Seismic Swarm PS20130831.1: Analysis of Activity Near Atka, Alaska

The seismic swarm designated PS20130831.1 occurred in the central Aleutian Islands, approximately 99 km south-southwest of Atka, Alaska. Registered over a 15-hour, 30-minute period from 06:43 to 22:14 UTC on 31 August 2013, the swarm comprised five earthquakes. This event cluster unfolded within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year.

The sequence began with a magnitude 5.5 event at 06:43:54 UTC at a depth of 25 km. Subsequent shocks included a magnitude 5.0 at 07:04:05 UTC (15 km depth), a magnitude 5.1 at 10:06:19 UTC (60 km depth), another magnitude 5.0 at 19:01:46 UTC (100 km depth), and a final magnitude 2.3 at 22:14:23 UTC (13 km depth). Depths ranged across crustal and upper mantle levels, consistent with typical subduction-related seismicity in the region.

This swarm followed closely after a magnitude 7.0 earthquake on 30 August 2013, located 101 km southwest of Atka and roughly 19 km from the swarm centroid. Such temporal proximity suggests possible triggering or stress redistribution along the subduction interface. The Aleutian arc has long been recognized for its elevated seismic hazard, with frequent moderate-to-large events driven by megathrust processes.

Historical records since 2000 indicate 12 swarms in the vicinity of Atka. These occurred in 2002 (one swarm), 2008 (four swarms), 2010 (five swarms), and 2013 (two swarms, including the present event). Swarm activity reflects episodic strain release along the plate boundary rather than isolated mainshock-aftershock sequences.

Geologically, the central Aleutians feature volcanic islands built atop the overriding plate, with Atka Volcano itself representing an active stratovolcano. The subduction zone here accommodates oblique convergence, producing both interface thrust earthquakes and intraslab events at varying depths. Updated regional assessments confirm ongoing seismic productivity, with monitoring networks providing refined hypocentral solutions for events in this remote area.

The 2013 swarm underscores the persistent seismic character of the central Aleutians, where clusters of moderate-magnitude events can occur without escalation to larger ruptures. Continued observation supports improved understanding of stress migration along this portion of the Pacific Ring of Fire.

References

USGS Earthquake Catalog
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification data