High resolution microseismicity and nearly-repeating events in the Marmara Sea

The Main Marmara Fault beneath the Marmara Sea has a prominent seismic gap that can produce a devastating earthquake and a serious risk for the surroundings. It is important to scrutinize the seismic activity in region and relate this activity to the deformation of the fault zone. In this study, a new micro-earthquake database is created for the Marmara Sea between 2014-2016 using the data mostly from ocean-bottom seismometers. The detected and located seismicity indicate that Tekirdağ Basin hosts a diffuse activity from ~7 km to about 18 km depth. A high micro-earthquake activity rate predominates beneath the Central Basin, at depths from 3 km to 15 km. The abundancy of earthquakes in the area can be attributed to a creeping zone, considering the conformity with the geodetic observations. On the other hand, Kumburgaz and the western part of Çınarcık Basins show sparse seismicity at depth ranges of 5-19 km and 3-18 km, respectively, signing to a locked fault compatible with the geodetic observations. In addition to micro-seismicity, the repeating events are detected using template matching method on the continuous waveforms from 2008-2021. The clusters of highly correlated detected earthquakes, which are closely spaced or partially overlapped, are attributed to the "near-repeating earthquakes". The nine nearly-repeating earthquake clusters beneath the Central Basin are observed at 8-13 km depths, suggesting seismic creep behavior together with a high seismicity rate. The fault mechanisms of the near-repeater clusters have strike-slip mechanism consistent with Main Marmara Fault zone. The nearly-repeating events have two different patterns of repeating intervals, as long-term and short-term type events. The amount of slip rates from the near-repeater clusters shows varying slip rates but comparable to geodetic rate. The number of near- repeating events decreased significantly after the 2018 and no repeating event is observed during 2019 which Mw 5.8 Silivri earthquake occurred.