Seismic Swarm S20220605.1 in the Rat Islands, Aleutian Islands

The Rat Islands region of the western Aleutian Islands, Alaska, experienced seismic swarm S20220605.1 between 13:22 on 4 June 2022 and 14:17 on 8 June 2022. In approximately 96 hours and 55 minutes, 74 earthquakes were recorded. This activity occurred along the Aleutian subduction zone, where the Pacific Plate subducts beneath the North American Plate at rates of 6–8 cm per year, generating frequent seismicity at intermediate depths within the downgoing slab.

The swarm events clustered predominantly at depths of 100–124 km, consistent with intraslab earthquakes in the subducting Pacific Plate. A smaller number of shallow events (depths of 3–13 km) occurred toward the later stages. Magnitudes ranged from 0.0 to 4.3, with the largest event (M4.3) recorded at 00:25 on 5 June at 110 km depth. The temporal distribution showed peak activity in the first 24 hours, followed by a gradual decline, characteristic of swarm behavior without a dominant mainshock-aftershock sequence.

Geologically, the Rat Islands form part of the Aleutian volcanic arc, shaped by long-term subduction since the Mesozoic. The region lies near the intersection of the Rat Fracture Zone and the trench, where slab geometry promotes clustered seismicity. Historical records indicate the area has hosted major earthquakes, including the 1965 M8.7 Rat Islands event, which produced extensive rupture along the megathrust. Intermediate-depth swarms like S20220605.1 reflect stresses within the cold, brittle core of the subducting slab, often linked to phase changes or dehydration reactions.

Since 2000, ten seismic swarms have been documented in the Rat Islands. Earlier episodes occurred in 2003 (two swarms), 2006 (one), 2014 (four), and 2021 (two). The 2022 swarm represents the most recent addition to this pattern, underscoring recurring episodic release of strain at depth. Such swarms contribute to the overall seismic hazard assessment for the Aleutian Islands, where intermediate-depth events can occasionally reach magnitudes capable of generating felt shaking across the arc.

Analysis of the event parameters reveals a narrow depth band for most events, suggesting localization along a specific structural horizon within the slab. The presence of both deeper and shallower outliers indicates minor vertical migration of activity, possibly influenced by fluid migration or stress transfer. No surface rupture or volcanic unrest was associated with this swarm, aligning with the tectonic rather than magmatic origin typical of Aleutian intermediate-depth sequences.

Ongoing monitoring by regional networks continues to track background seismicity in this highly active margin. Understanding swarm recurrence helps refine models of slab dynamics and improves probabilistic forecasts for larger events in the western Aleutians.

References

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