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Liquefaction Potential of the Settlement Area of Susurluk (Balıkesir, Turkey) In the Context of Earthquake Sensitive Urbanization

The settlement area of Susurluk (Balıkesir) is located in a region with a high seismic risk and its territory is in the first degree of earthquake zone according to the earthquake hazard map of Turkey. In addition, the area is suitable for liquefaction in terms of geological criteria. For this reason, the liquefaction potential maps of this settlement area have been prepared. Standard pentatlon test data provided by the Susurluk (Balikesir) municipality were used directly in the so-called simplified methods. According to the results of the study, Almost all of the area where the terrace is observed has “None” or “Low” liquefaction susceptibility while the liquefaction susceptibility of the area where the alluvium is observed range “Low” to “High”.


Introduction
Soil liquefaction is a major cause of damage during earthquakes. It is defined as the loss of strength due to transfer of inter granular stress from grains to pore water [1].The standard method for evaluating in situ liquefaction potential of soils has been proposed by Seed and Idriss [2] using the Standard Penetration Test (SPT) data. Later updates of this simplified procedure relied almost entirely on case histories involving liquefaction or non-liquefaction ( [1], [3], [4], [5], [6], [7], [8], [9]). The factor of safety against to the liquefaction of the soil layer is defined as the ratio of liquefaction resistance (e.g. cyclic resistance ratio, CRR) over cyclic stress of soil (e.g. cyclic stress ratio, CSR). The factor of safety (F L ) is not a sufficient parameter for evaluation of liquefaction and its damage potential at any site ( [6], [10], [11], [12]). However, the thickness and depth of the liquefiable layer and the factor of safety are very important inputs for damage potential based on liquefaction [12]. Since it was proposed by Iwasaki and co-workers [10], the liquefaction potential index (LPI) has been a very popular tool due to the inclusion of the thickness and depth of the liquefiable layer and the factor of safety as inputs (from [12]).
The settlement area of Susurluk (Balikesir), the study area is located in a region with a high seismic risk. It is mainly under the influence of the southern branch of The North Anatolian Fault Zone (NATZH) and the Havran-Balikesir Fault Zone. In the area, alluvium and terrace unit are observed and the depth of the ground water level ranging 0.5 to 6m. For this reason, the liquefaction potential analysis for this settlement area were carried out. In these analysis, the method purposed by Youd and co-workers [4], based on SPT data, was applied to obtain the factor of safety against to the liquefaction of the soil layer investigated. Also, the liquefaction potential of the soil profiles which represents the whole vertical 2

WMCAUS
IOP Publishing IOP Conf. Series: Materials Science and Engineering 245 (2017) 072037 doi:10.1088/1757-899X/245/7/072037 section at the boreholes were obtained by two methods; one of them given in [11] the other purposed Sonmez and Gokceoglu [6]. To prepare the liquefaction potential map for the study area, Inverse Distance Weighting (IDW) method, a type of deterministic method for multivariate interpolation with a known scattered set of points was applied.

Geological and Tectonic Setting
The Balikesir region including the settlement area of Susurluk is located in one of the geodynamically active parts of Turkey. The tectonic evolution of region is greatly affected by the collision of the Arabian and African plates with the Eurasian plate [13]. This compressional tectonic regime in the Miocene gave rise to the formation of North Anatolian and East Anatolian fault zones, along which the Anatolian block moved westwards. The study area, the settlement area of Susurluk (Balıkesir) is mainly under the influence of the southern branch of NATZH and the Havran-Balikesir Fault Zone (Fig.1). The North Anatolian Fault Zone (NAFZ) is made up of at least two fault splays at its western termination, namely the northern and southern fault splays in the Marmara region ( Fig. 1, [14]) .Towards the Biga Penninsula, the southern branch separates from the other by rightward step-overs. Yenice-Gönen Fault (YGF) is one of the main fault segment of the NAFS in Biga Peninsula. It is about 70 km long and generated destructive earthquake (M w =7.2) of 18th March, 1953.
The HBFZ is a 10-12 km wide, 100-120 km long, approximately N70°E-trending fault zone comprising several riddle faults that display a well-developed shear zone pattern with an echelon elongated hills (from [15]). The fault zone comprised six main segments and displays morphological and kinematic evidences for dominantly dextral movement during the Holocene (from [15]). According to Sözbilir and co-worker [15], geologic, geomorphologic and palaeo-seismologic studies along the HBFZ indicate that the Kepsut, Gökçeyazı and Ovacık fault segments are capable of generating an earthquake with a moment magnitude of up to 7.2 and have an average recurrence interval of 1000-2000 years for surface-ruptured earthquakes. Acidic and intermediate volcanic rocks exposed in the immediate east of the Susurluk settlement area. In the west of the area, granitic rocks exposed. The settlement area is built on Neogene units. Pleistocene aged Savastepe formation composed mainly of Conglomerate, sandstone, claystone, marl and tuffit uncomfortably overlie the Quaternary units. These units, alluvium and terrace are the youngest units in the study area.

