Earthquake Swarm S20120815.1 in the Rat Islands, Aleutian Islands

An earthquake swarm designated S20120815.1 was recorded in the Rat Islands region of the western Aleutian Islands, Alaska. The sequence began at 15:20 UTC on 15 August 2012 and concluded at 22:53 UTC on 16 August 2012, spanning 31 hours and 33 minutes. During this interval, 86 earthquakes were registered. The Rat Islands lie along the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate. This convergent margin drives the region’s high seismicity and volcanism. Subduction occurs at rates of 6–8 cm per year, generating frequent shallow to intermediate-depth earthquakes. Focal depths of events in the swarm ranged primarily between 5 km and 22 km, consistent with activity within the overriding plate and near the plate interface. The swarm’s largest event reached magnitude 4.7 at 20:41 UTC on 15 August, with an additional event of magnitude 3.8 recorded later that evening. Most events fell between magnitude 1.5 and 3.0, exhibiting a typical swarm pattern of clustered activity without a single dominant mainshock. Depths remained relatively stable, with the majority occurring between 15 km and 20 km. The Aleutian arc has a well-documented history of both great earthquakes and episodic swarm activity. Notable historical events include the 1965 magnitude-8.7 Rat Islands earthquake and earlier large shocks in 1899 and 1902. Since 2000, eight swarms have been identified in the Rat Islands area according to SeismoSight internal classification records: two in 2003, two in 2005, three in 2006, and one in 2007. These sequences illustrate the region’s tendency toward episodic, spatially concentrated seismicity superimposed on the background rate of plate-boundary earthquakes. Analysis of the 2012 swarm shows a rapid onset followed by a gradual decline in event rate over the 31-hour duration. Magnitudes displayed a broad distribution without clear aftershock decay, supporting classification as a swarm rather than a classic mainshock-aftershock sequence. Depths clustered around 17–19 km for the majority of events, suggesting a consistent source volume within the crust. Seismic swarms in subduction zones can arise from fluid migration, slow slip, or stress transfer along the plate interface. The 2012 Rat Islands swarm fits this tectonic context, occurring within a segment of the arc known for both moderate and great earthquakes. Continued monitoring remains essential given the potential for larger events along this highly active margin.

References

USGS Earthquake Hazards Program – Aleutian Islands tectonics and historical seismicity
Alaska Earthquake Center – Regional seismic monitoring summaries
SeismoSight internal swarm classification database (S20120815.1 parameters)