Seismic Swarm S20130830.1: Analysis of Seismicity Near Adak, Alaska

The seismic swarm designated S20130830.1 occurred approximately 105 km east-southeast of Adak in the Aleutian Islands, Alaska. It initiated at 16:25 UTC on 30 August 2013 and concluded at 05:38 UTC on 3 September 2013, spanning 85 hours and 13 minutes. During this interval, 180 earthquakes were recorded. The swarm began with a magnitude 7.0 event at 29 km depth, followed by a rapid sequence of aftershocks that included multiple events exceeding magnitude 4.0 within the first hours.

Adak lies along the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent moderate to great earthquakes and volcanic activity throughout the island arc. The Andreanof Islands segment, which includes the swarm epicentral area, has a well-documented history of large ruptures, including the 1957 magnitude 8.6 Andreanof Islands earthquake and subsequent events in 1986 and 1996. Crustal structure in the region features a well-defined Wadati-Benioff zone extending to depths exceeding 200 km, with many events occurring between 15 and 60 km depth.

Analysis of the first 100 events reveals a typical swarm decay pattern superimposed on the mainshock-aftershock sequence. Depths predominantly clustered between 13 and 25 km, with a secondary concentration near 60–80 km for several larger events recorded on 31 August. Magnitudes decreased rapidly after the initial 7.0 shock, with the largest subsequent events reaching 5.9, 5.5, and 5.4. The sequence contained 12 events of magnitude 4.0 or greater within the first 48 hours. Shallow events (less than 20 km) were more numerous in the initial 12 hours, while deeper activity (25–35 km) became relatively more prominent later in the swarm.

Historical records indicate limited swarm activity in the immediate vicinity since 2000. Only two prior swarms have been identified in the Adak region: one in 2008 and another in 2010. This low frequency suggests that the 2013 swarm represents a relatively uncommon clustered release of strain along a segment of the subduction interface or within the overriding plate.

The combination of a magnitude 7.0 mainshock and the subsequent prolonged aftershock sequence highlights the elevated seismic hazard along the central Aleutian arc. Continued monitoring by regional networks remains essential for understanding stress transfer and the potential for future large events in this tectonically active margin.

References
SeismoSight internal swarm classification S20130830.1
USGS Earthquake Catalog
Alaska Earthquake Center regional seismicity reports