Seismic Swarm VS20180909.1: Analysis of Activity Near Adak, Alaska
A seismic swarm designated VS20180909.1 was recorded 38 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 14:32 UTC on 8 September 2018 and concluded at 17:35 UTC on 11 September 2018, spanning 75 hours and 2 minutes. During this interval, 52 earthquakes were detected, all of low magnitude and shallow depth consistent with microseismicity patterns observed in subduction-zone environments.
The Aleutian arc forms part of the Pacific Ring of Fire, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–8 cm per year. This tectonic setting produces frequent earthquake swarms, volcanic activity, and associated hazards across the region. Adak Island lies within a zone of elevated seismicity, with historical records documenting both large-magnitude thrust events and episodic swarms of smaller events at shallow crustal depths.
Analysis of the swarm events shows magnitudes ranging from –1.1 to 0.2, with the majority clustered between –0.9 and –0.2. Focal depths were predominantly between 0 and 6 km, although several readings registered at or slightly above sea level, likely reflecting local velocity-model uncertainties common in island-arc settings. The temporal distribution indicates peak activity during the first 36 hours, followed by a gradual decline, a pattern typical of fluid-driven or stress-triggered swarm sequences rather than mainshock-aftershock sequences.
Since 1 January 2000, five seismic swarms have been identified in the immediate Adak region. These occurred in 2002 (one swarm), 2013 (one swarm), and 2018 (three swarms). The 2018 cluster, including VS20180909.1, represents the highest frequency of swarm activity in the instrumental record for this locale. Such recurrent swarms underscore the dynamic stress environment along the subduction interface and overlying crust.
Overall, swarm VS20180909.1 exemplifies the low-magnitude, high-frequency seismicity that characterizes the central Aleutians. Continued monitoring remains essential for distinguishing background swarm behavior from potential precursors to larger events in this tectonically active region.
References
- SeismoSight internal swarm classification records
- USGS Earthquake Catalog (Adak, Alaska region)
- Alaska Earthquake Center regional tectonic summaries