Seismic Swarm VS20200719.1: Analysis of Activity Near Adak, Alaska

The seismic swarm designated VS20200719.1 occurred approximately 40 km east-northeast of Adak in the Aleutian Islands of Alaska. It began at 07:01 UTC on 18 July 2020 and concluded at 12:14 UTC on 21 July 2020, spanning 77 hours and 13 minutes. During this interval, 57 earthquakes were recorded. Earthquake magnitudes ranged from -0.4 to 1.1, with the majority falling between 0.0 and 0.7. Focal depths were shallow, concentrated between 2 km and 8 km, indicating activity within the upper crust. The sequence showed clusters of events on 18–19 July, followed by a gradual decline through 20–21 July, consistent with swarm behavior rather than a mainshock-aftershock pattern. This region lies along the 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 resulting tectonic stress produces frequent earthquakes and volcanic activity across the Aleutian arc, part of the circum-Pacific Ring of Fire. Adak itself sits near the Andreanof Islands segment, an area historically prone to both moderate and great earthquakes, including events exceeding magnitude 8. Swarm activity has been documented in the broader Adak region since at least 2000, with 25 such episodes recorded through 2020. Notable years include 2018, which hosted 10 swarms, and 2020, which recorded 11. Earlier swarms occurred in 2002 and 2013. These episodes typically involve low-magnitude events at shallow depths, reflecting fluid migration or stress adjustments along local fault systems within the overriding plate. The 2020 swarm fits established patterns of microseismicity in the arc, where small events often occur without producing significant surface effects. Depths predominantly under 6 km suggest involvement of brittle crustal layers above the subduction interface.

References

SeismoSight internal swarm classification VS20200719.1.
USGS Earthquake Hazards Program regional tectonics summary for the Aleutian Islands.
Alaska Earthquake Center historical seismicity records.