NAF Experiment: Lithospheric Structure of the Central North Anatolia From S-wave Receiver Function Analysis

The North Anatolian Fault is a nearly 1400 km long major right-lateral strike-slip fault. It forms the northern margin of the Anatolian plate, which escapes westward as a result of the convergence between the northward moving Arabian plate and the relatively stable Eurasian plate. The North Anatolian Fault, which has caused many destructive earthquakes, is a young and active continental transform boundary located in part along an old suture zone. In order to investigate the lithospheric deformation and deep structure beneath the middle portion of the North Anatolian Fault, we deployed 39 broadband seismic stations as part of the North Anatolian Fault Passive Seismic Experiment (2005-2008), in collaboration with the Middle East Technical University, Istanbul Technical University and the Bogazici University Kandilli Observatory and Earthquake Research Institute. The deeper structure beneath the region is still poorly known. Our aim is to identify the differences in the lithospheric structure along and across the fault, and to determine how deep the surface structure and deformation along this major continental transform boundary extends into the mantle. The structure and thickness of the crust and mantle lithosphere is investigated using the high-quality teleseismic data from the NAF project through S-wave receiver function analysis. The S-wave receiver function technique (using the method of Hansen et al., 2007) isolates the S-to-P conversion generated at discontinuities beneath the station. To identify the converted phases more clearly, 30 events with epicentral distances between 60-75 degrees, magnitudes greater than 6.0 and depths shallower than 60 km have been processed. The Moho discontinuity is observed at all the stations between 4-5 seconds corresponding to a crustal thickness of 35- 40 km. The results do not indicate a distinctive change in the Moho across or along the fault zone, assuming a constant Vp/Vs ratio. A negative phase corresponding to 60-70 km depth is observed in several record sections, which we tentatively identify as the lithosphere-asthenosphere boundary (LAB). This suggests that under much of the region the lithosphere is thin and we do not see a mantle signature of the NAF.