# A Comparative Study on theVS30 and N30 Based Seismic Site Classification in Kahramanmaras, Turkey

Authors: Dalia Munaff Naji; Muge K. Akin; Ali Firat Cabalar
Journal: Advances in Civil Engineering
 (2020)

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## Abstract

Assessment of seismic site classification (SSC) using either the average shear wave velocity (VS30) or the average SPT-N values (N30) for upper 30 m in soils is the simplest method to carry out various studies including site response and soil-structure interactions. Either the VS30- or the N30-based SSC maps designed according to the National Earthquake Hazards Reduction Program (NEHRP) classification system are effectively used to predict possible locations for future seismic events. The main goal of this study is to generate maps using the Geographic Information System (GIS) for the SSC in Kahramanmaras city, influenced by both East Anatolian Fault and Dead Sea Fault Zones, using both VS30 and N30 values. The study also presents a series of GIS maps produced using the shear wave velocity (VS) and SPT-N values at the depths of 5 m, 10 m, 15 m, 20 m, and 25 m. Furthermore, the study estimates the bed rock level and generates the SSC maps for the average VS values through overburden soils by using the NEHRP system. The VS30 maps categorize the study area mainly under class C and limited number of areas under classes B and D, whereas the N30 maps classify the study area mainly under class D. Both maps indicate that the soil classes in the study area are different to a high extent. Eventually, the GIS maps complied for the purpose of urban development may be utilized effectively by engineers in the field.


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## Body

## 1. Introduction

Seismic site classification (SSC), which defines engineering properties of the soils by means ofVS or SPT-N values, is the simplest method to consider the site effects for numerous purposes including engineering projects and microzonation studies [1]. Many classification systems utilize the SPT-N and VS values measured in upper 30 m to provide an SSC assessment [2–4]. The National Earthquake Hazard Reduction Program (NEHRP), which is one of the widely used systems for the SSC applications, classified the soils by using their VS values in designing building codes and characterizing site response [5, 6].The objective of this study is to make an SSC assessment in the city of Kahramanmaras located in a seismically active region in southern-central Turkey. The region is influenced by the Eastern Anatolian Fault (EAF) and the Dead Sea Fault (DSF) [7]. No studies using SPT-N values have been performed in the city and vicinity area, although little efforts have been made using VS30 values obtained in the city centre [8]. Variations in the soil classification have been investigated at the depths of 5 m, 10 m, 15 m, 20 m, and 25 m by producing GIS maps for both the VS and SPT-N values obtained in the study area. The subsoil exploration has been conducted by means of seismic surface wave techniques (Multichannel Analysis of Surface Waves, MASW, and Microtremor Array Measurements, MAM) at 287 boreholes in 98 sites. The GIS maps of both weathered and engineering bed rock levels and average VS in overburden soils have also been produced for further studies. The present study is thought to be the first investigation carried out to estimate the SSC using both VS30 and N30 values in the study area.


