ALBERT

Description: The Marmara section of the North Anatolian Fault Zone (NAFZ) is late in its seismic cycle and can be expected to produce a magnitude M up to 7.4 earthquake during the next decades in direct vicinity to the 15-million population center Istanbul. This setting translates the seismic hazard into very high risk and makes a thorough understanding of the current seismotectonic setting of this NAFZ section a pressing task. The absence of near-fault stations along most part of the offshore Marmara section limits the reliability of existing seismicity catalogues for this region. For the first time we combine the different regional permanent networks thereby optimizing azimuthal coverage and present a refined hypocenter catalogue for the Sea of Marmara on this basis. Compared to the original locations, adoption of a refined automated technique to determine precise onset times for the different body waves and an iterative travel-time inversion scheme, lead to substantial improvement of 6.812 absolute earthquake locations, particularly in the epicentral distribution. The automated processing is explained in detail. Further optimization is achieved through relative relocation of 4,407 earthquakes. Our catalogue covers more than a decade (2006 – 2016) with a regional moment-magnitude of completeness of Mc = 2.1. The epicentral distribution delineates the Marmara Section, i.e. the northern NAFZ branch, as the seismically most active fault strand. We identify several aseismic fault patches that are interpreted to represent locked parts of the fault. Seismic activity in the past decade predominantly occurs off the main fault on the edges of the aseismic patches, supporting previous studies that the Marmara section of the NAFZ contains both locked and creeping fault portions. Single-event focal mechanisms (2.7 ≤Mw ≤ 4.5) indicate that currently both strike-slip and normal faulting occurs, confirming the transtensional setting of the region.