Seismic Swarm S20201201.2: Activity Southeast of Nikolski, Alaska

Seismic swarm S20201201.2 occurred 41 km southeast of Nikolski, Alaska, within the tectonically active Aleutian subduction zone. The sequence began at 16:22 UTC on 1 December 2020 and concluded at 02:14 UTC on 5 December 2020, spanning 81 hours and 51 minutes. During this period, 65 earthquakes were recorded, with magnitudes ranging from 1.9 to 6.4 and focal depths primarily between 14 km and 56 km.

The swarm initiated with a magnitude 6.4 event at 23 km depth, followed rapidly by numerous aftershocks and smaller events. Notable larger shocks included a magnitude 4.4 earthquake at 39 km depth on 1 December, a magnitude 3.9 event at 35 km on 2 December, and several magnitude 3.0–3.4 events distributed across the first three days. Depths clustered most frequently around 20–35 km, consistent with activity near the plate interface or within the overriding crust. Activity declined steadily after 3 December, with only isolated events recorded on 4 and 5 December.

Nikolski lies on Umnak Island in the central Aleutian arc, where the Pacific Plate subducts northwestward beneath the North American Plate. Convergence rates average 6–7 cm per year, driving frequent seismicity and volcanism along this segment of the Ring of Fire. Earthquake swarms in this setting often reflect episodic slip, fluid migration, or stress transfer along the megathrust or adjacent faults rather than a single mainshock-aftershock sequence.

Historical records indicate four prior swarms in the immediate region since 2000, occurring in 2007, 2009, 2012, and 2015. These events demonstrate recurring clustered seismicity southeast of Nikolski, likely tied to the same structural features accommodating subduction-related strain.

The December 2020 swarm provides insight into short-term strain release patterns. The initial 6.4 event appears to have triggered the subsequent cascade, with the majority of events occurring within the first 48 hours. Depth distribution suggests involvement of both crustal and upper-mantle structures, typical of Aleutian arc seismicity. Such swarms contribute to long-term hazard assessment by highlighting zones of elevated microseismicity that may precede or accompany larger subduction-zone earthquakes.

References
SeismoSight internal swarm classification S20201201.2
USGS Earthquake Catalog – Aleutian Islands regional seismicity
Alaska Earthquake Center – subduction zone tectonics summary