The present day stress states in the Marmara region

Abstract

Stress tensors with the aid of a large number of fault plane solutions provide important contributions for the evaluation of the seismotectonic setting of a region. In this study, recent earthquake activity at Ganos offshore, Tekirdağ Basin, Çınarcık Basin, Yalova Region and Gemlik Region, which are all located on the western branches of the North Anatolian Fault Zone, were analyzed using a current data set of fault plane solutions derived from a very dense seismic network. For the analysis of high quality fault plane solutions, data from KOERI and the TURDEP project was merged. Data from a total of 105 stations, including 5 continuous SBO stations of KOERI was used. The earthquakes were selected with the following criteria; minimum local magnitude of 2.0, number of minimum P-wave first motion polarity of 10 and toleration of maximum misfit of 1. During the study 85, 75, 73, 102, and 63 source mechanisms were determined in Ganos Offshore, Tekirdağ Basin, Çınarcık Basin, Yalova Region and Gemlik Region, respectively. Through the determination of 9226 high quality P-wave first motion polarities for the selected 398 earthquakes, the number of average polarity per earthquake was 23. Average error depth, latitude, longitude, and GAP values were also obtained as 2.75 km, 0.98 km, 1.25 km and 63˚, respectively. Furthermore, using the algorithm of Horiuchi et al., (1995), simultaneous focal mechanism solutions of individual earthquakes and recent stress regimes along with R-values have been determined for the five clusters. As a result, it was found out that NW-SE trending trans-tensional stress structures leading mostly normal and oblique faulting systems are predominant in Tekirdağ Basin, Çınarcık Basin, Yalova and Gemlik clusters. Nevertheless, Ganos Cluster was presented as a dextral strike-slip deformation system through the transition from normal to reverse faulting system. Vertically oriented Sigma-2 axis was obtained for the Ganos Offshore cluster, whereas in all other regions Sigma-1 axis was vertical. Furthermore, data from KOERI and the TURDEP project was also merged in order to analyze 25.07.2011 Marmara Sea and 16.08.2011 Gemlik Gulf Earthquakes with local magnitudes of 5.2 and 4.0, respectively. It was stated that the stress tensor solutions obtained using mostly small size earthquakes fit the source mechanisms of these two moderate size earthquakes. Consequently, in this research, I have proposed a new and comprehensive approach to the fault geometries, present stress state, and seismotectonic structures of the Marmara Region by a detailed analysis of the results of the large number of fault plane solutions of earthquakes in the Marmara Region using the most dense seismic network of Turkey.