Seismic Swarm PS20050614.1: Analysis of Activity Near Adak, Alaska

On 14 June 2005, a seismic swarm designated PS20050614.1 occurred approximately 284 km west-southwest of Adak, Alaska, within the tectonically active Aleutian Island arc. The sequence began at 08:03 UTC and concluded at 22:49 UTC, spanning 14 hours and 46 minutes. During this interval, seven earthquakes were recorded, with magnitudes ranging from 5.0 to 6.8 and focal depths between 17 km and 50 km.

The events unfolded as follows. The initial shock at 08:03:09 registered magnitude 5.2 at 50 km depth. A magnitude 5.0 event followed at 09:01:30 (40 km depth). The largest shock of the swarm, magnitude 6.8, struck at 17:10:12 from a shallow depth of 17 km. Subsequent activity included a magnitude 5.0 at 17:18:19 (47 km), magnitude 5.3 at 17:48:07 (26 km), and a final magnitude 5.6 at 22:49:17 (27 km). An additional magnitude 5.6 occurred at 11:49:02 (47 km depth). These clustered events reflect typical swarm behavior in subduction settings, where stress redistribution along the plate interface can trigger multiple moderate shocks without a single dominant mainshock-aftershock pattern.

The Aleutian Islands lie above the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic regime produces the region’s high seismicity and volcanism. The swarm epicenters fall within a segment of the arc known for both interplate thrust events and intraslab earthquakes. Historical records confirm recurrent large-magnitude activity; notable examples since 2000 include the magnitude 7.8 Rat Islands earthquake of 17 November 2003, centered 58 km from the 2005 swarm location, and the magnitude 7.2 event of 19 December 2007, located 80 km away.

Statistical context since 1 January 2000 indicates only two documented swarms in the immediate vicinity, with the earlier episode occurring in 2003. Such infrequent clustering underscores that while background seismicity remains elevated, discrete swarm episodes are relatively rare in this portion of the arc.

The 2005 swarm’s shallowest event (17 km) likely originated near the plate interface, whereas deeper shocks (up to 50 km) may reflect intraslab deformation. No surface rupture or tsunami generation was associated with these events, consistent with their moderate magnitudes and offshore locations.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center historical data
SeismoSight internal swarm classification PS20050614.1