Seismic Swarm VS20181028.1 Near Adak, Alaska: Geological Context and Event Analysis
The seismic swarm designated VS20181028.1 was recorded 37 km northeast of Adak, Alaska, in the central Aleutian Islands. It began at 23:40 on 27 October 2018 and concluded at 06:36 on 4 November 2018, spanning 174 hours and 55 minutes. During this interval, 253 earthquakes were detected. This activity occurred within the tectonically active Aleutian arc, where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year, generating frequent seismicity and volcanism along the Ring of Fire.
Adak Island lies on the Aleutian Ridge, a volcanic arc formed by this subduction. The region experiences both shallow crustal events and deeper Wadati-Benioff zone earthquakes, with historical records showing clusters often linked to stress transfer along the plate interface or volcanic systems. The swarm's location aligns with zones of known microseismicity influenced by fluid migration or fault interactions in the overriding plate.
Historical data indicate seven swarms in the area since 1 January 2000. These occurred in 2002 (one swarm), 2013 (one swarm), and 2018 (five swarms), underscoring episodic swarm behavior rather than isolated mainshock-aftershock sequences. Such patterns are characteristic of the central Aleutians, where swarm activity can precede or accompany volcanic unrest at nearby centers such as Great Sitkin or Kanaga volcanoes.
Analysis of the first 100 events reveals predominantly low-magnitude seismicity. Magnitudes ranged from -1.0 to 1.9, with the majority falling between -0.9 and 0.5. Depths clustered between -1 km and 6 km, indicating very shallow sources consistent with crustal fracturing above the subduction interface. Early events on 27–28 October showed mixed positive and negative magnitudes, transitioning to slightly higher values by late 29–30 October, including isolated events of 1.1 and 1.9. This distribution suggests a diffuse energy release without a dominant mainshock, typical of fluid-driven or aseismic slip-triggered swarms in subduction settings.
The swarm's internal classification by SeismoSight highlights its value for monitoring microseismic trends in remote Aleutian locations. No damage or felt reports were associated with these small events, yet the sequence contributes to understanding long-term strain accumulation along this high-hazard segment of the plate boundary.
References
SeismoSight internal swarm classification records for VS20181028.1 parameters and event list.
USGS Earthquake Hazards Program summaries of Aleutian arc tectonics and historical seismicity.
Alaska Volcano Observatory reports on regional volcanic-seismic interactions.