Abstract:
A series of earthquakes occurred along the North Anatolian Fault (NAF) during the 20th century, primarily migrating from east to west. The only part of the NAF that has not broken is under the Marmara Sea. The Main Marmara Fault (MMF), the NAF’s northern branch, is the most active one, with the highest slip rate amongst the several branches of the NAF. Since the seismic gap of ~150 km is beneath the sea, the geodetic data is not sufficient to constrain the full fault coupling, particularly in the Central Marmara. Nevertheless, the current data does imply that the GNSS vectors along the northern coast of the Marmara Sea are smaller than expected. One interpretation is that the MMF has heterogeneous interseismic coupling with creeping and locked segments. Another explanation is that the fault is locked, but the strain is asymmetrically localized around the MMF as a result of the deep basins. In this study, the competing effects of weak interseismic locking of the MMF and deep basins around the fault are studied by developing a 3-D finite element model for the Marmara Region, which includes a realistic topography, the 3-D geometry of the main fault, and basins, and using the geodetic data as a constraint. Our findings show that the deep basins confine the interseismic strain in the fault vicinity, and using a homogeneous half-space model leads to a slight underestimation of the locking depth. Our 3-D model shows that while the basins have some effects on strain localization, the heterogeneity of interseismic coupling is necessary to explain the observed GNSS data. We infer a change in the locking depth at the Ganos Bend between the strongly coupled Ganos and the weakly coupled Western Marmara. Seismic studies also indicate that these two segments vary considerably in background seismicity. The 50 km creeping segment coincides well with repeating earthquakes and higher rates of diffuse seismicity. Variations in regional stresses and earthquake focal mechanisms, including the 2019 Silivri earthquake sequence, are compatible with the dilatational quadrants in the region due to the loading caused by the interseismic creep of the Western Marmara.
