Crustal velocity and Vp/Vs structures beneath central Anatolia from local seismic tomography

We applied a seismic tomography technique to arrival time data generated by local crustal earthquakes in central Anatolia in order to study the three-dimensional velocity and Vp/Vs structures and their relation with the complex tectonic processes and seismic activity occurring in the study region. The relatively equal and large number of both P- and S-wave arrival times comprising a total of 51,650 arrivals and the relatively uniform distribution of the recording stations imply that the obtained velocity anomalies are reliable features down to a depth of 40 km. This is also evident from the results of the checkerboard resolution test, hit count, and the ray-path coverage. The inversion results indicate the existence of strong lateral heterogeneities in the crust and uppermost mantle beneath central Anatolia. Prominent low-velocity anomalies are clearly imaged at all layers especially beneath existing volcanoes and the active fault segments. Higher-than-average Vp/Vs ratios are widely distributed, indicating the possible existence of over-pressurized fluids that may be responsible for the triggering of the large crustal earthquakes along the north and east Anatolian fault zones. We noticed that the seismic activity occurs mainly at the low-velocity areas and to a lesser extent in some high-velocity zones, perhaps because of the complex tectonics and geological structures. These observations imply that all the zones with velocity anomalies—either low or high—are potential sites for strain energy accumulation and subsequent release. The obtained velocity and Vp/Vs models are consistent with previous geophysical measurements conducted beneath central Anatolia and give much deeper understanding of the current seismotectonic processes occurring in the region.