Seismic Swarm Near Unalaska, Alaska: January 2022 Analysis

A seismic swarm occurred 74 km south of Unalaska in the Aleutian Islands of Alaska from 05:17 UTC on 22 January 2022 to 04:13 UTC on 26 January 2022. Over 94 hours and 56 minutes, 78 earthquakes were recorded. The sequence began with a magnitude 6.2 event at 29 km depth, followed by numerous smaller events, the largest aftershock reaching magnitude 4.7 at 43 km depth.

The Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate, drives intense seismic activity in this region. Unalaska lies within the central Aleutians, an area characterized by frequent moderate-to-large earthquakes and active volcanism. The local geology features a complex arrangement of volcanic arcs, deep ocean trenches, and fault systems resulting from oblique subduction at rates exceeding 6 cm per year.

Historical records indicate only two swarms in the area since 2000, occurring in 2009 and 2022. Such swarms typically reflect fluid migration or stress adjustments along the plate interface rather than foreshock-mainshock-aftershock sequences. Depths in the 2022 swarm ranged primarily between 15 km and 48 km, consistent with the seismogenic zone of the subduction interface and overlying crust.

The initial magnitude 6.2 earthquake occurred at 05:17 on 22 January, with subsequent activity showing a rapid decline in magnitude and frequency after the first 24 hours. Events clustered at depths around 25–35 km, aligning with the expected location of the plate boundary. No damage or tsunami was reported, reflecting the offshore location and moderate energy release.

This swarm provides insight into episodic slip processes along the Aleutian megathrust. Similar patterns have been observed elsewhere in subduction zones, where swarms accommodate strain without producing great earthquakes. Continued monitoring by regional seismic networks remains essential for understanding long-term hazard in the Aleutians, one of the most seismically active regions on Earth.

References

USGS Earthquake Catalog Alaska Earthquake Center reports SeismoSight internal swarm classification data