studysite and in the current settlement area, respectively. Inaddition, preliminary evaluations based on the compar-isons between fundamental site and building periodsindicate that prime attention should be paid to resonancephenomena particularly for high-rise buildings in the town.

This study, which is related to a particular townlocated in the first-degree earthquake zone of Turkey,clearly shows the importance of microzonation mapsshowing various degrees of risk zones associated withdynamic soil behavior such as soil amplification andliquefaction. With the availability of such maps, theengineers and architects will be able to select appropri-ate and safe structural configurations.

Acknowledgements

This study was supported by the Research ProjectGrants Division of Hacettepe University (Project No.0302602008). The authors sincerely acknowledge theGeneral Directorate of Disaster Affairs of Turkey andthe geophysical team of this organization for the effortsexhibited during the geoseismic experiments and dataprocessing. The authors also wish to express theirgratitude to B. Hamdi Cingil, the mayor of Yenisehir,and architecture Sinan Suzgun and the personnel ofmunicipality for their kind interest and the logisticsupport they provided throughout the site investigations.Prof. Dr. Ahmet T. Basokur and research assistant IrfanAkca of Ankara University, and Dr. Nihat Sinan Isik of

ng Geology 87 (2006) 85104Gazi University are also acknowledged for their kind

103N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006) 85104contributions during the interpretations of the seismicrecords and numerical analysis, respectively.

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