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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]

[Display omitted] • Simulations indicate a possible advance in the failure time of the Main Marmara Fault's locked segment. • Dynamic triggering effects can increase the clock advance up to 1% of its recurrence time depending on the onset time. • Strong brittle-ductile coupling, smaller asperity size, and reduced clamping shorten the seismic cycle. The Mw5.8 earthquake in the Marmara Sea west of the locked segment of the North Anatolian Fault sparked a debate on whether this event may trigger or advance the expected large (Mw > 7) earthquake close to Istanbul in time. Its potential effect on the major earthquake cycle is analyzed using rate-and-state friction (RSF) dependent spring-slider models by considering miscellaneous views of friction and different simulation strategies. Dynamic and static effects are simulated using recorded seismic waveforms and computed static stress change. Results indicated that this moderate earthquake cannot produce instant triggering on the locked fault segment but can advance the failure time noticeably. Simulations display no sensitivity to the inertial damping term; thus, quasi-dynamic and full-dynamic approaches agree. On the other hand, strong brittle-ductile coupling, smaller asperity size, and reduced clamping due to normal stress variations shorten the seismic cycle and increase the induced failure time advances. If failure is far in time (> 5% of the recurrence time), the long-term effect on the seismic cycle is static and especially exceeds Coulomb failure predictions considerably when the system is strongly coupled. If fault failure is close, which is likely in this case, dynamic effects lead to responses that differ among state laws. The Ruina (slip) law which has a weakening rate more consistent with the laboratory velocity step tests, shows the highest failure time advance (≈ 1% of the recurrence interval), implying that the failure time is more imminent than before for the locked fault segment beneath the Marmara Sea. [ABSTRACT FROM AUTHOR]

High-resolution 3-D seismic data acquired in the Sea of Marmara on the Western High, along the northwestern branch of the North Anatolian Fault (also known as the Main Marmara Fault), shed new light on the evolution of the deformation over the last 500-600 ka. Sedimentary sequences in ponded basins are correlated with glacioeustatic cycles and transitions between marine and low sea/lake environments in the Sea of Marmara. In the 3 × 11 km2 of the 3-D seismic survey, deformation over the last 405-490 ka is localized along the main fault branch and north of it, where N130°-N140° trending normal faults and N40°-N50° folding accommodated strike-slip deformation associated with active argillokinesis. There is some evidence that deformation was more distributed further back in the past, at least over the depth range (<600 m below seafloor) of our survey. A N110° basin and buried ridge system were eventually cut by the presently active fault. The southern part of the basin was then uplifted, while the northern part was folded but continued to subside along the fault. A mass transport deposits complex dated between 405-490 ka shows a lateral displacement of 7.7 ± 0.3 km, corresponding to an estimated slip rate of 15.1-19.7 mm/a. We conclude that this strand of the Main Marmara Fault on the Western High has taken up most of the strike slip motion between the Anatolian and Eurasian plates over the last 405 ka at least. [ABSTRACT FROM AUTHOR]

Beneath the Marmara Sea, Turkey, the Main Marmara Fault (MMF), the offshore part of the North Anatolian Fault (NAF), is a well-known seismic gap for future M > 7 earthquakes. However, its detailed fault geometry and microearthquake activity have been debated for several decades. Using data acquired from long-term ocean bottom seismograph (OBS) observations, we made precise hypocenter estimations based on 3-D Vp and Vs velocity structures and assessed the fault geometry beneath the western and central parts of the MMF. The results indicate a segmentation boundary between the near-vertical western part and the south-dipping eastern part located around 28.10°E. Enriched OBS locations indicate microseismicity along both the inner and outer boundary faults of the Central Basin, especially on the western side. A comparison with previously published results suggests that the seismicity pattern has not changed for at least two years, between 2014 and 2016. Using a combined dataset of this and previous studies, lateral variations in the dip angle along the MMF fault segment from 27.4°E to 28.8°E were investigated. Based on this, we depicted on-fault seismicity along the MMF and defined three inactive areas of microseismicity. Two are located in the western segment, corresponding to the rupture area of the 1912 Ms. 7.4 earthquake, and the other, the largest, is located on the eastern segment. From a comparison of previous seismic and geodetic studies, it is considered that this area is a fully locked zone and has the potential for large earthquakes. Having compared the difference between hypocenter locations determined from OBSs and land-based stations, it is proposed that the epicentral locations of the mainshock and aftershocks of the September 26, 2019, M 5.7 earthquake are located much closer to the MMF than locations reported from only land-based results. • A segmentation boundary on the Main Marmara Fault is identified beneath the Central Basin. • Both inner and outer boundary faults of the Central Basin are developed in the western Central Basin. • Zones of no seismicity beneath the Kumburgaz Basin have the potential for a large earthquake. • The focal area of the September 2019 M 5.7 event was close to the Main Marmara Fault. [ABSTRACT FROM AUTHOR]