Earthquake Swarm VS20200515.1: Seismic Activity Near Adak, Alaska in May 2020

The Aleutian Islands region, where Adak is located, forms part of the tectonically active Pacific Ring of Fire. Here the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench, generating frequent earthquakes and volcanic eruptions. This subduction zone has produced some of the largest recorded earthquakes in North America, including the 1964 Great Alaska Earthquake. Adak itself sits on Adak Island in the Andreanof Islands group, an area characterized by steep volcanic terrain and ongoing seismic hazards due to its position directly above the subduction interface.

Swarm VS20200515.1 began at 08:54 on 14 May 2020 and concluded at 02:55 on 20 May 2020, lasting 138 hours and one minute. The events were centered 41 km east-northeast of Adak. During this period, 76 earthquakes were recorded, with magnitudes ranging from -0.8 to 2.1 and focal depths mostly between 0 and 10 km. The sequence featured numerous micro-earthquakes, many below magnitude 0.0, interspersed with slightly larger events on 16 and 17 May that reached 1.3, 2.0, and 2.1. Activity was most intense on 15 May, when dozens of events clustered within a few hours, then gradually declined through 19 May before a final small event on 20 May.

Such swarms are not uncommon in the Adak region. Since 2000, 21 swarms have been identified in the broader area, with notable clusters in 2018 (10 swarms) and 2020 (7 swarms). Earlier episodes occurred in 2002 and 2013. These swarms typically reflect fluid migration or stress adjustments along the subduction interface rather than foreshock sequences leading to a single large mainshock.

Geological monitoring in the Aleutians relies on regional seismic networks operated by the Alaska Earthquake Center and the U.S. Geological Survey. Depths recorded in this swarm are consistent with shallow crustal and upper-plate seismicity commonly observed above the subduction megathrust. Negative magnitudes and depths near or above sea level are typical for sensitive local networks capturing the smallest events in volcanic or tectonically fractured rock.

The swarm did not produce any reported damage or felt shaking on Adak, given the small magnitudes involved. Continued surveillance of the Andreanof Islands segment remains important because the region retains the potential for larger subduction-zone earthquakes.

References
Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity reports for the Aleutian Islands.
U.S. Geological Survey – Tectonic setting of the Aleutian subduction zone.
SeismoSight internal classification – Swarm VS20200515.1 parameters and historical swarm statistics.