3D crustal velocity and VP/VS structures beneath Southeast Anatolia and their geodynamic implications

We applied a seismic tomography method to arrival time data generated by local crustal earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and V-P/V-S structures beneath the area. Many of the previous seismological studies of the region are of a regional, or even global, scale. A total of 2150 carefully-selected events generating 13690 and 12560 P- and S-wave arrival times are finally used in the tomographic inversion. Results of the checkerboard resolution test imply that the obtained velocity and V-P/V-S anomalies are reliable features. In addition, hit count maps indicate that all parts are hit by an adequate number of rays to retrieve the crustal velocity structure. Strong lateral crustal heterogeneities are revealed beneath southeast Anatolia with many lower-than-average velocity anomalies. The low velocity anomalies are imaged especially near the active fault segments. In addition, high V-P/V-S ratios are mapped at most crustal layers especially at depths of 10 and 22 km which are consistent with the distribution of ophiolite belts. The high V-P/V-S zones are induced by the possible existence of over-pressurized fluids in the crust and perhaps the uppermost mantle. The existence of these fluids along with the intense tectonic activity could trigger large crustal earthquakes along the western segment of the East Anatolian fault zone. Although may occur in high velocity zones, the majority of the large crustal earthquakes are distributed near zones of average velocity/high V-P/V-S anomalies. Such mapped velocity and V-P/V-S zones are in agreement with many previous geophysical investigations beneath southeast Anatolia such as low P-n and S-n velocities, high S-n attenuation, high heat flow, and low L-g Q(0) values. Results beneath this region of the Anatolian Plateau are also similar to those observed beneath other continental plateaus such as the Tibet and point to a hot, partially-molten upper mantle.