Seismic Insights into Swarm S20200122.2 Near Adak, Alaska

The earthquake swarm S20200122.2 was recorded from 14:44 on 22 January 2020 until 04:00 on 2 February 2020. Centered 81 km west of Adak, Alaska, the sequence produced 627 events across 253 hours and 15 minutes. This activity occurred within the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting compressional stresses generate frequent crustal earthquakes, predominantly at depths of 5–15 km, consistent with the observed event depths ranging from 0 to 12 km. The first 100 events began with modest magnitudes between 0.7 and 3.3 at shallow depths of 6–11 km. Activity intensified on 23 January, featuring a magnitude-6.2 mainshock at 10 km depth at 05:53, accompanied by events reaching 4.2 and numerous aftershocks clustered around 3.0–3.8. Depths remained largely stable near 10 km during the peak phase, reflecting typical brittle failure in the overriding plate. The sequence then gradually declined, illustrating swarm-like behavior without a single dominant aftershock decay pattern. Geologically, the Adak area forms part of the Aleutian volcanic arc, marked by active volcanoes and high seismicity. Historical records since 2000 document only three prior swarms in the immediate vicinity—two in 2008 and two in 2017—highlighting the episodic nature of clustered seismicity in this segment of the arc.

References

U.S. Geological Survey Earthquake Catalog
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification database