3‐D crustal structure along the North Anatolian Fault Zone in north‐central Anatolia revealed by local earthquake tomography

SUMMARY 3‐D P‐wave velocity structure and Vp/Vs variations in the crust along the North Anatolian Fault Zone (NAFZ) in north‐central Anatolia were investigated by the inversion of local P‐ and S‐wave traveltimes, to gain a better understanding of the seismological characteristics of the region. The 3‐D local earthquake tomography inversions included 5444 P‐ and 3200 S‐wave readings obtained from 168 well‐located earthquakes between 2006 January and 2008 May. Dense ray coverage yields good resolution, particularly in the central part of the study area. The 3‐D Vp and Vp/Vs tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region. We observed the lower limit of the seismogenic zone for north‐central Anatolia at 15 km depth. Final earthquake locations display a distributed pattern throughout the study area, with most of the earthquakes occurring on the major splays of the NAFZ, rather than its master strand. We identify three major high‐velocity blocks in the mid‐crust separated by the İzmir‐Ankara‐Erzincan Suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of Neo‐Tethyan Ocean. These basement blocks may have in part influenced the rupture propagations of the historical 1939, 1942 and 1943 earthquakes. In addition, large variations in the Vp/Vs ratio in the mid‐crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures.