Foreshock Behaviors and Mainshock Rupture Properties Associated with the 2025 Mw 8.8 Kamchatka Earthquake Sequence

Phuc Mach, Xu Si, Dun Wang, Robert Shcherbakov, Ping He, Tuncay Taymaz, Chang Ding, Zhigang Peng 

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. Following the mainshock, tsunamis were recorded in multiple countries along the Pacific Ocean boundary and its islands, along with the eruption of several volcanoes in Kamchatka. The mainshock was preceded by a strong foreshock sequence that began with a Mw 7.4 event on July 20, located near the mainshock hypocenter. However, the physical mechanisms driving this foreshock sequence is not completely clear. Here, we present a detailed seismological and geodetic analysis of the sequence, combining teleseismic back projection, a single-station template matching detection for foreshocks and aftershocks, a statistical analysis of the entire sequence, and the GNSS and InSAR data for aseismic deformation. We find that the mainshock ruptured predominantly to the southwest direction with a total source duration of ~220 s. The M7.4 foreshock sequence triggered afterslip along the plate interface, which likely drove an expanding aftershock of its own and eventually triggered the mainshock nucleation. The aftershock productivity of the M7.4 event is also several times higher than several M7+ events before and after the M8.8 mainshock, which increases the probability of triggering a larger mainshock. We also find possible segmentation of the megathrust rupture zone along the Kuril-Kamchatka subduction zone that is separated by higher topography in the upper plate. The integrated approach provides an improved understanding of the physical mechanisms of foreshocks and segmentation of mainshock ruptures along major subduction zones, which is directly relevant to short-term forecasting and seismic hazard assessment of megathrust earthquakes in this and other regions.