Earthquake Swarm S20101226.1: Seismic Activity Near Adak, Alaska in December 2010

The Aleutian Islands region, located along the convergent boundary where the Pacific Plate subducts beneath the North American Plate, experiences frequent seismic activity due to its position on the Pacific Ring of Fire. Adak, Alaska, sits within the Andreanof Islands group, approximately 100 km north-northwest of the swarm's epicentral area. This tectonic setting produces both shallow crustal events and deeper Wadati-Benioff zone earthquakes, with the Aleutian Trench serving as the primary structural feature. Historical records document numerous moderate-to-large earthquakes and associated volcanic activity throughout the islands, reflecting ongoing plate convergence at rates of about 6–7 cm per year.

Swarm S20101226.1 began at 03:53 UTC on 25 December 2010 and concluded at 00:17 UTC on 29 December 2010, lasting 92 hours and 23 minutes. During this period, 102 earthquakes were recorded 100 km SSE of Adak. Analysis of the first 100 events reveals a sequence dominated by smaller-magnitude shocks interspersed with several events exceeding magnitude 4.0. The largest event reached magnitude 5.6 at a depth of 10 km early in the sequence, followed by additional notable shocks of magnitudes 5.4, 5.3, 5.2, 5.0 (three instances), 4.8, 4.7 (two instances), and 4.4. Depths ranged primarily between 10 km and 65 km, with many clustered around 25–35 km, consistent with intermediate-depth activity in the subduction zone.

Temporal distribution shows clustering in the initial 48 hours, with peak activity on 25 and 26 December. Magnitudes generally remained below 3.5 after the initial larger events, though several magnitude 4+ shocks occurred on 27 December. Depths exhibited modest variation, with shallower events (under 20 km) often associated with the higher-magnitude shocks. This pattern aligns with typical swarm behavior in subduction environments, where fluid migration or stress transfer along the plate interface can trigger multiple events without a single dominant mainshock-aftershock sequence.

Since 1 January 2000, two such swarms have been identified in the region, with this event representing the first recorded instance. The second swarm occurred after 2010. The 2010 activity underscores the persistent seismic hazard in the central Aleutians, where monitoring networks provide critical data for understanding subduction dynamics.

References

USGS Earthquake Catalog Alaska Earthquake Center SeismoSight internal swarm classification records