Seismic Swarm PS20250803.1: Analysis of Recent Activity Southeast of Severo-Kurilsk

A seismic swarm designated PS20250803.1 was recorded 183 km south-southeast of Severo-Kurilsk, Russia, in the Kuril Islands. The sequence began at 17:20 on 2 August 2025 and concluded at 19:47 on 3 August 2025, spanning 26 hours and 27 minutes. Nine earthquakes were detected during this period.

The events exhibited magnitudes ranging from 4.3 to 6.8, with focal depths between 10 km and 55 km. The largest shock reached magnitude 6.8 at a depth of 30 km. Multiple events clustered at shallow depths of 10 km, indicating activity within the upper crust. The sequence included two magnitude-5.0 events early on, followed by the peak magnitude and several aftershocks near magnitude 5.0–5.2.

This swarm occurred within the Kuril-Kamchatka subduction zone, where the Pacific Plate subducts northwestward beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. The region forms part of the Pacific Ring of Fire and hosts frequent moderate-to-large earthquakes due to plate interface slip, intraslab deformation, and volcanic arc processes. Severo-Kurilsk lies on Paramushir Island, near active volcanoes such as Ebeko and Chikurachki, where tectonic stress accumulation regularly produces seismic swarms.

Geological records show the Kuril arc has experienced repeated great earthquakes, including events exceeding magnitude 8 in the 20th century. The 1952 magnitude-9.0 event off the southern Kurils generated a major tsunami. Modern monitoring reveals persistent background seismicity, with the area classified as one of the highest seismic hazard zones globally.

Since 1 January 2000, four seismic swarms have been documented in the broader region. This swarm represents the first recorded in 2025. Such sequences typically reflect fluid migration or stress triggering along the subduction interface rather than a single mainshock-aftershock pattern.

The varying depths observed—ranging from shallow crustal events at 10 km to deeper activity at 55 km—align with the geometry of the subducting slab. Shallower events may relate to upper-plate fracturing or volcanic influence, while deeper shocks indicate intraslab processes.

SeismoSight classifies this sequence internally as swarm PS20250803.1 based on its temporal clustering and lack of a dominant mainshock.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Global CMT Catalog (globalcmt.org)
• Russian Academy of Sciences Far East Geological Institute reports
• International Seismological Centre (isc.ac.uk)