Seismic quiescence and activation prior to the 2025 M8.8 Kamchatka, Russia earthquake

Abstract

The 29 July 2025 Kamchatka earthquake, of magnitude M8.8 in the Kamchatka-Kuril subduction system, provides a unique opportunity to investigate the preparatory processes of a great subduction event. Despite Kamchatka's high seismic activity, the long-term evolution of seismicity preceding major ruptures has been poorly documented. Identifying temporal patterns-such as multiyear quiescence and short-term activation-is essential for understanding megathrust failure processes. We applied the Epidemic-Type Aftershock Sequence (ETAS) model and change-point analysis to earthquakes with M5 or larger within a 100-km radius of the 2025 mainshock epicenter, using the Advanced National Seismic System (ANSS) catalog spanning 1975-2025. This approach quantified temporal variations in the seismicity rate and detected statistically significant change points. We found a pronounced approximately 20-year quiescent interval beginning around mid-2003, followed by an abrupt activation that commenced with the M7.4 foreshock on 20 July 2025. Similar quiescence-to-activation sequences near the eventual hypocenter have been reported in previous studies for the 1997 M7.8 Kronotsky and 2006 M8.3 Simushirskoe earthquakes, which ruptured segments immediately north and south of the 2025 rupture, respectively. Together with these earlier studies, our findings suggest that heightened seismic activity following multiyear quiescence is a recurrent feature of the Kamchatka-Kuril subduction system hosting large megathrust earthquakes. Decadal-scale quiescence has also been reported off Hokkaido, Japan, overlapping the area assessed as having an elevated likelihood of a 17th-century-type M9-class earthquake. Given that Hokkaido is in the Kamchatka-Kuril subduction system, tracking a possible transition from quiescence to activation is important for situational awareness and time-dependent hazard assessment.