Progressive eastward rupture of the Main Marmara fault toward Istanbul.

The Main Marmara fault (MMF) in northwestern Türkiye poses the highest seismic risk in broader Europe. The 2025 moment magnitude (MW) 6.2 event was the largest earthquake along the MMF in >60 years. We integrated observations from multiple temporal scales including the decade-long evolution of M > 5 earthquakes, their rupture dynamics, and aftershock patterns. We show a series of eastward-propagating M > 5 events and a gradual eastward partial rupture of the MMF over the past ~15 years. The seismically active portion of the fault includes creeping and transitional segments with some of the most recent seismicity located near the presumably locked Princes' Islands segment south of Istanbul that has the potential to generate a M ~7 earthquake. Our analysis highlights the necessity of real-time monitoring of this part of the MMF. Editor's summary: Where the North Anatolian fault zone cuts into the Sea of Marmara, just south of Istanbul, lies the only western segment left unruptured over the past century. Martínez-Garzón et al. analyzed recent seismic patterns near the locked zone and noticed a pattern in rupture propagation since 2011. The epicenters and ruptures of moment magnitude (MW) >5 earthquakes, including the 2025 MW 6.2 event, have generally migrated from west to east, as have their aftershock sequences, breaching formerly quiet fault segments in what looks like a steady march toward a city of 18 million people. —Angela Hessler [ABSTRACT FROM AUTHOR]