Seismic Swarm VS20200203.1: Analysis of Activity Near Adak, Alaska

Seismic swarm VS20200203.1 occurred in the central Aleutian Islands, approximately 42 km east-northeast of Adak, Alaska. The sequence began at 14:01 UTC on 2 February 2020 and concluded at 04:18 UTC on 4 February 2020, spanning 38 hours and 17 minutes. During this period, 37 earthquakes were recorded, with magnitudes ranging from -0.5 to 1.5 and focal depths predominantly between 0 and 6 km. The events exhibited a typical swarm pattern, characterized by a lack of a dominant mainshock and relatively uniform energy release across numerous small events.

The swarm initiated with a magnitude 0.8 event at 3 km depth, followed by additional low-magnitude activity throughout 2 February. Magnitudes remained below 1.0 for most of the first day, with depths varying between surface and 6 km. On 3 February, the sequence included the largest event of the swarm, a magnitude 1.5 earthquake at 4 km depth, alongside numerous events clustered around 2–4 km. Activity tapered on 4 February, ending with two final events of magnitudes -0.4 and -0.5 at shallow depths. Overall, the distribution showed a concentration of events in the upper 4 km of the crust, consistent with shallow tectonic or hydrothermal processes.

This swarm aligns with the tectonic setting of the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year. The region experiences frequent seismicity due to plate boundary interactions, including thrust faulting along the megathrust and normal faulting in the overriding plate. Adak lies near the Adak Island segment of the arc, an area influenced by both subduction-related deformation and volcanic activity from nearby stratovolcanoes. Historical records indicate elevated microseismicity in this sector, often manifesting as swarms linked to fluid migration or stress transfer along preexisting faults.

Since 2000, 16 swarms have been documented in the vicinity. Earlier episodes occurred in 2002 (1 swarm) and 2013 (1 swarm). Activity increased markedly in 2018 (10 swarms), followed by two in 2019 and two in 2020, including VS20200203.1. These recurrent swarms reflect ongoing strain accumulation and release within the subduction environment, where small-magnitude events frequently cluster without producing larger mainshocks.

The shallow depths and low magnitudes observed in VS20200203.1 suggest a localized source, potentially associated with crustal fracturing or minor fluid involvement rather than deep slab processes. Such swarms contribute to the background seismicity that characterizes the central Aleutians, aiding in the mapping of active fault structures.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog (regional Aleutian tectonics summaries).
Alaska Earthquake Center annual reports on subduction zone activity.