Seismic Swarm S20101112.2: Analysis of Activity Near Nikolski, Alaska

Seismic swarm S20101112.2 was recorded 60 km west-southwest of Nikolski, Alaska, in the central Aleutian Islands. The sequence began at 11:57 on 11 November 2010 and concluded at 17:06 on 13 November 2010, spanning 53 hours and 9 minutes. During this interval, 44 earthquakes were registered, with magnitudes ranging from 1.8 to 3.5 and focal depths predominantly between 0 and 14 km.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a single dominant mainshock. Notable activity included a magnitude 3.5 event at 09:39 on 12 November at 4 km depth, followed closely by a 3.4 event at 09:41. Additional peaks occurred with magnitudes 3.4 at 15:13 and 3.3 at 17:01 on the same day. Depths remained shallow throughout, consistent with upper-crustal brittle failure.

Nikolski lies on Umnak Island within the Aleutian volcanic arc, formed by the ongoing subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent earthquake swarms due to fluid migration, stress transfer along the megathrust, and volcanic processes. Historical records document major events in the region, including the 1957 M8.6 Andreanof Islands earthquake and the 2006 M8.3 event near the central Aleutians, underscoring the persistent seismic hazard.

Since 1 January 2000, four swarms have been identified in the immediate area according to SeismoSight internal classification. Prior episodes occurred in 2007 (one swarm) and 2010 (three swarms total, including the present sequence). Such recurrent swarms reflect the dynamic stress regime of the subduction zone rather than isolated volcanic unrest.

The 2010 swarm’s shallow depths and rapid succession of events align with fluid-driven seismicity commonly observed along the Aleutian arc. No surface rupture or significant ground deformation was associated with this sequence. Monitoring by regional networks continues to track similar activity, contributing to improved understanding of subduction-zone behavior.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (Aleutian Islands tectonics summaries).
Alaska Earthquake Center regional reports on subduction-zone seismicity.