The Effect of Quaternary Alluvium on Strong Ground Motion: 2011van Earthquakes

Abstract

The distribution of damaged buildings observed from October 23 and November 9, 2011 Van earthquakes shows that a major portion of damage occurred at settlement areas underlain by alluvial soils. The effect of alluvial soil on strong ground motion and structural damage can be seen by comparing two strong ground motion records of the Van province, earthquake of 09 November 2011 (Mw:5.6). One record at Edremit on travertine bedrock had a peak horizontal acceleration of 0.10 g. The other one, at the city center of Van province, at a distance of 15 km from the epicenter in Quaternary alluvium, had the accelaration value of 0.25 g. The effect of soil on the ground motion is about 2.5. In this study, the lateral and vertical distribution of the soil were obtained using a total of 81 geotechnical borehole data with depths ranging from 10-25 m. Soil amplification values computed at the alluvial site based on measured shear wave velocity show reasonably good agreement with the observed motions. The earthquake damage was determined to be high at local areas of relatively poor soil conditions such as cohesionles and loose soil, high groundwater. In addition to the soil conditions, there is no doubt that the use of low quality materials and low construction technologies have caused higher damage at central districts of Van province. Considering the soil conditions, both ground deformation (soil amplification, liquefaction) and cycling loading will increase the loss of lives and property in the central districts of Van province. Therefore, in order to minimize the possible damage of a potential earthquake, ground conditions should be considered at the Van settlement area.