Seismic Swarm VS20200207.1: Activity Near Adak, Alaska

A notable earthquake swarm, designated VS20200207.1, occurred 41 km east-northeast of Adak, Alaska, from 15:11 UTC on 6 February 2020 to 12:51 UTC on 13 February 2020. Over 165 hours and 39 minutes, the sequence produced 144 earthquakes. This event highlights the persistent seismic character of the central Aleutian Islands, where tectonic forces drive frequent clustered activity. The first 100 events displayed predominantly low magnitudes, ranging from -0.7 to 1.9, with the majority below 1.0. Depths clustered in the shallow crust, typically between 0 and 6 km. Early activity featured very small events, including several negative magnitudes, followed by sporadic peaks such as magnitude 1.5 on 7 and 8 February and a maximum of 1.9 late on 12 February. Timing showed irregular clustering, with bursts separated by quieter intervals, consistent with swarm behavior rather than a mainshock-aftershock sequence. Historical records indicate 17 swarms in the region since 1 January 2000. These occurred in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), 2019 (2 swarms), and 2020 (3 swarms, including the present event). Such recurrence underscores the area's tendency toward episodic seismic swarms rather than isolated large shocks. The Adak region lies within the Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate. Convergence occurs at roughly 7–8 cm per year along this arc, generating intense crustal deformation and frequent seismicity. The central Aleutians host both volcanic and tectonic earthquakes, with shallow swarms often linked to fluid migration or stress redistribution along the plate interface and overlying crust. Adak Island itself occupies a tectonically active segment of the arc, positioned near the Adak Canyon and influenced by the oblique subduction geometry that promotes strike-slip and thrust faulting. Seismic monitoring in this remote area relies on regional networks operated by the Alaska Earthquake Center and the U.S. Geological Survey. These instruments capture microseismicity that would otherwise go unnoticed, revealing patterns of swarm activity that inform hazard assessment for the Aleutian chain. While individual swarms rarely produce damaging ground motion, their cumulative occurrence contributes to the elevated seismic hazard rating assigned to the central Aleutians. Ongoing tectonic loading along the subduction interface ensures that similar sequences will continue. Continued observation of parameters such as event rate, magnitude distribution, and depth migration provides valuable data for refining models of crustal stress in this dynamic margin.