Source and rupture properties of the 23 April 2025 Mw 6.3 Silivri High-Kumburgaz basin earthquake threatening İstanbul, NW Türkiye

The Main Marmara Fault (MMF), the westernmost segment of the North Anatolian Fault Zone (NAFZ), constitutes a prominent seismic gap beneath the Sea of Marmara, Türkiye. On 23 April 2025, a moderate earthquake of Mw 6.3 occurred offshore Silivri-Kumburgaz, at the transitional zone between the creeping Central Basin and the locked Kumburgaz segment. The present work provides a comprehensive investigation of the source properties, rupture complexity, and seismotectonic context of the 23 April 2025 Mw 6.3 Silivri High-Kumburgaz Basin earthquake and the subsequent aftershock activity. A combination of the advanced centroid moment tensor (CMT), finite-fault slip inversions, and precise relocations of the aftershock enabled to primarily resolve source and rupture properties, and to evaluate the co- and post-seismic deformation associated to the mainshock along the MMF. Moment tensor inversions performed on local, regional, and/or teleseismic data consistently indicate a dominantly right-lateral strike-slip rupture with a slight normal component. The spatiotemporal evolution of rupture kinematics, as inferred from the finite fault inversions suggests a bilateral co-seismic displacement for the 2025 earthquake that terminated toward the west, where a documented aseismic creep regime presumably reduces a substantial amount of stress. The observed post-seismic deformation, as identified by precise relocations of the aftershock sequence, indicates a distinct eastward propagation within the initial 24 h, extending up to approximately 20 km east from the epicenter of the mainshock. We observe that the mainshock nucleates within a low–high Vs transition zone between the partially creeping Central Basin and the locked Kumburgaz segment of the MMF. This implies the moderate mechanical strength of rocks can enable these transition zones as ideal spots where large crustal earthquakes nucleate. Our study favors an elevated probability of a future, large earthquake (M7 +), which could potentially pose a significant threat to the mega-city of Istanbul as the recent Mw 6.3 mainshock may have led to an increased stress on the locked part of the segment remained quiescent since 1766.