Seismic Swarm S20071222.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20071222.1 occurred in a tectonically active segment of the Aleutian Islands, approximately 182 km west-southwest of Adak, Alaska. The sequence began at 03:16 on 21 December 2007 and concluded at 03:52 on 23 December 2007, encompassing 31 earthquakes over 48 hours and 35 minutes. This event exemplifies typical swarm behavior in subduction zones, where clustered seismicity arises without a dominant mainshock.

The Aleutian region forms part of the Pacific Ring of Fire, driven by the oblique subduction of the Pacific Plate beneath the North American Plate at rates of about 6–8 cm per year. This convergence produces the Aleutian volcanic arc and generates frequent earthquakes across a wide range of depths. Adak itself sits near the Andreanof Islands segment, where historical records document recurrent moderate-to-large events linked to the plate interface and overlying crustal faults.

Data from the swarm reveal magnitudes ranging from 1.9 to 4.1, with the largest event (magnitude 4.1) occurring at 08:28 on 21 December at a depth of 21 km. Depths throughout the sequence varied between 9 km and 34 km, indicating activity both within the upper crust and near the subduction interface. Earlier events on 21 December showed a concentration around 10–25 km depth, while later activity on 22–23 December included slightly shallower and deeper foci. Such depth distributions align with the complex faulting patterns in the region, where intraslab and interface events commonly coexist.

Historical context underscores the recurrence of swarms in this area. Since 1 January 2000, four swarms have been documented, with three occurring in 2006 and one prior instance noted in 2007. These episodes reflect episodic stress release along the convergent margin, often influenced by fluid migration or aseismic slip transients that accompany subduction processes.

The swarm's temporal evolution followed a typical pattern of initial higher-magnitude triggers followed by a gradual decline in both frequency and size. No damage or felt reports were associated with these events, consistent with their offshore location and moderate magnitudes. Continued monitoring by regional seismic networks remains essential for distinguishing swarm activity from foreshock sequences that might precede larger earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center regional reports
Global CMT catalog for Aleutian tectonics
NOAA historical earthquake database for Adak vicinity