S-wave spectral analysis of the 1999 Kocaeli and Düzce Earthquake sequences

Abstract

Source parameters of the 1999 Kocaeli and Düzce, Turkey earthquake sequences are estimated by S-wave spectral analysis of acceleration data. Our data set consists of 484 strong motion acceleration records that correspond to 84 events with magnitudes changing between 1.93 and 6.04. The records are obtained during the period of 21 August 1999- 29 December 1999, from the permanent and temporary stations in the region. The seismological parameters are from a catalog compiled from the Kandilli Observatoıy and TÜBİTAK data. The records are uniformly corrected for distance, free surface and energy partition. The effect of whole path diminution is avoided by considering only records obtained within 40km of the hypocenter. We determine seismic moments (Mo), spectral corner frequencies (fc), source radii (r), stress drops (Acı), slip (s), radiated energy (Erad) and source duration(T). We obtain these parameters by (1) eye-fitting a straight line to the displacement spectra, and (2) fînding the best-fîtting theoretical spectra to the acceleration source spectra and compare the two methodologies. We systematically calculate k, the near surface attenuation factor, and study its dependence on site conditions, distance and magnitude. We correct for site conditions by (1) accounting for the amplification in the low-frequency part of spectrum only, (2) using the frequency-dependent site amplification factors proposed for Boore and Joyner (1997) and (3) by simply eliminating those stations having an average H/V ratio larger than 4. We discuss the differences that we observe in the source parameters introduced by utiliziation of different techniques for site effects.