3D Bedrock Structure of Bornova Plain and Its surroundings (İzmir/Western Turkey)

Abstract

An earthquake record is needed on engineering bedrock to perform soil deformation analysis. This record could be obtained in different ways (seismographs on engineering bedrock; by the help of the soil transfer function; scenario earthquakes). S-wave velocity (V s) profile must be known at least till engineering bedrock for calculating soil transfer functions true and completely. In addition, 2D or 3D soil, engineering–seismic bedrock models are needed for soil response analyses to be carried out. These models are used to determine changes in the amplitude and frequency content of earthquake waves depending on the seismic impedance from seismic bedrock to the ground surface and the basin effects. In this context, it is important to use multiple in situ geophysical techniques to create the soil–bedrock models. In this study, 2D and 3D soil–bedrock models of Bornova plain and its surroundings (Western Turkey), which are very risky in terms of seismicity, were obtained by combined survey of surface wave and microgravity methods. Results of the study show that the engineering bedrock depths in the middle part of Bornova plain range from 200 to 400 m and the southern and northern parts which are covered limestone and andesite show the engineering bedrock (V s > 760 m/s) feature. In addition, seismic bedrock (V s < 3000 m/s) depth changes from 550 to 1350 m. The predominant period values obtained from single station microtremor method change from 0.45 to 1.6 s while they are higher than 1 s in the middle part of Bornova plain where the basin is deeper. Bornova Plain has a very thick sediment units which have very low V s values above engineering bedrock. In addition, it is observed sudden changes at the interfaces of the layer in horizontal and vertical directions.