A new index for microzonation of earthquake prone settlement area by considering liquefaction potential and fault avoidance zone: An example case from Edremit (Balikesir, Turkey)

Abstract

In the present study, an approach is suggested for the settlement area where there is liquefaction and surface fault rupture hazard at the same time, which allows the assessment of land damage in case an earthquake occurred. Edremit (Balikesir NW Turkey), mainly under the influence the Edremit Fault Zone in the southern branch of the North Anatolian Fault Zone, was chosen as the study area. According to paleo-seismological findings from the Narli trench, at least three similar earthquakes occurred on the same rupture of Edremit Fault Zone passing in the studied area. In addition, generally the groundwater level in the liquefiable alluvial soil varies between 0.5 and 6 m, and also the ratio of areas of liquefiable soil varies between 56 and 78% at different depths in the area. The buffer zone for the Edremit Fault Zone ruptures was defined based on the distance from the surface fault rupture in the study area, and it was seen that 15% of the study area is within the first-degree fault avoidance zone, while 43.5% is located within free zone. The safety factor against liquefaction for the soil layers was determined by using simple procedure based on SPT-N values, and then, the spatial distribution of the liquefaction potential index was obtained. It is found that 43% of the study area has high or very high liquefaction potential while the rate of the area where liquefaction is not expected is 27.8%. The liquefaction potential and the map showing the fault avoidance zones are important and successful in terms of individual hazard related to earthquake. However, the said maps do not allow to assess, simultaneously and completely, the realistic extent of the possible land damage in case an earthquake occurs. So, using the liquefaction potential index and the distance from the surface fault rupture, a new index, namely land damage index, was defined to create the microzonation of the seismic hazard for liquefaction and surface fault rupture-induced land damage. According to the zonation of land damage index, 29.7% of the study area is consists of Land Damage Zone I where settlement is not allowed. The main goal of the preparation of the said microzonation for the study area is to recognize the hazard from active faults, with respect to liquefaction and surface fault rupture, and to provide guidance to planners on how to mitigate the risk for different types of buildings.