Seismic Swarm VS20200706.1: Analysis of Activity Near Adak, Alaska

The seismic swarm designated VS20200706.1 occurred in the Aleutian Islands region of Alaska, centered 42 km east-northeast of Adak. This sequence began at 05:00 on 5 July 2020 and concluded at 06:13 on 16 July 2020, spanning 265 hours and 13 minutes. During this period, 252 earthquakes were recorded, providing a detailed record of clustered seismic activity in a tectonically dynamic setting.

Adak 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 interaction drives frequent earthquakes and volcanic activity along the arc, with the crust characterized by oceanic island arcs, fault systems, and variable sediment cover. Depths in the swarm data ranged primarily from shallow levels near 0–10 km to occasional deeper events exceeding 100 km, consistent with both crustal faulting and deeper slab-related processes in the subduction environment.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes clustered between -1.0 and 2.2, with the majority negative or near zero, indicating microseismicity. Depths showed a concentration in the upper 10 km, though isolated events reached 104 km and 160 km. Early events on 5 July exhibited shallow foci and fluctuating magnitudes, while later ones on 7–9 July included slightly higher magnitudes up to 2.2 alongside deeper outliers. This pattern suggests initial fluid migration or stress adjustments along shallow faults, potentially transitioning to involvement of deeper structures.

The swarm fits into a broader historical context of seismic clustering in the region. Since 1 January 2000, 24 swarms have been documented near Adak. These occurred in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), 2019 (2 swarms), and 2020 (10 swarms). Such recurrent episodes underscore the area's persistent seismic productivity driven by ongoing plate subduction and associated fault reactivation.

Overall, VS20200706.1 exemplifies typical swarm behavior in the Aleutians, where numerous small events occur without a dominant mainshock. Continued monitoring remains essential given the region's high seismic hazard.

References

USGS Earthquake Catalog Alaska Earthquake Center SeismoSight internal swarm classification records