Geotechnical Characteristic of the Soil Units
In Susurluk (Balıkesir) settlement area, the topographic level increases to the east to the west. (Fig. 2a). In the area where alluvium is propagated, the slope is <5 °. The slope of the area where the terrace spreading is ranging mostly 0-10 degrees, reaching a maximum of 35 degrees ( Fig. 2a-b) The maps of the level of the groundwater table was created by using the level of the groundwater values measured in drilling wells; It is seen that the groundwater surface is approximately parallel to the topographic surface and the height from the shore to the west increases steadily to reach 64 m (Figure 2c-d). The depth of the groundwater table is ranging 0.5 to 6m and the mean of the said level is 3.3 m were classified as clayey silty sand, silty sand and gravel-sand. In the Figure 3, Examples to the grain distribution curves of the terrace and the alluvium in the study area were given.

Figure 3.The grain distribution curves of the terrace and the alluvium
The corrected SPT-N values of the terrace and alluvial soils in the study area were evaluated in the GIS, and the maps showing these values varying from 3, 6 and 9, 12 and 15 m depth from the topographic surface were produced (Fig. 4). Recorded data of SPT results collected provided by the Susurluk municipality should be corrected to as seen in Eq. 1 ([4]), and the correction factors will be calculated according to Table 1 1 where (N 1 ) 60 corrected standard penetration test blow count, N SPT represents the measured standard penetration resistance, C N is a factor to normalize, N SPT represents the effective overburden stress, C E , represents the correction for hammer energy ratio (ER), C B is the correction factor for borehole diameter, C R is the correction factor for rod length, and C S is the correction factor for samplers with or without liners (Table 1).  These maps were evaluated according to Terzaghi and Peck [16] classification, which defines the relationship between N SPT and relative density (D r ). The (N 1 ) 60 value ranges generally from 15 to 50 in terraces and from 10 to 30 in alluvium.

Safety factor against liquefaction
In this study, the method suggested by Youd and co-workers [4] was used in the calculation of factor of safety (FL) against liquefaction of the soil layers (Eq.2) where CSR is Cyclic stress ratio, CRR 7.5 is the cyclic resistance ratio for Mw = 7.5 earthquake and MSF is Magnitude scaling factor according to M w =7.5

Magnitude scaling factor and maximum ground acceleration
In the literature, there are several different methods of calculation for MSF. Idriss [17]  where the SRL is the length (km) of surface rupture (or the fault segment which produces the scenario earthquake) and the coefficients a and b depend on the fault type and are taken from Table 2.
, where, a max is the peak ground acceleration, S A and S B are site condition constants (S A =0 and S B =0 for rock sites, S A =1 and S B =0 for soil sites, S A =0 and S B =1 for soft soil sites) and Re is the distance to epicenter.
The length of the faults, the closest distances to the Susurluk settlement area (Fig.5), moment magnitudes obtained from Eq.5 and maximum horizontal accelerations of the scenario earthquakes obtained from Eq. 6 are given in Table 3. The most severe damage may occur if the scenario earthquake occurs on the southern branch of the NAFZ (Yenice-Gönen Faults) on which the highest maximum horizontal ground acceleration value was obtained among the earthquake scenario (Table 3). Therefore, Mw and a max considered in the liquefaction analyses are assumed to be equal to 7.3 and 302 gal, respectively   Table 3. The magnitude of possible scenario earthquakes and maximum ground acceleration based on the active faults affecting the Susurluk Balıkesir) settlement area

Determination of cyclic stress ratio
Cyclic stress ratio (CSR) characterizes the seismic demand induced by a given earthquake, and it can be determined from peak ground surface acceleration that depends upon site-specific ground motions [11]. The expression for CSR induced by earthquake ground motions formulated by Youd et al [4] is as follows: where 0.65 is a weighing factor to calculate the equivalent uniform stress cycles required to generate same pore water pressure during an earthquake; a max is the peak horizontal ground acceleration; g is acceleration of gravity; σ v and σ' v are total vertical overburden stress and effective vertical overburden stress, respectively, at a given depth below the ground surface; r d is depth-dependent stress reduction factor; MSF is the magnitude scaling factor.
This stress reduction factor (rd) accounts for the dynamic response of the soil column and represents the variation of shear stress amplitude with depth. The factor and is defined as following quation [4], (Eq. 8)

