Site-specific seismic hazard analyses in the Izmit region (Marmara, Turkey) through passive seismic tests

The Kocaeli region, located in the eastern Marmara near the North Anatolian Fault Zone, is well known as a zone of high seismic hazard due to its proximity to major fault zones and high seismic risk due to the dense distribution of critical industrial facilities in the area. The region experienced two major earthquakes in 1999 in Kocaeli (Mw = 7.4) and Duzce (Mw = 7.1). This study performs site-specific probabilistic and deterministic seismic hazard analyses at 14 sites in the Kocaeli basin. The multi-mode spatial autocorrelation, a modification to the original SPAC method, is performed to obtain the velocity profiles at the selected 14 sites. The updated method used here employs array-based measurement of seismic ambient noise and measures Rayleigh-wave velocity dispersion from which a one-dimensional shear-wave velocity profile is obtained using a series of small arrays of seismometers. In addition to the array data, the horizontal to vertical spectral ratios from the central stations of the array is used. After obtaining the velocity profiles at each site, site-specific seismic hazard analyses are performed. Probabilistic seismic hazard analysis results are compared to the design spectra from Turkey's current and previous seismic design codes. Analyses are also repeated with reference engineering bedrock conditions to investigate the contribution of site conditions to the seismic hazard. Numerical results of the study indicate the high seismic hazard in the region and highlight the significance of using local parameters in seismic hazard analyses.