Seismic Swarm S20220111.2: Analysis of Activity Southeast of Nikolski, Alaska

Seismic swarm S20220111.2 was recorded 105 km southeast of Nikolski, Alaska, on Umnak Island in the Aleutian chain. The sequence began at 11:56 on 11 January 2022 and concluded at 13:29 on 13 January 2022, spanning 49 hours and 33 minutes. A total of 53 earthquakes were detected during this period.

Magnitudes ranged from 2.0 to 4.8, with the largest event measuring 4.8 at a depth of 10 km on 11 January at 15:27:13 UTC. Depths clustered predominantly between 9 km and 13 km, consistent with shallow crustal seismicity, though a few events extended to 36 km. Multiple events exceeded magnitude 4.0, including 4.4, 4.3, and 4.2, occurring mainly within the first 24 hours. The temporal distribution showed peak activity on 11 January, tapering significantly by 13 January.

This swarm occurred within the tectonically active 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. The region forms part of the Pacific Ring of Fire and hosts the Aleutian volcanic arc, characterized by frequent earthquakes and active volcanism. Umnak Island lies near the intersection of the subduction interface and regional fault systems, contributing to both interplate and intraplate seismic events.

Earthquake swarms in subduction settings often reflect fluid migration, stress transfer along the megathrust, or minor slip on subsidiary faults rather than a single mainshock-aftershock sequence. Depths around 10 km align with the upper plate or shallow interface in this portion of the arc. Historical records since 2000 indicate only three documented swarms in the immediate vicinity: one each in 2007 and 2009, plus the 2022 event. These infrequent clusters suggest episodic rather than continuous swarm behavior in the area.

The 2022 swarm did not produce reported damage or trigger a tsunami, consistent with its moderate magnitudes and offshore location. Ongoing monitoring by regional networks continues to track background seismicity along the arc, where large megathrust earthquakes remain possible given the zone’s history of great events.

References

USGS Earthquake Catalog
Alaska Earthquake Center annual reports
NOAA National Centers for Environmental Information tectonic summaries