Seismic Swarm S20220111.1: Analysis of Earthquake Activity Southeast of Nikolski, Alaska

A significant seismic swarm, designated S20220111.1, occurred southeast of Nikolski, Alaska, from 11:41 UTC on 11 January 2022 to 02:55 UTC on 16 January 2022. Over 111 hours and 14 minutes, 135 earthquakes were recorded, with detailed parameters available for the first 100 events. This activity took place approximately 81 km southeast of Nikolski on Umnak Island in the Aleutian chain.

The region lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of about 6–7 cm per year. This tectonic setting produces frequent seismicity and volcanism along the 2,500 km island arc. Earthquakes here commonly originate at depths of 10–40 km, consistent with the observed swarm depths ranging from 1 km to 37 km, with the majority clustered between 8 km and 25 km.

Analysis of the initial 100 events reveals a rapid onset with several events exceeding magnitude 4.0 within the first hour, including a magnitude 5.8 earthquake at 16 km depth on 11 January at 12:53:45 UTC. Magnitudes in the swarm spanned 2.1 to 5.8, with 12 events of magnitude 4.0 or greater. Depths showed a bimodal distribution, with many shallow events near 10 km and a secondary cluster around 20–25 km. Temporal patterns indicated peak activity in the first 12 hours, followed by a gradual decline, characteristic of swarm behavior rather than a mainshock-aftershock sequence.

Historical data indicate this is the sixth swarm recorded in the area since 2000. Earlier swarms occurred in 2007 (three events), 2009 (one event), 2015 (one event), and 2022 (the current swarm). Such recurrent swarms highlight ongoing stress accumulation and release along the subduction interface near Umnak Island.

The Aleutian Islands experience some of the highest seismic hazard levels in the United States. The Nikolski vicinity has been shaped by both tectonic and volcanic processes, including the formation of calderas and stratovolcanoes such as Mount Vsevidof and Mount Recheshnoi on nearby Umnak. Updated monitoring by the Alaska Earthquake Center continues to track activity in this remote but tectonically active zone.

References
Alaska Earthquake Center. (2022). Earthquake catalog data for Aleutian region.
United States Geological Survey. (2023). Tectonic setting of the Aleutian subduction zone.
National Oceanic and Atmospheric Administration. (2022). Regional bathymetry and island geology of the Aleutians.