Identification of transtensional and transpressional features in the Sea of Marmara using onshore-offshore seismic and geodetic data

The scope of the study is to determine transtensional and transpressional features along the North Anatolian Fault beneath the Sea of Marmara, using seismic and geodetic data. For this purpose, focal mechanisms of small size NAF earthquakes, recorded by broadband stations and OBSs, have been derived and used as a tool to identify the transtensional and transpressional features. The focal mechanisms of: (1) small to moderate size events are obtained by the CMT inversion technique of Kuge (2003), using onshore waveform data from 2002-2015, (2) micro-earthquakes are obtained using the technique of Horiuchi (2015), using offshore waveform data recorded by 15 OBS stations from 2015-2016. Furthermore, published GPS velocities are used to determine the style of faulting and strain rates. The geodetic horizontal crustal strain rates are determined at equally spaced grid points by interpolating northing and easting components of the 112 GPS vectors from 1994-2013. The results indicate that extensional and strike-slip style deformation dominates the region, while contractional features are rare. Significant extension is observed in cinarcik Basin and the area between Marmara Island and Central Basin. Yalova-cinarcik region shows predominantly N-S extension while the cinarcik Basin events show NE-SW extension. Compressional or transpressional features are derived to the west of Marmara Island and Ganos and in the Central segment extending from Central Basin toward cinarcik Basin. The GPS strain rate results point out the highest values, 24 x 10-8/year, in cinarcik Basin, while the lowest values, 11 x 10-8/year, are observed in Central Marmara. The highest strain rates in both edges of the fault segment in cinarcik Basin can be indicative of a steadily creeping fault segment. In turn, lower strain rates in the Central Marmara region suggest that this segment of the NAF is locked.