Seismic Swarm S20200131.1 Near Aleneva, Alaska: Characteristics and Regional Context

Seismic swarm S20200131.1 was recorded 94 km NNW of Aleneva, Alaska, on Afognak Island in the Kodiak archipelago. The sequence began at 15:51 on 30 January 2020 and concluded at 12:17 on 1 February 2020, spanning 44 hours and 25 minutes. During this period, 69 earthquakes were registered.

Event magnitudes ranged from 1.4 to 3.5, with the largest shocks measuring 3.5 at 07:03 on 31 January (depth 4 km), 3.4 at 05:49 (depth 8 km), and another 3.4 at 06:58 (depth 6 km). Depths were predominantly shallow, concentrated between 2 km and 10 km, consistent with crustal activity above the subduction interface. The temporal distribution showed peak activity between 05:00 and 09:00 on 31 January, followed by a gradual decline.

This swarm occurred within the Alaska-Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. The Kodiak region lies above the megathrust fault responsible for great earthquakes, including the magnitude 9.2 event of 1964. Shallow crustal swarms in this setting often reflect localized stress adjustments or fluid migration along pre-existing faults rather than direct megathrust slip.

Historical records maintained by SeismoSight indicate that three prior swarms have occurred in the same general area since 1 January 2000, specifically in 2013, 2016, and 2017. Each earlier episode contained only a single event, making the 2020 sequence notably more productive.

Such swarms contribute to understanding background seismicity levels in a region that experiences frequent moderate earthquakes. Continued monitoring by networks such as the Alaska Earthquake Center remains essential for distinguishing swarm behavior from potential foreshock sequences preceding larger events.

References
SeismoSight internal classification records for swarm S20200131.1 parameters and event list.
USGS Earthquake Hazards Program regional tectonic summaries for the Alaska-Aleutian arc.
Alaska Earthquake Center historical seismicity database (2000–2020).