Analysis of Earthquake Swarm VS20200321.1 Near Adak, Alaska

Earthquake swarm VS20200321.1 occurred 43 km ENE of Adak, Alaska, in the central Aleutian Islands. It began at 10:24 on 20 March 2020 and concluded at 14:42 on 20 April 2020, spanning 748 hours and 18 minutes with a total of 579 recorded events. The Aleutian region lies within the Pacific Ring of Fire, where the Pacific Plate subducts northwestward beneath the North American Plate along the Aleutian Trench at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismic swarms and volcanic activity along the island arc. The swarm consisted predominantly of microearthquakes. Analysis of the first 100 events shows magnitudes ranging from -1.1 to 1.6, with the majority falling between -0.9 and 0.0. Depths clustered between 0 and 11 km, although isolated events reached 28 km. Early activity on 20 March featured very small events near the surface, transitioning to slightly deeper foci by late March. Notable events included a magnitude 1.6 quake at 3 km depth on 24 March and a magnitude 1.4 event at 8 km on the same day. Negative depth values in the catalog likely reflect location uncertainties common in sparse networks. Overall, the sequence displayed typical swarm characteristics: no dominant mainshock, clustered spatiotemporal occurrence, and rapid decay after peak activity. Adak Island and surrounding waters have a well-documented history of seismic swarms driven by fluid migration and stress transfer within the subduction interface. Since 1 January 2000, 19 swarms have been identified in the broader region, distributed as follows: one in 2002, one in 2013, ten in 2018, two in 2019, and five in 2020. These episodes underscore the persistent tectonic strain accumulation along the Andreanof Islands segment of the arc, which last hosted a great earthquake (Mw 8.6) in 1957. The 2020 swarm did not produce reported damage or tsunami effects, consistent with its modest energy release and shallow, distributed nature. Continued monitoring by regional seismic networks remains essential for distinguishing swarm behavior from potential foreshock sequences in this high-hazard subduction environment.

References

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