Earthquake Swarm S20060701.1: Seismic Activity Near Adak, Alaska

An earthquake swarm designated S20060701.1 was recorded 206 km west-southwest of Adak, Alaska, in the Aleutian Islands. The sequence began at 19:34 UTC on 1 July 2006 and concluded at 17:49 UTC on 4 July 2006, spanning 70 hours and 14 minutes. During this interval, 170 earthquakes were detected.

The first 100 events displayed magnitudes from 1.4 to 5.6 and focal depths predominantly between 4 km and 16 km, with occasional deeper occurrences reaching 54 km. Notable events included a magnitude 5.5 earthquake at 41 km depth at the swarm onset, followed by a magnitude 5.6 event at 49 km depth on 2 July. Several magnitude 4.5 events clustered within the initial hours, while the majority remained below magnitude 3.0. Depths showed a concentration in the upper crust, consistent with shallow tectonic adjustments along the subduction interface.

The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–8 cm per year. This tectonic setting produces frequent seismicity, including both mainshock-aftershock sequences and swarms. The region hosts the Aleutian volcanic arc, with active volcanoes such as those near Adak contributing to the broader geophysical environment. Historical records indicate elevated seismic productivity, with documented swarms reflecting fluid migration or stress transfer within the overriding plate and along the megathrust.

Since 1 January 2000, only one prior swarm had been identified in the immediate vicinity, occurring in 2005. The 2006 sequence therefore represents a relatively infrequent clustering pattern for the area. Such swarms typically lack a dominant mainshock and instead feature distributed energy release over days, distinguishing them from classic aftershock sequences.

Analysis of the initial 100 events reveals a rapid onset with multiple events above magnitude 4.0 within the first 24 hours, followed by a gradual decline in both rate and maximum magnitude. Shallow events (less than 10 km) predominated after the first day, suggesting progressive adjustment at shallower structural levels. No surface rupture or significant ground deformation was associated with the swarm.

This activity underscores the ongoing seismic hazard in the western Aleutians, where subduction-driven processes continue to generate both isolated events and episodic swarms. Monitoring networks remain essential for tracking such sequences and assessing potential escalation.

References

SeismoSight internal swarm classification records
USGS Earthquake Hazards Program regional tectonics overview
Alaska Earthquake Center historical seismicity summaries