Source parameters and seismic moment-magnitude scaling for northwestern Turkey

The source parameters of 523 aftershocks (0.5 < M-L < 5.9) of the 1999 Kocaeli earthquake are determined by performing a two-step spectral fitting procedure. The source spectrum, corrected for both site and propagation effects, is described in terms of a standard omega-square model multiplied by an exponential term of frequency. The latter term is introduced to estimate the high-frequency (f > 12 Hz) fall-off of the acceleration source spectra by computing the K parameter. The seismic moments obtained range between 1.05 x 10(14) and 2.41 x 10(17) N m, whereas the Brune stress drops are between 0.002 and 40 MPa. The K value varies between 0.00 and 0.08 sec, indicating a decay of the acceleration level at the higher frequency part of the spectrum greater than that assumed by the omega(-2) model. Both the stress drop and the K parameter show the tendency of increasing with aftershock magnitude. No evidence of self-similarity breakdown is observed between the source radius and M-0. Finally, both the seismic moment and the moment magnitude are compared with the local magnitude to derive new moment-magnitude relationships for the area.