The seismotectonics of the Marmara region (Turkey): results from a microseismic experiment

The Marmara region is an active tectonic zone characterised by the transiti on between the dextral strike-slip regime of the North Anatolian Fault (NAF ) and the extension regime of the Aegean Sea. Strong historical earthquakes (M > 7) and the presence of known seismic gaps imply a high level of seism ic hazard. A synthesis of recent studies of active tectonics in the region is presented, including inland and underwater observations. The branching o f the NAF is explained in terms of increasing influence of the extension. H istorical information and instrumental seismicity are reinterpreted in orde r to have a critical appraisal of the existence of large seismic gaps in th e central and eastern Marmara Sea. Focal mechanisms of strong earthquakes a re used to obtain orientation and shape factor of the deviator of the stres s tensor. The resulting tensor is in shear regime (sigma(2) vertical) but c lose to extension (R=0.93) with a, oriented N145 degrees 0. A microseismic experiment with 48 stations distributed around the Marmara Sea was carried out in October-December 1995. A total of 137 microearthquakes were located and 23 of those were selected to obtain focal mechanism solutions. The epic entral distribution indicates activity along the system of pull-apart basin s north of the Marmara Sea. The segment between Marmara Sea and the Saros B ay, activated in 1912, and the Gulf of Izmit, site of the 1754 earthquake, are now silent. Seismic activity is very linear along the northern branch o f the NAF, but it is more diffused on the Bursa and Iznik branches, southea st of the Marmara Sea. The stress tensor obtained from the focal mechanisms of the micro-earthquakes is compared to the one inverted from teleseismic data. The microseismic stress is compatible with a shear (intermediate) reg ime like the one obtained from strong earthquakes, though not as well const rained, one of the acceptable solutions having the same orientation but dif ferent shape (R=0.5). (C) 2000 Elsevier Science B.V. All rights reserved.