The July 29 2025 Mw 8.8 Kamchatka earthquake ruptured the plate interface off the east coast of the Kamchatka Peninsula along the Kuril-Kamchatka subduction zone and generated tsunamis across the Pacific Ocean, followed by volcanic eruptions in Kamchatka. The mainshock was preceded by an energetic foreshock sequence that began with a Mw 7.4 event on July 20 2025, located 40 km northeast of the mainshock hypocenter. However, the physical mechanisms connecting the foreshock to the mainshock are not completely clear. In this study, we present a detailed seismological and geodetic analysis of the sequence by investigating the rupture characteristics of the Mw 8.8 mainshock, the spatiotemporal evolution of the foreshock and early aftershock sequence, and the processes driving their interaction. Teleseismic back projection reveals that the mainshock ruptured predominantly toward the southwest with a total source duration of ∼220 s. The Mw 7.4 foreshock was accompanied by afterslip along the plate interface, which likely drove an expanding aftershock of its own and eventually triggered the mainshock nucleation. This event also shows anomalously high aftershock productivity relative to other M7+ events before and after the mainshock, indicating an elevated likelihood of triggering a larger earthquake. We also identify possible segmentation of the megathrust rupture zone along the Kuril-Kamchatka subduction zone that is separated by higher topography in the upper plate. These results provide new insight into the physical mechanisms of foreshocks and segmentation of mainshock ruptures along major subduction zones, with direct implications for short-term forecasting and seismic hazard assessment of megathrust earthquakes in this and other regions. Major points : 1. The 2025 M8.8 Kamchatka mainshock featured a predominant southwestward rupture of ∼500 km along the subducting plate interface. 2. Afterslip following the 2025 M7.4 foreshock drove its aftershock migration and likely triggered the M8.8 mainshock. 3. A possible soft barrier near Cape Shipunskiy likely separates the 2025 mainshock and other recent earthquake ruptures.
Mach et al. (Sun,) studied this question.