Seismic Swarm Activity in the Rat Islands, Aleutian Islands, Alaska

A seismic swarm designated S20210622.3 was recorded in the Rat Islands region of the Aleutian Islands, Alaska, beginning at 07:22 UTC on 22 June 2021 and concluding at 10:18 UTC on 23 June 2021. Over this 26-hour, 55-minute period, 48 earthquakes were detected. Magnitudes ranged from 0.0 to 2.1, with the majority occurring at shallow depths between 2 and 6 km. The sequence featured an initial cluster of events on 22 June, including several magnitude 2.0–2.1 shocks within the first hour, followed by a gradual decline in frequency and intensity through the following day.

The Rat Islands lie within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This setting produces frequent earthquakes, including both shallow crustal events and deeper Wadati-Benioff zone seismicity. The region’s volcanic arc geology, formed by this subduction, contributes to elevated seismic hazards, with historical records documenting great earthquakes exceeding magnitude 8.0.

Since 2000, seven swarms have been documented in the area, occurring in 2003 (two events), 2006 (one event), and 2014 (four events). These episodes typically involve low-magnitude earthquakes clustered over hours to days, consistent with fluid migration or stress transfer along the plate interface rather than a single mainshock-aftershock sequence.

Analysis of the 2021 swarm indicates predominantly shallow focal depths, suggesting activity within the overriding plate or upper portions of the subduction interface. Event rates peaked early, with 22 earthquakes recorded in the first four hours, then tapered to isolated smaller events by 23 June. Such patterns align with known swarm behavior in subduction zones, where episodic tremor and slip can occur without producing damaging ground motion.

The Aleutian Islands remain one of the most seismically active regions on Earth, monitored continuously by regional and global networks to improve understanding of subduction dynamics and hazard assessment.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
Global CMT Catalog (globalcmt.org)