Seismic Swarm Analysis: Komandorskiye Ostrova Region, December 2018

A notable seismic swarm occurred in the Komandorskiye Ostrova region of Russia between 17:01 on 20 December 2018 and 03:12 on 23 December 2018. Over 58 hours and 10 minutes, the sequence included 11 earthquakes, beginning with a magnitude 7.3 mainshock at a depth of 16 km. Subsequent events ranged from magnitude 5.0 to 5.9, predominantly at depths of 10 km, with one event at 18 km. This activity highlights the persistent tectonic stresses in the western Aleutian arc.

The Commander Islands occupy a critical position at the junction of the Pacific Plate and the Bering Plate. Subduction along the Aleutian Trench drives frequent seismic events, with the region experiencing both thrust and strike-slip faulting. The swarm's shallow focal depths align with typical rupture zones in this convergent margin setting.

Geological records indicate that the islands formed through volcanic and tectonic processes associated with plate convergence over millions of years. The area's history features recurrent large-magnitude earthquakes, reflecting ongoing plate motion rates of several centimeters per year. Updated monitoring data confirm elevated background seismicity in the broader Kamchatka-Komandorsky segment.

The sequence unfolded rapidly after the initial magnitude 7.3 shock at 20 Dec 2018 17:01:55. A magnitude 5.9 event followed within minutes at 17:09:04, succeeded by magnitudes 5.4, 5.6, and another 5.6 through 17:35:10. Additional events included a magnitude 5.1 at 18:30:59 on 20 December, then magnitudes 5.0 and 5.5 on 21 December. On 22 December, two events of 5.6 and 5.3 occurred in close succession, concluding with a magnitude 5.0 at 03:12:48 on 23 December. These clustered occurrences represent a classic swarm pattern without a single dominant aftershock decay.

A strong earthquake of magnitude 7.3 was recorded since 2000, located 187 km southeast of Ust’-Kamchatsk Staryy and approximately 6 km from the swarm center. Such proximity underscores the interconnected nature of fault systems in the region.

This swarm provides insight into stress transfer along the plate boundary, where moderate events can cluster following a larger rupture. The data support models of episodic seismic release in subduction zones.

References:
SeismoSight internal swarm classification PS20181220.1
USGS Earthquake Catalog (regional tectonics summaries)
Global Seismographic Network reports on Aleutian arc activity