Seismic Swarm PS20051024.1 in the Kuril Islands

The Kuril Islands form part of a volcanic island arc along the Pacific Ring of Fire, where the Pacific Plate subducts beneath the Okhotsk Plate at rates exceeding 8 cm per year. This tectonic setting produces frequent earthquakes at varying depths, from shallow crustal events to deeper Wadati-Benioff zone activity. The region has long been recognized for its high seismicity, with historical records documenting destructive events that have shaped both the landscape and monitoring efforts.

Between 20:29 UTC on 23 October 2005 and 13:09 UTC on 24 October 2005, a seismic swarm designated PS20051024.1 was recorded in the Kuril Islands. Over 16 hours and 40 minutes, five earthquakes occurred. The sequence began with a magnitude 5.3 event at 10 km depth, followed 7 seconds later by a magnitude 5.1 shock at 40 km. Roughly 69 minutes afterward, a magnitude 5.4 event struck at 10 km depth, succeeded within seconds by a magnitude 5.0 quake at 37 km. The swarm concluded with a magnitude 3.9 event at 35 km depth on 24 October.

Such swarms represent clusters of seismicity without a dominant mainshock-aftershock pattern, often linked to fluid migration or stress adjustments along the subduction interface. Depths ranging from 10 km to 40 km align with the transition from shallow megathrust segments to intermediate slab depths in this arc.

Since 1 January 2000, only one prior swarm had been documented in the area, occurring in 2003. The 2005 swarm preceded two major earthquakes in close proximity: the magnitude 8.3 event of 15 November 2006, centered 47 km away, and the magnitude 8.1 event of 13 January 2007, located 56 km distant. These larger ruptures highlight the ongoing accumulation and release of strain along the Kuril-Kamchatka trench.

The Kuril subduction zone remains one of the most active seismic regions globally, with potential for both intraslab and interface events capable of generating tsunamis. Continued monitoring supports improved understanding of precursory swarm behavior in this tectonic environment.

References

USGS Earthquake Catalog
SeismoSight internal swarm classification records