Seismic Swarm Near Adak, Alaska: Analysis of the December 2023 Event

The Aleutian Islands form a volcanic arc resulting from the subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–8 cm per year. This tectonic setting produces frequent earthquakes, including swarms, along the plate interface and within the overriding crust. Adak Island lies within this highly active zone, where historical seismicity includes both great subduction-zone events and shallower crustal sequences.

Between 07:31 UTC on 19 December 2023 and 22:11 UTC on 20 December 2023, a swarm of 30 earthquakes occurred 37 km west of Adak. Magnitudes ranged from 0.3 to 2.9, with the majority of events recorded at depths between 2 km and 13 km. The sequence lasted 38 hours and 40 minutes, exhibiting a typical swarm pattern of clustered activity without a single dominant mainshock.

Such swarms in the central Aleutians are often linked to fluid migration or minor slip along pre-existing faults within the accretionary complex. Depths predominantly in the upper crust are consistent with brittle failure above the deeper plate interface. The low-magnitude character of the events aligns with background microseismicity levels documented across the region since instrumental monitoring began.

Only one prior swarm has been identified in the same locale since 1 January 2000, occurring in 2017. This rarity underscores the episodic nature of swarm activity west of Adak compared with more continuous aftershock sequences following larger mainshocks elsewhere in the arc.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the area. No surface deformation or volcanic unrest has been associated with the December 2023 sequence, indicating a primarily tectonic origin.

References

USGS Earthquake Hazards Program – Aleutian Arc tectonics summary
Alaska Earthquake Center – Regional seismicity reports
NOAA National Centers for Environmental Information – Historical earthquake catalog