Seismic Swarm VS20210712.1 Near Adak, Alaska: Geological Context and Event Analysis

A notable earthquake swarm, designated VS20210712.1, occurred in the central Aleutian Islands beginning at 10:27 UTC on 11 July 2021 and concluding at 15:34 UTC on 16 July 2021. Located approximately 45 km east-northeast of Adak, Alaska, the swarm produced 120 earthquakes over 125 hours and 7 minutes. This sequence reflects the intense tectonic activity characteristic of the region.

The Adak area lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates exceeding 6 cm per year. This tectonic setting generates frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. Adak Island itself is part of the Andreanof Islands group, formed through volcanic arc processes associated with subduction. Historical large-magnitude events, such as the 1957 Andreanof Islands earthquake of magnitude 8.6, underscore the zone’s capacity for great earthquakes, while smaller swarms represent common background deformation.

Analysis of the first 100 events in the swarm reveals predominantly low-magnitude activity. Recorded magnitudes ranged from −1.1 to 1.7, with the majority falling between −0.8 and 0.5. Focal depths clustered between 3 km and 10 km, although a few events reached 14 km. The sequence exhibited typical swarm characteristics, featuring numerous small events without a single dominant mainshock. Peak activity occurred on 12–14 July, including several events of magnitude 0.4–1.7 on 14 July at depths of 4–7 km.

Such swarms have occurred repeatedly in the Adak region. Since 2000, 29 swarms have been documented, with notable concentrations in 2018 (10 swarms) and 2020 (13 swarms). Earlier episodes were recorded in 2002 and 2013, with additional activity in 2019 and 2021. These recurrent swarms indicate ongoing stress release along fault networks within the overriding plate and near the plate interface.

The 2021 swarm’s shallow depths and low magnitudes suggest fluid migration or aseismic slip may have contributed to triggering, processes frequently observed in subduction-zone environments. No damage or felt reports were associated with the sequence, consistent with the small event sizes.

Continued monitoring of the Aleutian arc remains essential given its history of both swarms and great earthquakes. Data from networks such as the Alaska Earthquake Center provide critical insights into precursory patterns and long-term seismic hazard.

References

Alaska Earthquake Center, University of Alaska Fairbanks.
U.S. Geological Survey Earthquake Catalog.
SeismoSight internal swarm classification records.