Seismic Swarm S20200424.1: Analysis of Activity Near Adak, Alaska
Seismic swarm S20200424.1 was recorded 41 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 18:53 UTC on 23 April 2020 and concluded at 03:56 UTC on 9 May 2020, spanning 369 hours and 3 minutes. During this period, 279 earthquakes were detected.
Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -1.1 to 1.3, with the majority falling between -0.5 and 0.5. Depths remained shallow, concentrated between 0 and 10 km, consistent with upper-crustal faulting. Early events clustered tightly in time, with several occurring within minutes of one another on 23–24 April, indicating rapid stress release along localized structures.
The Aleutian Islands form part of the Pacific Ring of Fire, where the Pacific plate subducts beneath the North American plate at rates exceeding 6 cm per year. This convergent margin produces frequent seismicity, volcanic arcs, and occasional earthquake swarms driven by fluid migration or magmatic processes. Adak lies near the Andreanof Islands segment, a region historically associated with both great subduction earthquakes and smaller swarm sequences.
Historical records indicate 20 swarms in the area since 1 January 2000. These occurred in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), 2019 (2 swarms), and 2020 (6 swarms). The elevated frequency in 2018 and 2020 suggests episodic increases in crustal permeability or stress perturbations within the subduction interface and overlying volcanic basement.
Such swarms contribute to ongoing monitoring of seismic hazard in the Aleutians, where even low-magnitude events can provide insights into fault mechanics and precursory activity ahead of larger ruptures.
References
- Alaska Earthquake Center, University of Alaska Fairbanks
- USGS Earthquake Hazards Program
- SeismoSight internal swarm classification records