Seismic Swarm VS20210623.1 Near Adak, Alaska: Analysis and Regional Context
Seismic swarm VS20210623.1 occurred 43 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 03:56 on 22 June 2021 and concluded at 11:57 on 24 June 2021, spanning 56 hours and 1 minute. During this interval, 34 earthquakes were recorded, with magnitudes ranging from -0.7 to 1.5 and focal depths mostly between 2 km and 11 km. One outlier event reached 28 km depth. The swarm featured low-magnitude events clustered in time, typical of fluid-driven or stress-transfer processes in subduction settings.
The Aleutian Islands lie above the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic regime produces the Aleutian volcanic arc and generates frequent earthquakes, both megathrust events and shallower crustal swarms. Adak Island itself occupies a position within the Andreanof Islands segment, an area characterized by intermediate-depth seismicity and active volcanism.
Historical records indicate 28 swarms have occurred in the region since 1 January 2000. Earlier episodes took place in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), 2019 (2 swarms), and 2020 (13 swarms), with the 2021 event representing the first of that year. Such recurrent swarms reflect ongoing strain accumulation and release along the subduction interface and overlying crust.
Within the 2021 swarm, activity initiated with a 0.8-magnitude event at 7 km depth, followed by numerous microearthquakes. Notable events included a 1.5-magnitude shock at only 2 km depth early on 23 June and a later 1.3-magnitude event at 28 km. The shallow clustering of many events suggests possible involvement of hydrothermal fluids or localized faulting within the volcanic edifice.
Seismic swarms in this setting provide valuable indicators of stress changes that may precede larger earthquakes or volcanic unrest. Continued monitoring by regional networks remains essential for assessing evolving hazard potential in the central Aleutians.
References
SeismoSight internal swarm catalog VS20210623.1
USGS Earthquake Catalog (Adak, Alaska region)
Alaska Earthquake Center regional reports