## 2. Seismic Activity and Geological Setting of Kahramanmaras Area

Kahramanmaras basin developed by tectonic-origin surface deformations in the Quaternary and Holocene is located in south of the Bitlis-Poturge Massif area nearby the triple junction of the Arabian, African, and Anatolian plates [9, 10]. Due to the collision of Arabian and Eurasian plates over the Bitlis Suture, the basin was eventually filled out by heavy alluvial sediments and dense turbiditic flysch sequences [11–14]. In the basin, Alacik formation overlies the older units with unconformable contact starting with base conglomerate [15]. In the north, the formation shows different lithological properties that contain marl, clayey limestone, coal band alternations, and claystone. The quaternary age alluviums, dominant materials available in the basin, are composed of gray/light gray, gravel, sand, and silt, which are loose textured and cementless deposited in horizontal and vertical directions. The alluviums thickness in the region was found to increase up to 300 m, which can increase the earthquake intensity by 2-3 degrees [7] (Figure 1).Figure 1
Geological map of the Kahramanmaras area [16].The collision along Arabian and Anatolian Plates formed the Kahramanmaras territory. The available structural schemes set the triple junction of Arabian, Anatolian, and African plates at the northern Karasu valley, nearby the study area [17]. The study area is surrounded by active faults of the EAFZ (i.e., Surgu Segment, Savrun Segment, Cardak Segment, Toprakkale Segment, Cokak Segment, Amanos Segment, and Golbasi Segment), Kahramanmaras Fault Zone, Engizek Fault Zone, and Narli segment of the Dead Sea Fault Zone (DSFZ) [18]. The Engizek Fault Zone with a length of 66 km locates in north of the study area [19, 20]. The Kahramanmaras Fault Zone (KFZ) begins from east of the study area and passes through the northern part of Kahramanmaras city and extends until the Kilavuzlu Dam [19, 20]. Yilmaz et al. [16] revealed that the Golbasi-Turkoglu segment has suffered from numerous devastating earthquakes (Table 1). An earthquake with a magnitude of 6.8 occurred in 1544 on Cardak Fault [21]. Nalbant et al. [22] studied the KM zone (the area between Kahramanmaras and Malatya), which has gathered significant amount of stress during the last 200 years; thus the magnitude of a possible earthquake along the segment ruptured by 1114 (Mw > 7.8) and 1513 (Mw > 7.4) earthquakes is expected to be larger than 7.3. Palutoglu and Sasmaz [18] pointed out that another destructive earthquake in the vicinity of study area took place in 1795 with a magnitude of 7.0 (Figure 2). Accordingly, Golbasi-Turkoglu Segment, the KFZ, the Engizek Fault, and the Cardak Fault come to the forefront. Therefore, Kahramanmaras and its vicinity area are defined as an area with a seismic gap, and large earthquakes are expected to occur in a near future.Table 1
Earthquake events took place on Golbası-Turkoglu segment [16, 22, 38, 39].
DateMagnitudeLength of rupture (km)29 November, 1114M = 7.8—28 March, 1513M = 7.41032 March, 1893M = 7.1454 December, 1905M = 6.838Figure 2
Surface ruptures along the EAFZ [18].


## 3. Methodology

One of the most widely used surface wave analysis methods to calculateVS30 and site response analysis is the Multichannel Analysis of Surface Wave (MASW) method [23, 24]. The MASW is a seismic survey employed to record Rayleigh waves on a multichannel record [25]. Subsurface wave velocities at 88 sites were measured using MASW in the study area by the municipality of Kahramanmaras in order to complete the geophysical studies. The test equipment consists of a 12-channel geometrics seismograph with 4.5 Hz frequency vertical geophones spaced 3 m apart from each other and a sledge hammer with 9 kg weight. Locations, where the 88 MASW tests were carried out, are shown in Figure 3.Figure 3
Locations of field tests [8, 10].Microtremor Array Measurements (MAM) carried out by Ozmen et al. [8] at 10 different locations in the study area were used to estimate the shallow S-wave velocity profiles. The Spatial Auto Correlation (SPAC) method was performed to recover Rayleigh wave phase velocity in a 1–30 Hz frequency range. Microtremor data were recorded simultaneously by means of a 24-bit A/D portable recorder on sensors mounted in the field over various durations. The MAM tests were carried out on the clastic and carbonate rocks except sites 4624 and 4625 lying in a quaternary soil. Ozmen et al. [8] revealed that most of the profiles in the study area have top layers with less than 10 m thickness. VS of the top layers were found to be high at many sites, apart from three sites (4617, 4624, and 4626) with less than 200 m/s in the study area. Figure 4 shows an inverted 1D S-wave velocity profile up to 100 m depth by means of a hybrid heuristic.Figure 4
TheVS profiles implemented by MAM test in the study area [8].
(a)(b)


