Seismicity, focal mechanisms and active stress field around the central segment of the north anatolian fault in turkey

We analysed locations and focal mechanisms of events with magnitude ≥3, which are recordedby 39 broad-band seismic stations deployed during the North Anatolian Passive SeismicExperiment (2005-2008) around central segment of the North Anatolian Fault (NAF). UsingP- 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 andpresent seismicity in the southern block indicates widespread continental deformation. In thenext 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 focalmechanisms indicate mainly strike-slip motions apart from some normal and few thrust eventsthat are related to complex local fault geometry. Calculated pressure/tension axes are mainlysubhorizontal and maximum horizontal stress directions (SH max) are oriented predominantlyin 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 themain strand whereas in the west, antithetic N-S trending faults show left lateral strike-slipmotions. The seismic cluster that converged near Çorum after relocation indicates a dominantright-lateral strike-slip mechanism along the E-W trending fault. These focal mechanisms areused to perform stress tensor inversion across the region to map out the stress field in detail.Overall, maximum (σ1) and minimum (σ3) principal stresses are found to be subhorizontaland the intermediate principle stress (σ2) is vertically orientated, consistent with a dominantstrike-slip regime. These directions point to the clockwise rotation of stress trajectories from N to S where NW-SE directed σ1 in the north turns towards N-S in the south away from theNAF. Moreover, the 200-km-long Ezinepazar-Sungurlu Fault which is previously mapped asan active strike-slip fault is characterized by minor seismic activity and trends perpendicularto 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.