The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. 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 behavior along its 150 km length. In September 2019, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes > Mw3.5 in a week. The Mw5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 Mw6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (~100 m) and spread to ~7 km following an Mw4.7 foreshock within ~2 days. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the Mw5.8 earthquake did not occur on the MMF, but it ruptured ~60 degree north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the mainshock 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 he Mw5.8 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.