Seismic Swarm Activity in the Rat Islands, Aleutian Islands

Seismic swarm S20230609.1 occurred in the Rat Islands region of the Aleutian Islands, Alaska, from 20:33 UTC on 8 June 2023 to 02:14 UTC on 11 June 2023. Over 53 hours and 40 minutes, the swarm comprised 38 earthquakes, with the majority recorded at depths of approximately 10 km. The largest events reached magnitudes of 4.5 and 4.2 early in the sequence, followed by numerous smaller shocks between magnitudes 2.1 and 4.0.

The swarm exhibited a typical pattern of clustered seismicity without a dominant mainshock, beginning with higher-magnitude events and transitioning to lower-magnitude activity. Events were distributed across the first two days, with reduced frequency toward the conclusion on 11 June. Depths remained consistent near 10 km for most shocks, indicating activity within the upper crust or shallow subduction interface.

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 great megathrust events, as well as volcanic activity along the arc. The region has a well-documented history of seismic swarms, with five such episodes recorded since 2000, occurring in 2003 (two swarms), 2006 (two swarms), and 2014 (one swarm).

Such swarms are common in subduction environments and may reflect fluid migration, stress transfer along the plate interface, or minor slip on subsidiary faults. The Rat Islands segment has hosted notable historical earthquakes, underscoring its potential for both swarm activity and larger events.

References

• USGS Earthquake Hazards Program: Aleutian Islands tectonic overview
• Alaska Earthquake Center regional seismicity reports