Seismic Swarm S20080222.1: Activity Near Adak, Alaska

A seismic swarm designated S20080222.1 occurred approximately 98 km south-southwest of Adak, Alaska, in the Aleutian Islands. The sequence began at 23:51 on 21 February 2008 and concluded at 19:06 on 22 February 2008, spanning 19 hours and 14 minutes. During this interval, 29 earthquakes were recorded.

The events exhibited magnitudes ranging from 1.7 to 5.4, with the largest shock measuring 5.4 at a depth of 15 km. Subsequent notable events included a magnitude 4.7 earthquake at 40 km depth and a magnitude 4.4 event at 47 km depth. Most events clustered between 15 km and 30 km depth, consistent with activity along the subduction interface.

The Aleutian Islands lie within an active subduction zone where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. Adak and surrounding islands experience elevated seismic hazard due to their position along the arc.

Historical records document major earthquakes in the region, including the 1957 magnitude 8.6 Andreanof Islands event and the 1965 magnitude 8.7 Rat Islands earthquake. These great events highlight the potential for large-magnitude ruptures along the Aleutian megathrust. Modern monitoring by the Alaska Earthquake Center and USGS confirms ongoing background seismicity in the area, with swarms occasionally occurring in response to stress transfer or fluid migration.

Analysis of the 2008 swarm shows a rapid onset followed by a gradual decline in event rate. The distribution of magnitudes follows a typical Gutenberg-Richter relationship for the region, with the majority of events below magnitude 3.0. Depths indicate activity primarily within the overriding plate and near the plate interface.

Such swarms contribute to understanding local stress regimes and may precede or accompany volcanic unrest, although no associated volcanic activity was reported in 2008. Continued instrumentation in the Aleutians improves detection and characterization of these sequences.

References

USGS Earthquake Hazards Program
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification S20080222.1