Seismic Swarm Analysis: Adak, Alaska Region, December 2007

The Aleutian Islands form part of a highly active subduction zone where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent earthquakes, volcanic activity, and seismic swarms along the arc. Adak, located on Adak Island in the central Aleutians, lies near the Andreanof Islands segment, a region with a well-documented history of large-magnitude events due to the shallow subduction interface.

Seismic swarm PS20071219.1 was recorded 211 km west-southwest of Adak. The sequence began at 06:47 on 19 December 2007 and concluded at 02:18 on 20 December 2007, spanning 19 hours and 30 minutes. During this interval, 12 earthquakes were registered. The events clustered at depths ranging from 6 km to 60 km, consistent with activity along the subduction interface and within the overlying crust.

The sequence included one mainshock of magnitude 7.2 at 09:30:27 on 19 December at 34 km depth. This event was followed within minutes by several aftershocks exceeding magnitude 5.0. Subsequent events maintained magnitudes between 5.0 and 5.6, with the final recorded earthquake of magnitude 5.0 occurring at 02:18:20 on 20 December at 14 km depth. Depths remained predominantly in the 12–60 km range, indicating slip distributed across the megathrust and adjacent crustal volumes.

Historical data for the Adak region since 1 January 2000 show 12 prior swarms. These occurred in 2003 (1 swarm), 2005 (3 swarms), 2006 (7 swarms), and 2007 (1 swarm). The December 2007 swarm aligns with this episodic pattern, which reflects periodic stress accumulation and release along the subduction zone. A magnitude 7.2 event on 19 December 2007, located 207 km west-southwest of Adak and 36 km from the swarm centroid, represents the strongest earthquake recorded in the immediate area since 2000.

Such swarms in subduction zones often precede or accompany larger ruptures by redistributing stress along the plate interface. The 2007 sequence’s timing and magnitude distribution provide a snapshot of short-term seismic behavior in an arc segment capable of generating great earthquakes. Ongoing monitoring by regional networks continues to track similar activity to refine hazard assessments for the central Aleutians.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
NOAA National Centers for Environmental Information (ncei.noaa.gov)