Seismic Swarm VS20200615.1: Analysis of Activity Near Adak, Alaska

The seismic swarm designated VS20200615.1 occurred 41 km northeast of Adak, Alaska, in the central Aleutian Islands. Registered events began at 13:05 on 14 June 2020 and concluded at 14:39 on 18 June 2020, spanning 97 hours and 34 minutes. During this interval, 69 earthquakes were recorded, with magnitudes ranging from -0.8 to 2.6 and focal depths predominantly between 0 and 10 km.

The sequence featured low-magnitude events clustered in the initial 48 hours, followed by a gradual decline. A magnitude 2.6 earthquake on 15 June at 11:57 represented the largest event, occurring at a depth of 9 km. Subsequent activity included numerous events below magnitude 0.0, with several at or near the surface. The swarm exhibited typical characteristics of fluid-driven or stress-release sequences common in subduction environments, with events tightly grouped in both time and space.

Adak lies within the Aleutian volcanic arc, formed by the oblique subduction of the Pacific Plate beneath the North American Plate along the Aleutian Trench. This tectonic setting produces frequent seismicity, including both mainshock-aftershock sequences and earthquake swarms. The central Aleutians host active volcanoes such as Great Sitkin and Kasatochi, whose magmatic systems can influence shallow crustal stresses and contribute to swarm activity. Depths observed in the swarm align with the brittle upper crust above the subduction interface, where hydrothermal circulation and minor fault slip often generate such clusters.

Historical records indicate elevated swarm frequency in the region since 2000, with 23 documented swarms through 2020. Notable years include 2018 (10 swarms) and 2020 (9 swarms), suggesting episodic increases in seismic unrest possibly linked to regional stress changes or volcanic processes. Earlier swarms occurred in 2002 and 2013, underscoring the persistent nature of clustered seismicity along this portion of the arc.

Analysis of VS20200615.1 reveals a short-duration swarm dominated by microseismicity, consistent with background tectonic loading in a highly active subduction zone. No significant surface deformation or volcanic unrest was associated with the sequence based on available monitoring data. Such events provide valuable insights into the mechanics of swarm generation, highlighting the role of heterogeneous fault networks and fluid migration in modulating earthquake rates.

Continued monitoring by regional networks remains essential for distinguishing swarm activity from potential foreshock sequences preceding larger events. The Aleutian Islands' high seismic hazard underscores the importance of understanding these patterns for improved forecasting and hazard assessment.

References

SeismoSight internal swarm classification VS20200615.1.
USGS Earthquake Catalog (regional Aleutian seismicity).
Alaska Earthquake Center tectonic summaries.