S. Parolai, D. Bindi, E. Durukal, H. Grosser, and C. Milkereit
Source parameters and seismic moment-magnitude scaling for northwestern Turkey
Bulletin of the Seismological Society of America (April 2007), 97(2):655-660

Abstract:
The source parameters of 523 aftershocks (0.5L <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 kappa parameter. The seismic moments obtained range between 1.05X10 ¹⁴ and 2.41X10 ¹⁷ N m, whereas the Brune stress drops are between 0.002 and 40 MPa. The kappa 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 (super -2) model. Both the stress drop and the kappa 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.