The Effect Of Fault-Zone Guided Waves On Site Amplification: A Case Study From The Izmit Basin, Turkey

The İzmit basin located in the eastern Marmara region of Turkey was formed in Plio-Quaternary period and is known to be a pull-apart basin controlled by the northern branch of the North Anatolian Fault Zone (NAFZ). During the 1999 Mw=7.4 Kocaeli earthquake, the greatest damage and casualties occurred in and around the İzmit basin, where the industrial facilities and population are mostly concentrated. For such a region with high seismic hazard potential, the effect of fault-zone guided waves should also be considered in seismic hazard studies investigating site amplification. In this study, the fault-induced site effect for the İzmit basin is investigated by carrying out 2D seismic wave propagation simulations using the bedrock geometry and the shear-wave velocity profiles describing the velocity structure of the sediments. In order to quantify the effect of fault zone on seismic waveforms, we use velocity models with and without the fault zone. Fault-zone guided waves can easily be seen on both time domain waveforms and the results of the frequency-wavenumber analysis. The fault-zone guided waves cause an amplification level of about 4-4.5 in the frequency range of 0.25 to 2 Hz.