Estimation of optimum velocity model and precise earthquake locations in NE Aegean: Implications for seismotectonics and seismic hazard

This study relocates the seismicity in NE Aegean during the period 2011-2017 using data recorded by the Hellenic Unified Seismic Network (HUSN) in order to elucidate the relationship between seismicity and active faults in this area. P- and S-phase travel times of well-recorded events were first inverted in order to derive a minimum 1D velocity model with station delays using VELEST. Absolute locations of 4450 events were obtained by use of the nonlinear probabilistic algorithm NLLOC and the newly derived velocity model. Precise relative locations with horizontal and vertical uncertainties that do not exceed 1.2 km were calculated for 3354 events using the double-difference algorithm. The relocated seismicity delineates active faults to the south of Lesvos island, at the tip of Biga peninsula in Turkey and along the parallel strands of strike-slip faults that accommodate the westward motion of Anatolia. The comparison of the seismicity distribution with known active faults and the regional stress field shows that the strike-slip faults represent either principal shear zones, or Riedel shears oriented obliquely to the minimum stress axes. Normal faults are oriented almost perpendicular to the direction of the minimum stress axes in accordance with the transtensional deformation model. The seismogenic layer thickness derived from the depth distribution of relocated seismicity was found to be in the range of 14.8-15.8 km. By combining this thickness with geometrical characteristics of active faults and with a relationship that connects moment magnitude with rupture area, it is possible to estimate expected magnitudes of earthquakes. These magnitudes range from 6.3 to 7.2 depending on the rupture scenario that is considered for each fault. Of particular concern are the faults of Agia Paraskevi in Lesvos and Mastichochoria in Chios island that traverse densely populated areas and can produce large events with magnitudes from 6.4 to 6.9. Very little seismicity can be observed along these faults in the past 7 years, which may indicate either that they are creeping, or that they are locked and accumulate strain energy.