Seismic Swarm Analysis: Adak, Alaska Region – January 2023 Event

A seismic swarm designated S20230102.2 was recorded 47 km east-northeast of Adak, Alaska, in the Andreanof Islands of the central Aleutian chain. The sequence began at 01:43 UTC on 2 January 2023 and concluded at 00:51 UTC on 3 January 2023, encompassing 30 earthquakes over 23 hours and 8 minutes.

The Aleutian Islands lie above the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces the Aleutian Trench and a well-developed volcanic arc, rendering the region one of the most seismically active on Earth. Adak itself occupies a position within the Andreanof Islands block, where oblique subduction and crustal faulting contribute to both deep and shallow seismicity.

Event magnitudes ranged from 0.4 to 2.3, with the largest shock (M 2.3) occurring at 12:51 UTC on 2 January at a depth of 7 km. The majority of hypocenters clustered between 5 and 6 km depth, although a few events registered at shallower (3–4 km) or greater (7 km) levels. Depths near 6 km are consistent with brittle failure within the overriding plate or the upper portion of the subducting slab interface. No events exceeded magnitude 3.0, indicating a low-energy swarm rather than a mainshock–aftershock sequence.

Temporal distribution showed two modest peaks: one between 09:00–12:00 UTC and another near 21:00 UTC on 2 January. Most events occurred within a narrow 10 km radius, suggesting a localized source volume. The swarm terminated abruptly after the final recorded event at 00:51 UTC on 3 January.

Regional historical data indicate that swarm activity is recurrent in this sector. Since 1 January 2000, 33 swarms have been identified near Adak. Notable years include 2018 (10 swarms) and 2020 (13 swarms), with additional activity in 2002, 2013, 2019, and 2021. These episodes reflect episodic fluid migration or stress transfer along the subduction interface and overlying crustal faults.

The January 2023 swarm fits the established pattern of low-magnitude, short-duration clusters that do not typically culminate in larger mainshocks. Continued monitoring by regional seismic networks remains essential for distinguishing background swarm behavior from potential precursory activity.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification database