Seismic Swarm S20101220.1: Activity Near Adak, Alaska

A seismic swarm designated S20101220.1 occurred approximately 91 km south-southeast of Adak, Alaska, from 01:37 UTC on 19 December 2010 to 20:11 UTC on 21 December 2010. Over 66 hours and 33 minutes, the sequence produced 48 earthquakes. Depths ranged primarily between 17 km and 53 km, consistent with intermediate-depth events in a subduction setting. Magnitudes varied from 2.1 to a peak of 5.2, with several events exceeding magnitude 4.0 clustered in the first 24 hours.

The swarm began with a magnitude 4.7 shock at 27 km depth, followed by a rapid succession of events. The largest earthquake, magnitude 5.2 at 25 km depth, occurred at 22:11 UTC on 19 December. Subsequent activity included multiple magnitude 4.1–4.9 events through the evening, then tapered into smaller shocks. By 21 December, the sequence concluded with a final magnitude 3.2 event. Depths showed modest variation but remained concentrated in the 20–40 km range for most events, typical of the Wadati-Benioff zone beneath the central Aleutians.

This region lies within the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting compressional tectonics generate frequent seismicity along the arc, including both shallow thrust events and deeper intraslab earthquakes. Adak and the surrounding Andreanof Islands have recorded numerous moderate-to-large earthquakes historically, reflecting the high strain accumulation characteristic of the arc. Volcanic centers nearby, such as those on Great Sitkin and Kanaga islands, further illustrate the active margin setting.

No prior swarms matching the internal classification criteria have been recorded since 2000, indicating that S20101220.1 represents an isolated episode of clustered activity rather than a recurring pattern in the immediate vicinity. Such swarms can arise from fluid migration or localized stress transfer along the slab interface, though detailed source mechanisms for this sequence remain outside the scope of basic catalog analysis.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center, University of Alaska Fairbanks
NOAA National Centers for Environmental Information, historical seismicity reports