Seismic Swarm in the Rat Islands, Aleutian Islands: The 2014 Event and Regional Context

The Rat Islands form part of the western Aleutian arc in Alaska, situated above the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate. This subduction drives the region's intense seismicity, with the Pacific Plate descending at rates of 6–7 cm per year. Intermediate-depth earthquakes commonly occur within the Wadati-Benioff zone at depths of 70–150 km, reflecting dehydration and phase changes in the downgoing slab.

Swarm S20140623.2 began at 21:03 UTC on 23 June 2014 and concluded at 03:40 UTC on 20 July 2014, lasting 630 hours and 37 minutes. During this period, 992 earthquakes were recorded in the Rat Islands area. Analysis of the first 100 events reveals a rapid onset of activity, with the largest magnitude reaching 4.9 within the initial hours. Depths ranged primarily from 81 km to 135 km, clustering around 95–120 km, consistent with intraslab seismicity rather than shallow crustal faulting. Magnitudes varied from 1.9 to 4.9, showing a typical swarm pattern of numerous smaller events punctuated by occasional larger shocks, without a single dominant mainshock.

This swarm represents one of only five documented seismic swarms in the region since 2000. Earlier episodes occurred in 2003 (two swarms), 2005 (one swarm), and 2006 (one swarm). Such swarms remain infrequent relative to the steady background of subduction-related earthquakes, suggesting episodic triggering mechanisms possibly linked to fluid migration or stress transients within the slab.

The Aleutian arc's tectonic setting has produced major historical earthquakes, including the 1965 Rat Islands event of magnitude 8.7, which generated a significant tsunami. Ongoing monitoring highlights the area's persistent potential for both swarm activity and larger megathrust ruptures.

References
USGS Earthquake Catalog
Alaska Earthquake Center regional reports
Subduction zone studies of the Aleutian arc