3-D shear wave velocity structure beneath the European Side of Istanbul from seismic noise arrays analysis

It has been shown that during the 1999 August 17 Izmit earthquake local site effects affected ground motion in distant areas such as the damage observed in Avcilar, west of. Istanbul. Despite efforts to study soil conditions in Istanbul in recent years, site effect problems in the deeper soil are not completely defined. Besides this, the main problems expected in the city of. Istanbul will be the result of amplification of ground motion due to an earthquake. To understand these effects, studies were carried out which showed either limited measurements or provided data for shallow depth by obtaining only the depth of engineering bedrock or the average shear wave velocity down to 30 m depth (V-s30). In this study, we determine seismological bedrock depth and the soil properties using microtremor array measurements on 25 sites on the European side of Istanbul. The results are evaluated and compared with previous studies using a joint inversion of the Rayleigh wave dispersion curve and horizontal/to vertical (H/V) spectrum to describe the deeper subsurface and prove the reliability of the resulting models. It has been also proved that the joint inversion approach is more sensitive in predicting the bedrock depth when compared with the Rayleigh wave analysis results. 2-D and 3-D structures of the underground are constructed to define the bedrock depth of the study area. In the south of the study area (sites: Buyukcekmece, Avcilar, Esenyurt, Florya, Yesilkoy Zeytinburnu and Yedikule) shear wave velocities of the subsurface materials near to the surface are < 300 m s(-1) and the engineering bedrock is reached at 200-400 m. The shear velocity models point to a thick sedimentary cover and sediment thickness of 600 m in the south which decreases to the north to 100 m. 3-D shear wave velocity models reflect evidence of different basin structures in the study area, which are of great importance due to resonance effect as well as focusing effects of the seismic waves on the basin edges, non-linear behaviour and phase transition of seismic waves.