Coseismic faulting complexity of the 2019 M (sub w) 5.7 Silivri earthquake in the central Marmara seismic gap, offshore Istanbul

The submarine Main Marmara fault is overdue for an M>7 earthquake in direct vicinity to the Istanbul megacity and the only segment of the right-lateral North Anatolian fault zone that has not been activated since 1766. On 26 September 2019, an M (sub w) 5.7 earthquake occurred offshore Silivri (west of Istanbul), stimulating the discussions on a future major earthquake. The predominant faulting style for this pending earthquake remains enigmatic. Here, we study the coseismic rupture evolution of the 2019 Silivri earthquake and decipher the multitype-faulting aspects by a nonstandard approach. The event was dominated by a large compensated linear vector dipole component, of about -50%, that we interpret as a consecutive strike-slip and thrust double-couple episode, closely collocated in space and time. Because of local variations of the fault geometry at the eastern boundary of the central basin, crustal shortening and related thrust faulting are expected. A striking observation is the almost synchronous occurrence of both faulting types during a single earthquake. The earthquake complexity here is reported for the first time in the Sea of Marmara region and has to be considered in future rupture scenarios of an expected M>7 event, with subsequent consequences for tsunami hazard and risk.