Seismic Swarm VS20220126.1: Rat Islands, Aleutian Islands, Alaska

The Rat Islands form part of the western Aleutian archipelago in Alaska, situated along the convergent boundary where the Pacific plate subducts beneath the North American plate. This tectonically active region experiences frequent seismicity due to the oblique subduction and associated crustal deformation. The Aleutian subduction zone has produced some of the largest earthquakes recorded in North America, including the magnitude 8.7 event of 1965 centered near the Rat Islands.

Between 07:09 UTC on 25 January 2022 and 16:33 UTC on 28 January 2022, a swarm comprising 91 earthquakes was recorded in this area. The sequence lasted 81 hours and 23 minutes. Events ranged in magnitude from 1.1 to 4.9, with the majority occurring at shallow depths between 0 and 14 km. Notable events included a magnitude 4.9 earthquake at 12 km depth on 26 January at 01:02 UTC and a magnitude 4.8 event at 10 km depth later that day at 04:31 UTC. Depths remained predominantly crustal, consistent with activity along the subduction interface and overlying plate.

Swarm sequences such as this often reflect localized stress adjustments rather than mainshock-aftershock patterns. In subduction settings, they may arise from fluid migration along faults, aseismic slip, or minor changes in the regional stress field. The 2022 swarm exhibited a rapid onset followed by sustained low-to-moderate magnitude activity, with no single dominant mainshock. Most events clustered within the first 48 hours, after which rates gradually declined until termination.

Historical records indicate nine comparable swarms in the Rat Islands since 1 January 2000. These occurred in 2003 (two swarms), 2006 (one swarm), 2014 (four swarms), and 2021 (two swarms). The recurrence suggests episodic clustering of seismic energy release along this segment of the arc, potentially linked to the heterogeneous locking of the subduction megathrust.

Overall, the January 2022 swarm represents a typical expression of background tectonic processes in one of Earth’s most seismically productive regions. Continued monitoring of such sequences contributes to improved understanding of precursory patterns and long-term hazard assessment along the Aleutian chain.

References SeismoSight internal swarm classification VS20220126.1 USGS Earthquake Hazards Program – Aleutian Arc tectonics summary