Earthquake Swarm S20100804.1: Seismic Activity West of Adak, Alaska

An earthquake swarm designated S20100804.1 was recorded 147 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 13:03 on 4 August 2010 and concluded at 12:39 on 7 August 2010, spanning 71 hours and 36 minutes. During this period, 60 earthquakes were registered, with magnitudes ranging from 1.9 to 3.7 and focal depths predominantly between 25 and 35 km.

The swarm initiated with a 3.2-magnitude event at 35 km depth, followed rapidly by additional shocks. Peak activity occurred within the first several hours, including a 3.7-magnitude earthquake at 29 km depth and multiple events exceeding magnitude 3.0. Subsequent days showed a gradual decline in both frequency and intensity, with the final recorded event measuring 2.9 at 26 km depth. Depths remained relatively consistent throughout, clustering in the mid-crustal range typical of subduction-related seismicity.

This swarm represents one of five documented swarms in the region since 2000. Earlier episodes occurred in 2005 (one swarm), 2006 (two swarms), and 2007 (two swarms). Such recurrent patterns indicate episodic stress release along the plate interface without progression to a mainshock-aftershock sequence.

The location lies within 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 shallow to intermediate-depth earthquakes along the megathrust and within the overriding and subducting plates. The central Aleutians, including the area near Adak, host both volcanic and seismic hazards associated with the arc’s active volcanoes and fault systems.

Seismic swarms in this environment often reflect fluid migration, aseismic slip, or localized stress perturbations rather than immediate precursors to larger events. Depths observed in the 2010 swarm align with the seismogenic zone of the subduction interface, where brittle failure occurs before transition to ductile behavior at greater depths.

References

USGS Earthquake Catalog Alaska Earthquake Center NOAA National Centers for Environmental Information