Seismic Swarm Analysis: Rat Islands, Aleutian Islands, December 2022

The Rat Islands region of the western Aleutian Islands, Alaska, lies within a highly active subduction zone where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent intermediate-depth earthquakes within the Wadati-Benioff zone, typically between 50 and 150 km depth, as well as shallow crustal events and volcanic activity associated with the Aleutian arc. The December 2022 swarm occurred in this environment, highlighting the ongoing stress accumulation and release along the subducting slab.

SeismoSight recorded swarm S20221214.1 beginning at 18:40 on 14 December 2022 and concluding at 01:55 on 17 December 2022. Over 55 hours and 14 minutes, the network detected 63 earthquakes. The sequence initiated with a magnitude 6.3 event at 73 km depth, followed by a rapid succession of aftershocks and smaller events clustered between 62 and 82 km depth. Magnitudes ranged from 2.0 to 6.3, with the majority falling between 2.0 and 3.5. Depths remained consistently in the intermediate range, consistent with intraslab seismicity rather than interface rupture.

The temporal distribution showed highest activity in the first 12 hours, with 20 events recorded on 14 December alone. Activity then declined gradually through 15 and 16 December before tapering to a final event on 17 December. This pattern is characteristic of swarm behavior, where events occur without a single dominant mainshock-aftershock sequence, often driven by fluid migration or localized stress transfer within the slab.

Historical data indicate that such swarms are recurrent in the Rat Islands. Since 1 January 2000, six swarms have been documented: two in 2003, one in 2005, two in 2014, and one in 2020. These episodes underscore the region’s persistent seismic productivity, which has also included great earthquakes such as the 1965 M8.7 Rat Islands event. Intermediate-depth swarms like the 2022 sequence contribute to understanding slab dehydration and phase transitions that influence rupture dynamics at greater depths.

Overall, the 2022 swarm reinforces the Aleutian subduction zone’s role as one of Earth’s most seismically active margins. Continued monitoring remains essential for refining hazard assessments in this remote but tectonically significant area.

References

USGS Earthquake Catalog
Alaska Earthquake Center
SeismoSight internal swarm classification database