Seismic Swarm VS20210724.1 Near Adak, Alaska

Seismic swarm VS20210724.1 was recorded 43 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 04:11 UTC on 23 July 2021 and concluded at 08:27 UTC on 28 July 2021, spanning 124 hours and 16 minutes. During this interval, 83 earthquakes were registered.

Event magnitudes ranged from -0.9 to 3.2, with the great majority falling below 1.0. Depths were predominantly shallow, concentrated between 3 km and 12 km, although one outlier reached 145 km. The largest event, magnitude 3.2, occurred at 09:46 UTC on 24 July at 145 km depth. Negative-magnitude detections reflect microseismicity typical of swarm sequences, while the temporal clustering without a single dominant mainshock distinguishes the activity from classic aftershock sequences.

The Adak region lies above the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate along the Aleutian Trench. This convergent margin produces frequent earthquakes and volcanic activity throughout the Aleutian arc. The central Aleutians host several historically active volcanoes, and crustal stresses generated by plate convergence regularly trigger both isolated events and swarm-like clusters at shallow to intermediate depths.

Historical records maintained since 1 January 2000 document 30 swarms in the immediate vicinity. Annual counts include one swarm each in 2002 and 2013, ten in 2018, two in 2019, thirteen in 2020, and three in 2021. These episodes illustrate the recurrent nature of swarm activity along this segment of the arc, often linked to fluid migration or localized stress adjustments within the overriding plate and subducting slab.

Analysis of swarm VS20210724.1 shows a rapid onset followed by sustained low-magnitude seismicity that gradually declined over five days. The predominance of events at depths of 4–8 km suggests activity within the upper crust, consistent with the tectonic regime of the central Aleutians. No damage or felt reports were associated with the sequence, underscoring its minor energetic release relative to larger regional earthquakes.

Continued monitoring of such swarms contributes to refined understanding of subduction-zone processes and improves hazard assessment for communities in the Aleutian Islands.

References

SeismoSight internal swarm classification records VS20210724.1.
USGS Earthquake Catalog, Aleutian Islands region.
Alaska Earthquake Center, regional tectonics overview.