Seismic Swarm S20140624.4: Rat Islands, Aleutian Arc

The Rat Islands region of the western Aleutian Islands, Alaska, lies within one of Earth’s most active subduction zones. Here the Pacific Plate converges with and descends beneath the North American Plate at rates exceeding 6 cm per year, generating the Aleutian volcanic arc and frequent seismicity. The tectonic setting produces both shallow thrust events along the megathrust and deeper intraslab earthquakes within the subducting slab, with hypocenters commonly between 70 and 130 km depth.

Seismic swarm S20140624.4 began at 22:14 UTC on 23 June 2014 and continued until 06:22 UTC on 30 June 2014, spanning 152 hours and 7 minutes. During this interval, 79 earthquakes were recorded. Magnitudes ranged from 1.3 to 4.1, with the majority falling between 1.5 and 2.5. Focal depths clustered between 74 and 106 km for most events, although a few late-stage events occurred at shallower levels near 11–27 km. The sequence exhibited a typical swarm pattern: an initial burst of activity followed by fluctuating but persistent low-to-moderate magnitude events without a single dominant mainshock.

The swarm coincided with a major regional earthquake—an M 7.9 event on 23 June 2014 located only 8 km from the swarm centroid. This megathrust rupture likely altered local stress conditions, triggering the subsequent swarm through dynamic or static stress transfer within the subducting slab.

Historical records indicate that seven swarms have occurred in the Rat Islands since 1 January 2000. Earlier swarms took place in 2003 (two events), 2005 (one), 2006 (one), and three additional swarms in 2014, underscoring the region’s recurrent clustered seismicity. Such swarms are characteristic of the Aleutian subduction environment, where fluid migration and slab dehydration can promote episodic earthquake sequences.

The 2014 swarm illustrates the interplay between large-magnitude plate-boundary ruptures and ensuing intraslab activity. Continued monitoring of the Aleutian arc remains essential for understanding stress evolution along this highly active margin.

References

SeismoSight internal swarm catalog S20140624.4
USGS Earthquake Hazards Program regional tectonics summary (Aleutian arc)
Alaska Earthquake Center annual seismicity reports (2014)