Seismic Swarm VS20210729.1: Activity 45 km Northeast of Adak, Alaska

A notable earthquake swarm, designated VS20210729.1, occurred 45 km northeast of Adak, Alaska, within the tectonically active Aleutian Islands. The swarm initiated at 21:35 on 28 July 2021 and concluded at 10:57 on 6 August 2021, spanning 205 hours and 22 minutes. During this period, 161 earthquakes were recorded, providing valuable data on regional seismicity in a subduction zone setting. The Aleutian Islands form part of the Pacific Ring of Fire, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. This convergent boundary drives frequent seismic and volcanic activity across the archipelago, including Adak Island. Adak lies near the central Aleutian arc, characterized by intermediate-depth earthquakes and occasional swarms linked to fluid migration or stress adjustments in the subducting slab. Depths in the recorded events ranged primarily from 3 to 13 km, consistent with crustal processes, though isolated deeper events reached 146 km and 210 km, reflecting the arc's deeper seismogenic zone. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values between -0.9 and 1.9. The majority clustered around magnitudes of -0.5 to 0.5, indicating microseismic behavior typical of swarms rather than mainshock-aftershock sequences. Temporal distribution showed initial sparse events on 28 July, followed by increased frequency on 29–31 July, with many events at depths of 7–10 km. Later events in the sequence maintained similar characteristics, underscoring persistent low-level energy release. Historical records indicate 31 swarms have occurred in the region since 1 January 2000. These include single events in 2002 and 2013, ten in 2018, two in 2019, thirteen in 2020, and four in 2021. Such recurrence highlights the area's ongoing seismic productivity within the Aleutian subduction environment. This swarm contributes to understanding stress dynamics in the central Aleutians, where swarms often precede or accompany volcanic unrest. Continued monitoring supports hazard assessment for nearby communities and infrastructure.

References

SeismoSight internal swarm classification records for VS20210729.1.
Regional geological framework from Aleutian arc tectonics literature.