The moderate size 2019 September Mw 5.8 Silivri earthquake unveils the complexity of the Main Marmara Fault shear zone

The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of. Istanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behaviour along its 150 km length. In 2019 September, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes >= M-w 3.5 in a week. The M-w 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 M-w 6.3 Cmarcik earthquake. Our analyses reveal that the activity started in a narrow zone (similar to 100 m) and spread to similar to 7 km following an M-w 4.7 foreshock within similar to 2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the M-w 5.8 earthquake did not occur on the MMF, but it ruptured similar to 60 degrees north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the main shock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10-13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the M-w 5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading.