Abrupt variations in brittle-ductile transition depth and lower crustal properties beneath two branches of the north Anatolian fault zone, Turkey.

As part of the multi-disciplinary Faultlab project, we present new detailed images of the crust and upper mantle beneath a major continental strike-slip fault system. Our study region samples the north Anatolian fault zone (NAFZ) near the epicentres of two large earthquakes that occurred in 1999 at Izmit (M7.5) and Düzce (M7.2) and where estimates of current slip rate are 20-25 mm/yr. We calculated receiver functions from teleseismic earthquakes that were recorded by a rectangular seismometer array spanning the NAFZ with 66 stations at a nominal inter-station spacing of 7 km and 7 additional stations further afield. We use a combination of H-K stacking, common conversion point migration and non-linear inversion of receiver function stacks to constrain the subsurface velocity structure and illuminate major changes in the architecture and properties of the upper crust, lower crust and upper mantle, both across the two NAFZ branches and along the NAFZ, at length scales of less than 20 km. We show that the northern NAFZ branch depth extent varies from the mid-crust to the upper mantle and it is likely to be less than 5 km wide. A high velocity lower crust and a region of crustal underthrusting appear to add strength to a heterogeneous crust and play a role in dictating the variation in faulting style and postseismic deformation. Furthermore, we show a direct relationship between crustal terrane, seismicity rate and seismicity depth, indicating that the brittle-ductile transition is likely to vary over horizontal length scales of less than 10 km.