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Seismicity, focal mechanisms and active stress field around the central segment of the North Anatolian Fault in Turkey

Karasozen, E.
Özacar, Atilla Arda
Biryol, C. Berk
Beck, Susan L.
We analysed locations and focal mechanisms of events with magnitude >= 3, which are recorded by 39 broad-band seismic stations deployed during the North Anatolian Passive Seismic Experiment (2005-2008) around central segment of the North Anatolian Fault (NAF). Using P- and S-arrival times, earthquakes are relocated and a new 1-D seismic velocity model of the region is derived. Relocated events in the area are mainly limited to a depth of 15 km and present seismicity in the southern block indicates widespread continental deformation. In the next step, focal mechanisms are derived from first motions (P, SH) and amplitude ratios (SH/P) using a grid-search algorithm in an iterative scheme. Analysis of our well-constrained focal mechanisms indicate mainly strike-slip motions apart from some normal and few thrust events that are related to complex local fault geometry. Calculated pressure/tension axes are mainly subhorizontal and maximum horizontal stress directions (SH max) are oriented predominantly in NW-SE direction which corresponds well with the slip character of NAF and its splays. In the east, E-W trending splays show right-lateral strike-slip mechanisms similar to the main strand whereas in the west, antithetic N-S trending faults show left lateral strike-slip motions. The seismic cluster that converged near Corum after relocation indicates a dominant right-lateral strike-slip mechanism along the E-W trending fault. These focal mechanisms are used to perform stress tensor inversion across the region to map out the stress field in detail. Overall, maximum (sigma(1)) and minimum (sigma(3)) principal stresses are found to be subhorizontal and the intermediate principle stress (sigma(2)) is vertically orientated, consistent with a dominant strike-slip regime. These directions point to the clockwise rotation of stress trajectories from N to S where NW-SE directed sigma(1) in the north turns towards N-S in the south away from the NAF. Moreover, the 200-km-long Ezinepazar-Sungurlu Fault which is previously mapped as an active strike-slip fault is characterized by minor seismic activity and trends perpendicular to the computed maximum stress direction in the southwest away from the main strand of NAF suggesting that the Sungurlu segment is either compressional in nature or inactive.