Determination of cyclic strength ratio
In order to calculate the cyclic strength ratio of the soil layer investigated, the equations in the methods given in [4] was applied (Eqs. 9-12) where CRR 7.5 is the cyclic resistance ratio for M w =7.5 , (N 1 ) 60CS is the (N 1 ) 60CS value corrected for fines content(FC), α and β are coefficients that depend on the fines content.

Liquefaction potential index and Liquefaction severity index of the soil profile investigated
LPI was originally proposed by Iwasaki and co-workers ( [10], [20]) to evaluate the potential for liquefaction to cause foundation damage. LPI assumes that the severity of liquefaction is proportional to (1) the thickness of the liquefied layer; (2) proximity of the liquefied layer from the ground surface; and (3) amount by which the factor of safety (FL) is less than 1.0. LPI is defined as: where z is depth from the ground surface in meters. F(z) values given by Iwasaki and co-workers ( [10], [20]) was rearranged by Sonmez [11] as following equations (Eq. 16) The liquefaction potential of the study area was prepared by two different methods. In the first (Fig.6a), the approach proposed by Youd et al [4] is based on (Eqs 9-12) and the method given in [11] was applied. In the other map (Fig. 6b), the method given in [6] was applied to obtain the liquefaction potential of the study area. Sonmez and Gokceoglu [6] introduce the probability of liquefaction (P L ) proposed by Juang et al. [21] based on the factor of safety to the LPI concept. The calculation of the LPI based on P L modified by Sonmez and Gokceoglu [6] was given following equations (Eqs. [17][18][19]. In addition, Sonmez and Gokceoglu [6] rearranged the classification of liquefaction severity by considering Chen and Juang's [22] probability of liquefaction classes based on P L values. (or the soil layer FL ≤ 1.411can be considered as non-liquefiable considering clay content and liquid limit). (19) where L S is Liquefaction severity index, the term W(z) is as same as those Eq. 14 (a) (b) Figure 6. Liquefaction susceptibility maps of the Susurluk (Balikesir) settlement area on the liquefaction potential index classification modified by Sonmez [11] (a) and the Liquefaction severity index purposed Sonmez and Gokceoglu [6]

Result and Conclusion
In the study area, terraces and alluvium are observed on the surface. Scenario earthquakes were created taking into account the lengths of active faults near the survey area (within a 100 km circle). The maximum ground acceleration (a max ) is obtained by using the attention relationship. As a result of these analysis, to calculate the factor of safety (F L ) against liquefaction of the soil layers, Mw and a max considered are assumed to be equal to 7.3 and 302 gal, respectively.
In the liquefaction analyses for the study area, to obtain liquefaction susceptibility maps of the study area, liquefaction potential index classification modified by Sonmez [11] and the Liquefaction severity index purposed Sonmez and Gokceoglu [6] were applied. Iwasaki et al. ( [10], [20]) defined the effect of the factor of safety on liquefaction potential as linear from zero to one. It is obviously known that the layer having the smallest factor of safety may cause more damage on the surface. The studies carried out by Sonmez [11] and Sonmez and Gokceoglu [6] were performed to overcome the use of linear relation for impact of the factor of safety on liquefaction potential.
Almost all of the area where the terrace is observed has "None" or "Low" liquefaction susceptibility according to the methods purposed by Sonmez [11] and as the same, these are has "Very low" susceptibility according to the methods purposed by Sonmez and Gokceoglu [11]. 21.1% of the areas where alluvium is observed have "Low", 43.3% "Moderate ", 27.4 % have "High" and 6.3% have "Very High" liquefaction susceptibility according to the methods proposed by Sonmez [11]. Considering liquefaction severity index purposed Sonmez and Gokceoglu [6] values, 85% of the area where the terrace is observed has "None" or "Very Low" liquefaction susceptibility while 15% of the area has "Low" liquefaction susceptibility. And, 18.7% of the areas where alluvium is observed have "Moderate" while 81.3% of the areas have "Low" and "Very low" liquefaction susceptibility.
Taking into account these liquefaction susceptibility maps in the urban planning for the Susurluk (Balıkesir) settlement area is the useful in the reducing of earthquake hazards.