Passive seismic methods for obtaining shear-wave velocity profiles and depth to basement in an urban-industrial area, Marmara region, Turkey

The Kocaeli region is located in the eastern Marmara region of Turkey, near the North Anatolian fault zone. As part of a program to assess seismic hazard in this earthquake-prone area, a series of passive seismic (microtremor) studies were made with the objective of acquiring shear-wave velocity (Vs) profiles of the upper few hundred meters of soft and weakly consolidated sediments, over a 5x5 km area. A set of 14 sites were studied using nested triangular passive seismic arrays. Combined use of spatially averaged coherency (SPAC) methods and horizontal/vertical particle motion ratios was successful in documenting thickness and Vs variations over depth ranges from 2 m to 500 m. Particular challenges addressed in this survey were in the processing of data from an environment with highly variable local seismic noise sources in this urban and industrial setting. Apart from successful definition of softness of sediments in the upper 100 m, the survey resolved seismic basement (Vs ~1200 m/s) at depths ranging from 160 m in the north to 500 m in the south-west. While the number of sites is limited there is an indication that the trend of deep basement estimates is in a north-easterly direction which corresponds with the direction of expected secondary tensional faulting associated with the primary dextral east-west shear faults of the North Anatolian fault zone.