We have simulated the waveforms of three aftershocks of the İzmit (17 August 1999) earthquake, with magnitudes greater than M 5.0, to determine the lithospheric structure of the Gulf of İzmit, Marmara region. Using the discrete wavenumber technique (Bouchon, 1981), different layered crustal models have been tested for the simulation of the waveforms, and the best crustal model was determined by a best-fit criterion between the observed and simulated seismograms. Our results indicate that the average thickness of the crust is 32 km and that Pn and S velocities are 8.0 and 4.60 km/sec, respectively, in the Gulf of İzmit, Marmara region.
We also computed synthetic waveforms of the 21 April 2000 Denizli (Honaz) and the 9 July 1998 İzmir (Doğanbey) earthquakes in the Aegean in order to compare the lithospheric structures of the Marmara and the Aegean regions. The Aegean region has an average crustal thickness of 33 km and Pn and S velocities of 7.85 and 4.53 km/sec, respectively. Although thicknesses of the crusts are comparable and suggest an approximately equal amount of E-W stretching in the Marmara and N-S stretching in the Aegean regions, a patchy midcrustal low-velocity zone exists in the Aegean.
The upper-mantle Pn velocity variation between the Marmara and the Aegean regions is interpreted as the effect of a thinning continental lithosphere toward the active Aegean arc and the establishment of a consequent upper-mantle temperature gradient, increasing from the north to the south. Additionally, the Black Sea oceanic lithosphere that steeply dips southward beneath the Marmara region, as evidenced by the seismic tomographic results obtained by Gülen and Kuleli (1995), can contribute to the advective cooling of the upper mantle that causes relatively high Pn velocities in the Marmara region.