## 4. Seismic Site Classification for the Kahramanmaras Area

Site classification using NEHRP system is determined based on the average properties of the soil within upper 30 m in the ground, includingVS30, N30, and undrained shear strength (su30) [26]. Table 2 lists the five different site classes in terms of VS30 and N30 proposed by the NEHRP system. The present investigation proposes a series of SSC maps prepared using a GIS software in order to classify the study area based on VS30 for 76 locations and N30 for 287 boreholes.Table 2
Definition of NEHRP site classes in terms ofVS30 and N30 [3].
Site classGeneral descriptionVS30 (m/s)N30AHard rock>1500—BRock760 <VS30 ≤ 1500—CVery dense soil and soft rock360 <VS30 ≤ 760>50DStiff soil180 <VS30 ≤ 36015 ≤N30 ≤ 50ESoft soilVS30 < 180˂15


### 4.1. VS30-Based Seismic Site Classification

VS30, a parameter used for site classification in building codes, can be estimated by dividing 30 m with the required time of the wave travelling over an upper 30 m depth [2, 27]. The VS30 values required for the SSC could be estimated by using the following equation [26]:(1)VS30=30∑i=1ndi/Vsi,where di is the thickness of the ith layer, VSi is VS of the ith layer, and n is the number of the strata within the upper 30 m.Boore [28] proposed four different methods to estimate the values of VS30 for the profiles which do not have a 30 m depth. The method employed in the present study was the correlation between shallow velocities and VS30, where the key parameter is time-averaged velocity to a certain depth d (VSd). VSd can be calculated using the following equation:(2)Vsd=dttd,where tt(d) is travel time to a depth d and it is described as follows:(3)ttd=∫0ddzVsz.The datasets of 15 MASW and 10 MAMVS sites with actual depths of measured shear wave velocity models reaching to 30 m were employed to extend the depths with less than 30 m. Actually, VS30 for shallow models were estimated by correlating VS30 using these readily available 25 sites against their VSd for the depths (d) assumed. The depths (d) were found to be ranging from 10 m to 24 m in such approach over the study area. As recommended by Boore [28], the authors have ignored the sites with less than a 10 m depth and produced straight regression lines for various depths from 10 m to 24 m (Figure 5). It seems that the results obtained are consistent with those obtained by Boore [28]. As can be seen from Figure 5, coefficient of correlations (R2) is higher for the larger depths likely because of the increase in effective stress resulting in decrease in void ratio [29–32]. The form of the equations for these regression lines was found to be as follows:(4)logVS30=a+blogVSd,where VS30 is the average shear wave velocity, VSd is the time-averaged velocity at a certain depth, a and b are the coefficients. The authors have estimated the VS30 values for the 76 sites by employing equations (1)–(4) and plotted their findings using ArcGIS software on Figure 6. The Vs estimates made by the authors were found to be consistent with the studies by Biricik and Korkmaz [7], Ozmen et al. [8], and Perincek and Kozlu [12] carried out on the nearby area. It can be seen that the study area was primarily classified as C with a 78% of the entire area, while classes B and D were seen in much narrow regions through the study area.Figure 5
log (VS30) as a function of log (VSd), for d = 10 m, 15 m, 20 m, and 24 m.
(a)(b)(c)(d)Figure 6
VS30 map of Kahramanmaras area.Figure7 has been plotted in order to characterize the variation of soil types at the depths of 5 m, 10 m, 15 m, 20 m, and 25 m. A 2.9% of the soils at 5 m depth was classified as E (red regions close to the airport), an 11.6% of those was classified as B, a 39% of those was classified as D, and 46.5% was classified as B (the northern regions) (Figure 7(a)). Similarly, variation of the soil types at a depth of 25 m was found to be about 6.1%, 28.6%, and 65.3% for classes B, D, and C, respectively (Figure 7(e)).Figure 7
AverageVS values for the depths of (a) 5 m, (b) 10 m, (c) 15 m, (d) 20 m, and (e) 25 m in Kahramanmaras area.
(a)(b)(c)(d)(e)


