Seismic Swarm PS20140623.2: Rat Islands, Aleutian Islands, Alaska

The Rat Islands region forms part of the western Aleutian arc, where the Pacific Plate subducts beneath the North American Plate at rates exceeding 6 cm per year. This convergent margin produces intense seismicity across a wide depth range, with intermediate-depth events commonly occurring within the subducting slab between 70 and 150 km. The June 2014 swarm occurred in this setting, centered near the location of the magnitude 7.9 mainshock.

Swarm PS20140623.2 began at 20:53 UTC on 23 June 2014 and concluded at 00:03 UTC on 25 June 2014, spanning 27 hours and 9 minutes. During this interval, 11 earthquakes were recorded. The sequence initiated with the magnitude 7.9 event at 109 km depth, followed by a series of aftershocks whose magnitudes ranged from 5.0 to 6.0 and depths remained between 100 and 122 km. All events clustered tightly in both space and time, consistent with a localized release of stress within the downgoing slab.

The temporal progression showed the largest event at the outset, succeeded by six events exceeding magnitude 5.0 within the first nine hours. Subsequent activity included isolated events on 24 June at depths near 100–121 km and a final magnitude 5.2 shock at 122 km depth that marked the swarm’s termination. This pattern reflects rapid aftershock triggering at intermediate depths rather than a prolonged mainshock-aftershock cascade.

Since 1 January 2000, twelve seismic swarms have been documented in the Rat Islands area. These occurred in 2003 (two swarms), 2005 (one), 2006 (seven), 2007 (one), and 2012 (one). The 2014 swarm aligns with this recurrent behavior, indicating episodic clusters of intermediate-depth seismicity superimposed on the region’s background rate.

Three strong earthquakes have occurred near the swarm center since 2000. The 23 June 2014 magnitude 7.9 event lies 20 km from the swarm centroid. Earlier events include a magnitude 7.8 shock on 17 November 2003 located 85 km away and a magnitude 7.1 event on 17 March 2003 situated 77 km distant. These large intraslab ruptures underscore the capacity of the subducting Pacific slab to generate significant seismicity at depths exceeding 100 km.

The combination of frequent swarms and occasional magnitude 7+ events highlights the Rat Islands segment as one of the most seismically active portions of the Aleutian subduction zone. Continued monitoring of intermediate-depth clusters provides valuable insight into slab stresses and rupture processes in this tectonically dynamic environment.

References
USGS Earthquake Catalog
Alaska Earthquake Center
SeismoSight internal swarm classification PS20140623.2