Seismic Swarm S20200531.1: Analysis of Activity Near Adak, Alaska

A seismic swarm designated S20200531.1 was recorded 43 km ENE of Adak, Alaska, beginning at 02:10 on 30 May 2020 and concluding at 00:09 on 10 June 2020. Over 261 hours and 59 minutes, the swarm encompassed 210 earthquakes, providing a detailed record of microseismic activity in the central Aleutian Islands.

The first 100 events reveal a pattern dominated by low-magnitude earthquakes. Magnitudes ranged from -0.9 to 3.1, with the majority falling between -0.6 and 0.7. Depths clustered primarily between 5 and 9 km, though some events occurred at shallower levels near 0-2 km or slightly deeper at 10-11 km. Activity intensified on 31 May, featuring a peak event of magnitude 3.1 at 07:51:30 UTC with a focal depth of 11 km. Earlier phases on 30 May showed scattered events with frequent negative magnitudes, indicative of very small ruptures. Subsequent sequences maintained shallow depths around 6-8 km, suggesting brittle failure within the upper crust. The distribution points to a diffuse swarm rather than a mainshock-aftershock sequence, consistent with fluid migration or stress triggering along pre-existing faults.

This swarm fits into a broader historical context of seismic swarms in the region. Since 1 January 2000, 22 swarms have been documented near Adak. Notable years include 2018 with 10 swarms and 2020 with 8, alongside earlier episodes in 2002, 2013, and 2019. Such recurrent activity underscores the dynamic nature of the local fault systems.

Geologically, the area lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6-7 cm per year. This tectonic setting produces the Aleutian volcanic arc and drives frequent seismicity through megathrust earthquakes, intraslab events, and shallow crustal faulting. Adak Island sits atop this arc, where the plate interface reaches depths conducive to both volcanic and tectonic processes. Historical records document major events, such as the 1957 magnitude 8.6 Andreanof Islands earthquake, which originated nearby and generated significant tsunamis. Ongoing monitoring highlights persistent background seismicity linked to slab dehydration and mantle wedge dynamics.

The 2020 swarm's characteristics align with typical subduction-related microseismicity, where small events often occur in clusters without producing surface rupture. Depths predominantly under 10 km indicate activity above the plate interface, likely on subsidiary faults within the overriding plate. Updated seismic catalogs confirm continued low-level activity in the Aleutians post-2020, reinforcing the zone's status as one of Earth's most active convergent margins.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (www.aeic.alaska.edu)
SeismoSight internal swarm classification data