Earthquake Swarm VS20181221.1: Seismic Activity Near Adak, Alaska

An earthquake swarm designated VS20181221.1 was recorded 47 km east-northeast of Adak in the Aleutian Islands, Alaska. The sequence began at 11:14 on 20 December 2018 and concluded at 12:41 on 22 December 2018, spanning 49 hours and 27 minutes. During this period, 30 earthquakes were detected, with the majority exhibiting very low magnitudes and shallow focal depths.

The events displayed a characteristic pattern of low-energy seismicity. Initial activity on 20 December included micro-earthquakes with magnitudes between -0.7 and 1.9 at depths of 3–9 km. A notable outlier occurred at 19:18 with a magnitude 2.5 event at 77 km depth. Subsequent activity on 21 December consisted primarily of events with magnitudes from -0.7 to 0.1 at depths of 4–7 km. The swarm tapered off on 22 December with two final events of magnitudes 0.6 and 1.2 at depths of 0–1 km.

This swarm occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The region around Adak lies along the Andreanof Islands segment of the arc, known for frequent seismic swarms driven by stress transfer, fluid migration, and frictional instabilities along the plate interface and overlying crustal faults. Historical records indicate elevated swarm activity in this segment, consistent with the 11 swarms documented since 2000, including isolated events in 2002 and 2013, followed by nine swarms in 2018 alone.

Such swarms provide insight into transient stress conditions in subduction environments. The predominance of shallow, low-magnitude events suggests possible involvement of hydrothermal fluids or localized crustal adjustments rather than a single large rupture. Deeper events, such as the magnitude 2.5 shock, may reflect slip along the deeper plate interface. Continued monitoring by regional networks helps distinguish these swarms from foreshock sequences that could precede larger mainshocks.

References
Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismic catalog.
U.S. Geological Survey Earthquake Hazards Program – Aleutian arc tectonics and subduction parameters.
SeismoSight internal classification records for swarm VS20181221.1.