Seismic Swarm S20071219.2: Earthquake Activity near Adak, Alaska

Seismic swarm S20071219.2 occurred approximately 207 km west-southwest of Adak, Alaska, in the Andreanof Islands region of the Aleutian arc. The sequence began at 09:38 on 19 December 2007 and concluded at 03:10 on 24 December 2007, spanning 113 hours and 31 minutes. During this period, 64 earthquakes were recorded, with the largest events reaching magnitude 4.9.

The swarm exhibited a typical pattern of clustered seismicity, with initial events concentrated on 19 December. Magnitudes ranged from 2.1 to 4.9, while focal depths varied between 0 km and 59 km. Several events clustered near 30 km depth, including the two magnitude-4.9 shocks at 09:55 and 10:05 on 19 December. Subsequent activity showed a gradual decline in both frequency and magnitude, with isolated events persisting through 23 December before a final magnitude-2.6 quake on 24 December.

This swarm reflects the dynamic stress regime of 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. The resulting megathrust interface and overlying crustal faults generate frequent earthquake sequences, including swarms that lack a single dominant mainshock. Depths recorded in the swarm align with both intraslab and interface seismicity common to the central Aleutians.

The Andreanof Islands segment has a well-documented history of large earthquakes, including the magnitude-8.6 event of 1957 that ruptured a 1200 km section of the arc. Modern monitoring since 2000 has identified eight additional swarms in the broader region, occurring in 2005 (one swarm), 2006 (six swarms), and 2007 (one swarm). These episodes underscore the persistent, episodic nature of seismicity driven by ongoing plate convergence and fluid migration within the subduction system.

Analysis of swarm S20071219.2 provides insight into short-term seismic clustering along the arc. The concentration of events within the first 24 hours, followed by rapid decay, is consistent with swarm behavior observed in other subduction settings. Depths predominantly between 10 km and 40 km suggest activation of both shallow crustal structures and intermediate-depth slab seismicity.

References

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