Seismic Swarm S20091013.2 Near Nikolski, Alaska: Event Analysis and Regional Context

Seismic swarm S20091013.2 was recorded 155 km east-southeast of Nikolski, Alaska, in the central Aleutian Islands. The sequence began at 06:07 UTC on 13 October 2009 and concluded at 14:25 UTC on 15 October 2009, spanning 56 hours and 17 minutes. During this interval, 41 earthquakes were detected, with magnitudes ranging from 1.8 to 4.2 and focal depths predominantly between 10 km and 57 km.

The swarm exhibited a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock. Early activity on 13 October included events of magnitude 3.5 at 06:46 and 3.2 at 09:49, followed by a peak magnitude 4.2 earthquake at 23:48. Subsequent days showed continued but declining activity, with notable events of magnitude 3.6 on 14 October and 3.8 on 15 October. Depths clustered around 20–30 km for most shocks, consistent with crustal deformation within the overriding plate.

This location 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. The resulting megathrust interface generates frequent seismicity and arc volcanism. Nikolski sits on Umnak Island, part of a volcanic arc characterized by andesitic stratovolcanoes and active fault systems. Historical records document repeated earthquake swarms in this segment, often linked to stress transfer along the plate boundary or fluid migration in the forearc crust.

Since 1 January 2000, three swarms have been classified in the immediate region according to SeismoSight criteria. One swarm occurred in 2007, while two took place in 2009, indicating episodic clustering rather than steady background seismicity. Such swarms are common in subduction settings and typically do not culminate in great earthquakes, though they provide valuable data on local stress regimes.

The 2009 sequence contributed to ongoing monitoring efforts that track microseismicity for potential precursory signals ahead of larger tectonic events. Depths and magnitudes align with the expected range for intraslab and interface events in this portion of the arc.

References
SeismoSight internal swarm classification records
USGS Earthquake Catalog for Aleutian regional seismicity
Alaska Earthquake Center tectonic summaries