### 4.2.N30-Based Seismic Site Classification

The SPT-N values, which were corrected for hammer energy variation, show the presence of dense to very dense sands across the entire study area apart from Yavuz Selim region, a recently settled industrial area in the southern region, where loose sands with clay and silt inclusions were found. The SPT-N values up to 30 m depth can be estimated by using the equation proposed by Kumar et al. [33] as follows:(5)N30=∑i=1ndi∑i=1ndi/Ni,where N30 is the average SPT-N through 30 m, di is thickness of the ith layer, n is number of the layers in 30 m depth, and Ni is the value of SPT-N for layer i. The authors had employed equation (5) in order to generate the SPT-N based SSC maps through the 287 boreholes across the study area. Following N30 estimations, the SSC maps over the study area using the NEHRP classification system have been produced (Figure 8) [3]. As can be seen from the Figure 8, 57.5% of the soils were classified as D, while the rest (42.5%) were classified as C. Average SPT-N maps for the depths of 5 m, 10 m, 15 m, 20 m, and 25 m have been plotted in Figure 9. For example, types of the soils at 5 m depth were found to be class E (dark brown colour in the central, southern, and eastern parts), class C (northern parts), and class D with the percentages of 8.4, 11.1, and 80.5, respectively (Figure 9(a)). In general, class D was found to be the dominant soil type at any depth in the study area. Soft soil deposits were mainly observed at relatively shallow depths in the study, and their amounts were decreased by increasing depth.Figure 8
N30 based seismic site classification map in Kahramanmaras area.Figure 9
Average SPT-N values for the depths of (a) 5 m, (b) 10 m, (c) 15 m, (d) 20 m, and (e) 25 m in Kahramanmaras area.
(a)(b)(c)(d)(e)


## 4.1. VS30-Based Seismic Site Classification

VS30, a parameter used for site classification in building codes, can be estimated by dividing 30 m with the required time of the wave travelling over an upper 30 m depth [2, 27]. The VS30 values required for the SSC could be estimated by using the following equation [26]:(1)VS30=30∑i=1ndi/Vsi,where di is the thickness of the ith layer, VSi is VS of the ith layer, and n is the number of the strata within the upper 30 m.Boore [28] proposed four different methods to estimate the values of VS30 for the profiles which do not have a 30 m depth. The method employed in the present study was the correlation between shallow velocities and VS30, where the key parameter is time-averaged velocity to a certain depth d (VSd). VSd can be calculated using the following equation:(2)Vsd=dttd,where tt(d) is travel time to a depth d and it is described as follows:(3)ttd=∫0ddzVsz.The datasets of 15 MASW and 10 MAMVS sites with actual depths of measured shear wave velocity models reaching to 30 m were employed to extend the depths with less than 30 m. Actually, VS30 for shallow models were estimated by correlating VS30 using these readily available 25 sites against their VSd for the depths (d) assumed. The depths (d) were found to be ranging from 10 m to 24 m in such approach over the study area. As recommended by Boore [28], the authors have ignored the sites with less than a 10 m depth and produced straight regression lines for various depths from 10 m to 24 m (Figure 5). It seems that the results obtained are consistent with those obtained by Boore [28]. As can be seen from Figure 5, coefficient of correlations (R2) is higher for the larger depths likely because of the increase in effective stress resulting in decrease in void ratio [29–32]. The form of the equations for these regression lines was found to be as follows:(4)logVS30=a+blogVSd,where VS30 is the average shear wave velocity, VSd is the time-averaged velocity at a certain depth, a and b are the coefficients. The authors have estimated the VS30 values for the 76 sites by employing equations (1)–(4) and plotted their findings using ArcGIS software on Figure 6. The Vs estimates made by the authors were found to be consistent with the studies by Biricik and Korkmaz [7], Ozmen et al. [8], and Perincek and Kozlu [12] carried out on the nearby area. It can be seen that the study area was primarily classified as C with a 78% of the entire area, while classes B and D were seen in much narrow regions through the study area.Figure 5
log (VS30) as a function of log (VSd), for d = 10 m, 15 m, 20 m, and 24 m.
(a)(b)(c)(d)Figure 6
VS30 map of Kahramanmaras area.Figure7 has been plotted in order to characterize the variation of soil types at the depths of 5 m, 10 m, 15 m, 20 m, and 25 m. A 2.9% of the soils at 5 m depth was classified as E (red regions close to the airport), an 11.6% of those was classified as B, a 39% of those was classified as D, and 46.5% was classified as B (the northern regions) (Figure 7(a)). Similarly, variation of the soil types at a depth of 25 m was found to be about 6.1%, 28.6%, and 65.3% for classes B, D, and C, respectively (Figure 7(e)).Figure 7
AverageVS values for the depths of (a) 5 m, (b) 10 m, (c) 15 m, (d) 20 m, and (e) 25 m in Kahramanmaras area.
(a)(b)(c)(d)(e)


