Seismic Swarm S20200214.1: Activity Southwest of Atka, Alaska

Seismic swarm S20200214.1 was recorded 126 km south-southwest of Atka, Alaska, in the central Aleutian Islands. The sequence began at 06:53 UTC on 13 February 2020 and concluded at 13:46 UTC on 14 February 2020, spanning 30 hours and 52 minutes. During this interval, 30 earthquakes were detected, with magnitudes ranging from 2.2 to 5.0 and focal depths between 10 and 35 km.

The swarm initiated with a magnitude 4.6 event at 18 km depth. Subsequent activity included multiple events clustered between magnitudes 2.5 and 3.7 during the morning of 13 February, with depths varying from 13 to 35 km. A notable escalation occurred at 15:26 UTC on 13 February, when a magnitude 5.0 earthquake struck at 24 km depth. This was followed within minutes by three events of magnitudes 4.1, 4.1, and 4.2, all at shallow depths of 10–20 km. Later activity on 13 February featured events between magnitudes 2.3 and 3.4, primarily at 10–22 km depth. On 14 February, the sequence tapered with 10 events of magnitudes 2.5–3.8, mostly at depths of 10–16 km, ending with a final magnitude 2.5 event at 10 km.

This pattern reflects typical swarm behavior in subduction-zone settings, where numerous events occur without a single dominant mainshock-aftershock sequence. Depths remained predominantly in the upper to mid-crust, consistent with brittle failure along the plate interface or within the overriding crust.

The central Aleutian arc lies above the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. Atka Island and surrounding waters form part of a tectonically active region marked by both frequent seismicity and Holocene volcanism. Historical records indicate persistent seismic swarms in this sector, with 11 documented episodes since 1 January 2000. These occurred in 2008 (1 swarm), 2010 (1), 2013 (5), 2014 (1), 2015 (2), and 2016 (1). Such recurrent swarms underscore the ongoing stress accumulation and release along the subduction megathrust and associated crustal faults.

Monitoring of this swarm contributes to improved understanding of precursory signals and swarm statistics in remote arc environments. Continued observation supports hazard assessment for nearby communities and maritime traffic in the Aleutian region.

References

SeismoSight internal swarm classification S20200214.1 dataset.
USGS Earthquake Catalog (regional Aleutian arc tectonics).
Alaska Volcano Observatory geologic summaries (Atka Island arc setting).