Post-500 ka and Holocene activity on distributed faults of the North Anatolian Fault system along the southern shelf of Marmara Sea, Turkey

Geodetic monitoring and patterns of seismicity indicate that the Northern Branch of the North Anatolian Fault (NAF) absorbs the majority of the relative motion between the Eurasia and Anatolia plates along the northern Marmara Sea. Nonetheless, historical seismicity documents that the Central Branch of NAF is also hazardous, with earthquakes diffusely occurring in the southern Marmara Sea. In order to better assess the seismic hazards facing large cities along the southern coast, we recently collected geophysical data across the adjacent shelf. These data include closely spaced high-resolution multichannel seismic profiles, sparker seismic profiles, CHIRP sub-bottom profiles, and multibeam bathymetric data. The stratigraphic and structural analyses of this new dataset and prior datasets highlight the geometry of three long faults and many shorter, discontinuous faults. The three longer faults are interpreted as primarily strike-slip fault zones. All the mapped faults are Late Quaternary active and, in fact, many fault segments are Holocene active. Smaller discontinuous faults are present in Gemlik and Erdek bays, some of them clearly active during Holocene time. This pattern of Late Quaternary active faults can account for the well-documented dispersed seismicity on the southern shelf. Based on the lengths of the various fault segments, we estimate that earthquakes with moment magnitude as high as 7.4 may occur along the southern shelf of the Marmara Sea. Therefore, the system of distributed faults that constitutes the Central Branch of the NAF in that area represents a significant seismic hazard for the southern coastal cities.