Seismic Swarm VS20200226.1: Analysis of Activity Near Adak, Alaska

The seismic swarm designated VS20200226.1 occurred 42 km east-northeast of Adak, Alaska, in the central Aleutian Islands. It began at 14:51 UTC on 25 February 2020 and concluded at 11:44 UTC on 17 March 2020, spanning 500 hours and 53 minutes. During this interval, 580 earthquakes were recorded. This event unfolded in a tectonically active subduction zone where the Pacific Plate descends beneath the North American Plate, generating frequent seismic and volcanic activity along the Aleutian arc. Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -0.6 to 1.9, with the majority below 1.0 and only a few reaching 1.6 or higher. Depths were consistently shallow, clustered between 0 and 8 km, with several events at or near 0 km and occasional negative depth values likely reflecting location uncertainties. Temporal distribution showed an initial burst on 25 February, followed by sustained low-level activity through 27 February, featuring clusters of events within minutes of one another. This pattern aligns with swarm characteristics, lacking a dominant mainshock and instead exhibiting diffuse, repetitive energy release. The Adak region forms part of the Aleutian volcanic arc, shaped by ongoing plate convergence at rates exceeding 6 cm per year. Historical records since 2000 indicate 18 prior swarms in the area, with notable occurrences in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), 2019 (2 swarms), and 2020 (4 swarms including the present event). Such sequences reflect stress adjustments within the overriding plate and along the subduction interface, often linked to fluid migration or frictional heterogeneity. Geological context underscores the arc's position as one of Earth's most seismically productive zones, where shallow crustal events like those in VS20200226.1 commonly accompany deeper Wadati-Benioff zone activity. The swarm's shallow focus suggests involvement of the upper crust rather than the deeper megathrust. No significant surface deformation or volcanic unrest was associated, consistent with the modest energy release. This episode contributes to understanding swarm recurrence in the central Aleutians, where monitoring networks provide high-resolution catalogs essential for hazard assessment. Continued observation supports refined models of subduction-related seismicity.

References

• SeismoSight internal swarm classification records for VS20200226.1 parameters and event catalog.
• U.S. Geological Survey Earthquake Hazards Program documentation on Aleutian arc tectonics and subduction dynamics.