Earthquake Swarm VS20230217.1 Near Adak, Alaska

The Aleutian Islands form part of a tectonically active subduction zone where the Pacific Plate descends beneath the North American Plate, generating frequent seismic events. Adak, located on the Andreanof Islands, sits amid this dynamic setting characterized by volcanic arcs and deep oceanic trenches. Earthquake swarms are common in such environments due to fluid migration and stress adjustments along fault systems. Swarm VS20230217.1 was recorded 91 km west of Adak. It began at 02:21 on 17 February 2023 and concluded at 15:42 on 23 February 2023, spanning 157 hours and 20 minutes with a total of 309 earthquakes. This sequence aligns with patterns observed in the region since 2000, during which nine swarms have occurred. Prior episodes took place in 2008 (two swarms), 2017 (one), 2020 (one), 2021 (one), 2022 (three), and 2023 (one). Analysis of the first 100 events reveals predominantly shallow foci, with depths clustered between 1 km and 8 km and occasional negative or deeper readings up to 11 km. Magnitudes ranged from 0.7 to 3.5, with the majority falling between 1.0 and 2.3. Early activity featured low-magnitude events that gradually included stronger shocks, such as a 3.5 magnitude quake at 11 km depth on 17 February at 08:14:58. Temporal distribution showed bursts of activity interspersed with quieter intervals, consistent with swarm behavior rather than a single mainshock-aftershock sequence. These characteristics underscore the swarm's role in ongoing tectonic adjustment within the Aleutian subduction zone. Historical seismic records indicate that similar swarms contribute to the cumulative release of strain in this high-seismicity corridor, which has produced notable events throughout the 20th and 21st centuries.

References

USGS Earthquake Catalog Alaska Earthquake Center reports SeismoSight internal swarm classification data