## 4.2.N30-Based Seismic Site Classification

The SPT-N values, which were corrected for hammer energy variation, show the presence of dense to very dense sands across the entire study area apart from Yavuz Selim region, a recently settled industrial area in the southern region, where loose sands with clay and silt inclusions were found. The SPT-N values up to 30 m depth can be estimated by using the equation proposed by Kumar et al. [33] as follows:(5)N30=∑i=1ndi∑i=1ndi/Ni,where N30 is the average SPT-N through 30 m, di is thickness of the ith layer, n is number of the layers in 30 m depth, and Ni is the value of SPT-N for layer i. The authors had employed equation (5) in order to generate the SPT-N based SSC maps through the 287 boreholes across the study area. Following N30 estimations, the SSC maps over the study area using the NEHRP classification system have been produced (Figure 8) [3]. As can be seen from the Figure 8, 57.5% of the soils were classified as D, while the rest (42.5%) were classified as C. Average SPT-N maps for the depths of 5 m, 10 m, 15 m, 20 m, and 25 m have been plotted in Figure 9. For example, types of the soils at 5 m depth were found to be class E (dark brown colour in the central, southern, and eastern parts), class C (northern parts), and class D with the percentages of 8.4, 11.1, and 80.5, respectively (Figure 9(a)). In general, class D was found to be the dominant soil type at any depth in the study area. Soft soil deposits were mainly observed at relatively shallow depths in the study, and their amounts were decreased by increasing depth.Figure 8
N30 based seismic site classification map in Kahramanmaras area.Figure 9
Average SPT-N values for the depths of (a) 5 m, (b) 10 m, (c) 15 m, (d) 20 m, and (e) 25 m in Kahramanmaras area.
(a)(b)(c)(d)(e)


## 5. Spatial Variability of Rock Depth

Classification of rock based on the shear wave velocity (VS) in geotechnical earthquake engineering is still in practice. According to the NEHRP classification system, soft rock and very dense soil at upper 30 m have the average velocities of 360 m/s to 760 m/s. The 30 m average VS of a value more than 760 m/s is classified as a hard rock [34]. For the purpose of seismic microzonation, Nath [35] defined the seismic bed rock with an average VS value of 3000 m/s and higher and from 400 m/s to 700 m/s for the engineering bed rock. Miller et al. [36] had assigned the level of bed rock through considering VS as 244 m/s and above by using MASW survey. For the present study, velocity of shear wave of 330 ± 30 m/s has been specified for the weathered rock (WR) and 760 ± 60 m/s has been appointed for the engineering rock (ER). Among the 98 VS locations for the MASW and MAM surveys, only 83 locations were able to be utilized to assign the upper level of the stratum corresponding to VS of 330 ± 30 m/s. The estimated depths were tabulated (Table 3) and employed to generate the map for weathered rock level up to 14 m depth via ArcGIS Software V 10.4.1 (Figure 10). About 67% of the region has weathered rock level at the entire ground surface except the central part and a small area at the eastern side. The weathered rock level between 0 m and 5 m was estimated about 17.7% of the region. About 12.7% of the region was observed with weathered rock from 5 m to 10 m depth (dark orange colour). Around 2.6% of the region has weathered rock level from 10 m to 14 m (around the airport). In general, more than half of the study area was found to be a very dense soil or soft rock at the surface. Engineering bed rock strata were observed at 52 different locations, where VS values correspond to 760 ± 60 m/s. Data in those locations have been employed to generate the map of engineering bed rock level (Figure 11). Engineering bed rock level in the study area was found to be at the ground level in northern region (Ahir Dagi mountain) with a percentage of 9.7% of the boreholes. Depth of the engineering rock between 0 m and 10 m depth in the northern-central and western regions was about 17.3% of the whole boreholes. Almost half of the study area (48%) has an engineering depth ranging from 10 m to 20 m (dark green color). The engineering rock level was found to be between 20 m and 32 m for 23% of the entire study area (light green colour). For a depth greater than 100 m, the engineering rock level was found to be about 1.92% of the whole data (site 4624 shown in brown colour).Table 3
Coordinates of MASW and MAM test stations implemented in Kahramanmaras city with depth and equivalent shear wave velocity corresponding to weathered and engineering rock.
Station no.CoordinatesDepth ofVS profile (m)Depth of rock (m)EquivalentVS of the overburden (m/s)XYWeatheredEngineeringWeatheredEngineeringJF-1303102416313430017.7∗426JF-230324341644826.705.4∗461JF-3304612416547921016.4∗589JF-43065794165972220No hard rock∗∗JF-5306304416497330018.4∗399JF-6307600416486230021.2∗469JF-73061234163156100No hard rock∗∗JF-8304969416166480No hard rock∗∗JF-93073154161457205.1No hard rock230∗JF-103095834160651300No hard rock∗∗JF-1130936141623655.50No hard rock∗∗JF-12308079416331922No soft rock0∗∗JF-13309225416389019No soft rock0∗∗JF-14305264416010416012.7∗542JF-1530703041582371908.8∗688JF-163083324157312245.9No hard rock155∗JF-17311198416307213010.23∗537JF-183115624161736100No hard rock∗∗JF-193138974162043184.38No hard rock231∗JF-20316116416211522017∗374JF-21———————JF-22315959416343622No soft rock0∗∗JF-233183074163431300No hard rock∗∗JF-2431925741645317.50No hard rock∗∗JF-25318626416077421No soft rock0∗∗JF-2631687741611169.507.4∗516JF-27313473416005430021.3∗373JF-283113444160024100No hard rock∗∗JF-293186764158305309.2No hard rock242∗JF-3031667141564665.5No soft rockNo hard rockNo hard rock∗JF-313161244158640139.9No hard rock233∗JF-32313724415783564.82No hard rock270∗JF-333137974156346107.89No hard rock256∗JF-3431114841562507No soft rockNo hard rockNo hard rock∗JF-353123194157883806.4∗391JF-3631431541538261209.7∗364JF-37316375415366311.50No hard rock∗∗JF-38316335415119210.50No hard rock∗∗JF-39315632414980730023∗523JF-4031468241475116.50No hard rock∗∗JF-41315362415172319015∗428JF-4231431441502798.50No hard rock∗∗JF-433173964152137140No hard rock∗∗JF-443203194164198906.9∗450JF-45321427416474620011.8∗414JF-4632016041625716.505∗301JF-4732228141639461908.72∗510JF-48323630416395516012.7∗544JF-49303102416313420015.4∗356JF-50303243416448219No soft rock0∗∗JF-5130461241654791511.7No hard rock232∗JF-523065794165972118.7No hard rock260∗JF-5330630441649736.5No soft rockNo hard rockNo hard rock∗JF-543076004164862308.519.4304381JF-55306123416315680No hard rockNo hard rock∗JF-56304969416166419015.2∗505JF-5730731541614576.53.9No hard rock187∗JF-58309583416065110No soft rockNo hard rockNo hard rock∗JF-593093614162365221013.2209239JF-6030807941633191814No hard rock257∗JF-6130922541638902213.62No hard rock305∗JF-62305264416010415011.6∗438JF-633070304158237110No hard rock∗∗JF-643083324157312169.46No hard rock261∗JF-65311198416307210No soft rockNo hard rock∗∗JF-66311562416173617No soft rock0∗∗JF-67313897416204318010.6∗492JF-68316116416211512No soft rock0∗∗JF-69329780416466630016.8∗504JF-70315959416343622No soft rock0∗∗JF-71318307416343170No hard rock∗∗JF-72319257416453170No hard rock∗∗JF-7331862641607741007.8∗624JF-743168774161116130No hard rock∗∗JF-75313473416005490No hard rock∗∗JF-76311344416002430022.8∗535JF-77318676415830523018.2∗512JF-78316671415646618No soft rock0∗∗JF-79316124415864064.65No hard rock261∗JF-80313724415783530021.4∗572JF-813137974156346308.920.5298328JF-823111484156250150No hard rock∗∗JF-833123194157883300No hard rock∗∗JF-843143154153826225.517236325JF-8531637541536637.55.8No hard rock273∗JF-8631633541511928.74.1No hard rock270∗JF-87315632414980714011.4∗417JF-88314682414751130022.8∗423JF-461730836941621071003.87.6140222JF-46183121204163601100017443473JF-46193115494162084100028.8∗437JF-46203144324161874100522.8245407JF-46213171064162758100018.5∗441JF-46223214224161623100621210299JF-46233175034159998100023∗508JF-462431588141563831008.5No hard rock170∗JF-46253216174156597100832220364JF-462631582241607791004No hard rock230∗Figure 10
Weathered rock level in Kahramanmaras area using the MASW and MAM data.Figure 11
Engineering rock level in Kahramanmaras area using the MASW and MAM data.Figures12 and 13 present the overburden soil depth to the weathered rock and engineering bed rock levels, respectively. The equivalent VS for the depth of soil is indicated as VH and computed as follows [37]:(6)VH=∑i=1ndi∑i=1didi/Vsi,where H = ∑ and di is accumulative depth up to the level of weathered rock or engineering bed rock levels in m; di and vsi indicate the thickness in meters and velocity of shear wave of the ith layer in m/s, respectively. Applying equation (6) has revealed equivalent velocity of the overburden soil ranging from 140 m/s to 443 m/s up to weathered rock and from 222 m/s to 688 m/s up to engineering bed rock. According to the NEHRP classification system, about 85% of the study area has an average overburden VS from 180 m/s to 360 m/s and is classified as stiff soil over a depth ranging from 3.9 m to 14 m. 11% of the study area has an average VS less than 180 m/s and is classified as soft soil over a depth from 3.8 m to 8.5 m (yellow areas in Figure 12). Also, average VS value from ground surface to weathered rock level over the 3.8% of the study area was estimated as higher than 360 m/s. Furthermore, average VS value for the dense overburden soil and/or soft rock from 5 m to 32 m depth, about 85% of the study area, was found to be more than 360 m/s.Figure 12
AverageVS of overburden soil to the weathered bed rock in Kahramanmaras area.Figure 13
AverageVS of overburden soil to the engineering bed rock of Kahramanmaras area.


