The Effect of the 3‐D Structure on Strain Accumulation and the Interseismic Behavior Along the North Anatolian Fault in the Sea of Marmara.

The 150‐km seismic gap in the Sea of Marmara is the longest section of the North Anatolian Fault that has not ruptured in the last century. Recent geodetic studies suggest that the interseismic coupling along the Main Marmara Fault (MMF) is heterogeneous with locked and creeping segments. Alternatively, the MMF is fully locked, but the strain is localized asymmetrically around the fault due to the deep basins. In this study, we build a 3‐D model of the region, including the basins and the 3‐D fault geometry, to study the competing effects of deep basins and the interseismic locking distribution using the observed Global Navigation Satellite System velocities. We conclude that the basins along the fault localize the interseismic strain, leading to slightly deeper‐estimated locking depths. The on‐land data are too far to constrain the full interseismic coupling, especially in the Central Marmara. The Western Marmara is better resolved, where we infer a ∼50‐km creeping zone. The transition from the creeping to the locked behavior in Western Marmara is around the Ganos Bend. The creeping zone correlates well with a higher rate of diffuse seismicity and repeating earthquakes, implying that the seismicity could be an indicator of interseismic behavior. Variations of earthquake focal mechanisms and associated regional stresses are also consistent with compressional and dilatational quadrants due to the creeping segment. The significant thrust components observed in the 2019 Silivri earthquake sequence north of the MMF can also be explained by the compressional loading due to the interseismic creep further west. Plain Language Summary: The Sea of Marmara, on the western extension of the North Anatolian Fault, is the only part of the fault that has not ruptured in the last century. Recent studies suggest that the interseismic locking or coupling, which is the ratio of interseismic slip rate deficit to the long‐term plate velocity, changes along the fault. Another suggestion is that the Main Marmara Fault is fully locked, and the deep basins cause strain localization and asymmetrical strain accumulation around the fault. In this study, we build a 3‐D model of the region to understand the fault locking and the basin effects on the deformation by using geodetic data. When the basin effects are considered, the estimated best‐fitting locking depths increase slightly. Especially in the Central and Kumburgaz Basin segments, the on‐land data are too far to constrain the full interseismic coupling behavior. Western Marmara is better resolved, where a transition from the 50‐km‐long creeping segment to the locked segment is well‐constrained to be located at the Ganos Bend. This unlocked segment correlates well with a high rate of earthquake activity and repeating earthquakes. We suggest that the heterogeneous interseismic coupling pattern is consistent with off‐fault focal mechanisms. Key Points: We consider the competing effects of the heterogeneous structure and interseismic behavior along the Main Marmara FaultThe basins that surround the fault lead to a more localized strain and slightly deeper locking depth estimatesThe existing data show creep in Western Marmara, which correlates well with the focal mechanisms, distribution of seismicity and repeaters [ABSTRACT FROM AUTHOR]