Foreshock behaviors and mainshock rupture properties associated with the 2025 M_w 8.8 Kamchatka earthquake sequence

The July 29, 2025 M _W 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 M _W 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 M _W 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 M _W 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 Mbl7+ 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.