## 6. Conclusions

The purpose of this study is to generate an intensive series of Geographic Information System (GIS) maps for the seismic site classification (SSC) in Kahramanmaras city, influenced by both west part of East Anatolian Fault and northern part of Dead Sea Fault Zones with a capability of producing the strongest future earthquakes in southern-central Turkey. The study presents the GIS maps for average SPT-N values (N30), average shear wave velocity values (VS30), variation in depths to weathered and engineering rock levels, and average VS of overburden soil. The VS30 based SSC maps indicated that majority of the soils in the study area were found to be class C, although N30 based SSC maps indicated that more than 50% of the study area was class D. The differences among the findings by two different approaches could be explained by the accuracy of both geophysical tests of MASW and MAM at the field because of some difficulties in opening 287 boreholes and recording their SPT-N drops. Another reason could be the method employed for extending the VS profiles up to 30 m. This approach increases the queries on consistency of NEHRP classification system. Furthermore, another series of raster maps generated for average VS and SPT-N values at 5 m, 10 m, 15 m, 20 m, and 25 m depths revealed that 85% of the study area, according to the weathered bed rock level, was classified as soil type D with depths ranging from 3.9 m to 14 m, while 84% of the study area, according to the engineering bed rock level, was classified as C from 5 m to 32 m depth.



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*Source: 8862827-2020-11-04.xml*