Distribution of Seismic Velocities and Attenuation in the Crust beneath the North Anatolian Fault (Turkey) from Local Earthquake Tomography

We investigate the crustal structure beneath the western part of the North Anatolian fault zone (NAFZ), an area where at least five damaging earthquakes occurred during the twentieth century. This study is based on local earthquake tomography using the data from aftershocks of the Izmit event (17 August 1999, M 7: 4) recorded by stations of permanent and temporary networks. We derive the distribution of V(P), V(S), and the V(P)/V(S) ratio based on the iterative inversion for both V(P) - V(S) and V(P) - V(P)/V(S) using the LOTOS code. Innovatively, in this study we perform an inversion for frequency-dependent S-wave attenuation (1/Q(S)). The reliability of the results is assessed through synthetic tests. The distributions of the resulting seismic parameters (V(P), V(S), V(P)/V(S), and Q(S)) highlight important geodynamical features in the study area. The low-velocity and high-attenuation patterns mostly correlate with the fracturing zones of the NAFZ. Low velocities are also observed beneath the main sedimentary basins (e.g., Adapazari, Duzce, and Kuzuluk). High-velocity and low-attenuation patterns correlate with blocks presumed to be rigid (Kocaeli, Armutlu, and Almacik blocks). The rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities, while moderate and weak seismicity is mostly concentrated in low-velocity areas. Based on these results we propose and discuss the role that the Almacik block could have played in producing the largest earthquakes in the study area in the twentieth century.