Seismic Swarm PS20220111.1 Near Nikolski, Alaska

A seismic swarm designated PS20220111.1 was recorded on 11 January 2022, southeast of Nikolski on Umnak Island in the Aleutian Islands, Alaska. The sequence began at 11:35 UTC and concluded at 15:27 UTC, spanning 3 hours and 51 minutes. During this interval, eight earthquakes were registered at a location approximately 113 km southeast of Nikolski.

The events exhibited the following parameters: two magnitude 6.8 shocks occurred within seconds of each other at depths of 20 km and 21 km; subsequent activity included a magnitude 5.4 event at 16 km depth, followed by smaller shocks of magnitudes 2.8, 6.6, 5.8, 5.3, and 5.0 at depths ranging from 10 km to 19 km. These closely spaced occurrences reflect typical swarm behavior, where multiple events cluster temporally and spatially without a single dominant mainshock-aftershock sequence.

The region lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. Umnak Island and surrounding areas form part of a volcanic arc influenced by this subduction, with historical activity contributing to the broader seismic hazard of the Aleutian chain.

Since 2000, 28 swarms have been documented in the vicinity. These occurred in the following years with the indicated counts: 2006 (1), 2007 (4), 2008 (1), 2009 (4), 2010 (7), 2012 (1), 2014 (3), 2015 (2), 2016 (1), 2017 (1), 2018 (2), and 2020 (1). Such recurrent swarms underscore the persistent tectonic stress accumulation along the plate interface.

References

USGS Earthquake Catalog
Alaska Earthquake Center, University of Alaska Fairbanks
NOAA National Centers for Environmental Information