Seismic Swarm VS20200118.1: Analysis Near Adak, Alaska
Seismic swarm VS20200118.1 was recorded 41 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 01:23 on 18 January 2020 and concluded at 06:58 on 23 January 2020, spanning 125 hours and 35 minutes. During this period, 119 earthquakes were detected.
The Aleutian Islands lie along an active subduction zone where the Pacific Plate descends beneath the North American Plate. This tectonic setting produces frequent seismicity, including both isolated events and swarm activity. Adak itself sits on the Andreanof Islands segment, a region characterized by intermediate-depth Wadati-Benioff zone earthquakes and occasional shallow crustal swarms linked to fluid migration or stress transfer along the plate interface.
Historical records since 2000 document 14 swarms in the immediate area. These occurred in 2002 (1 swarm), 2013 (1 swarm), 2018 (10 swarms), and 2019 (2 swarms). Such clustering indicates episodic swarm behavior rather than steady background seismicity, consistent with the complex fault networks and hydrothermal systems present in the central Aleutians.
Analysis of the first 100 events reveals a classic swarm pattern. Magnitudes ranged from –0.6 to 1.8, with the large majority below 1.0. Depths were predominantly shallow, concentrated between 0 and 5 km, although a few events reached 17 km. The sequence showed no dominant mainshock; instead, activity built gradually with multiple small events occurring in tight temporal clusters, particularly on 18–19 January. Early events included several negative-magnitude detections at very shallow or slightly negative reported depths, likely reflecting the sensitivity of the local network to microseismicity.
The absence of a clear magnitude progression and the rapid decay after the initial 36-hour peak align with fluid-driven swarm mechanisms commonly observed in subduction-related volcanic arcs. No events exceeded magnitude 2.0 within the examined subset, underscoring the low-energy character of this episode.
This swarm adds to the documented pattern of recurrent, low-magnitude seismic bursts in the Adak region. Continued monitoring remains essential given the area’s position within the Pacific Ring of Fire and its history of both swarm activity and larger subduction-zone earthquakes.
References
SeismoSight internal swarm catalog VS20200118.1
USGS Earthquake Catalog (Adak, Alaska region)
Alaska Earthquake Center regional reports