Seismic Swarm S20110902.1 in the Aleutian Islands

SeismoSight recorded swarm S20110902.1 beginning at 11:02 on 2 September 2011 and concluding at 18:28 on 4 September 2011. The sequence occurred 187 km east of Atka, Alaska, and comprised 65 earthquakes over 55 hours and 26 minutes.

The Aleutian Islands lie along an active subduction zone where the Pacific Plate descends beneath the North American Plate. This tectonic setting produces frequent seismic events, both shallow and intermediate-depth, as well as persistent volcanic activity. Atka itself sits on the central Aleutian arc, a region characterized by complex faulting and magma movement that can trigger earthquake clusters.

Within the swarm, magnitudes ranged from 2.5 to 5.1. The largest event reached magnitude 5.1 at 25 km depth late on 2 September. Several other notable shocks included a magnitude 4.9 at 17 km depth and additional events of magnitude 4.8 and 4.3. Depths clustered predominantly between 40 km and 45 km, although a subset of shallower events occurred between 17 km and 29 km. The temporal distribution showed the highest rate of activity during the first twelve hours, followed by a gradual decline through 4 September.

This swarm represents the earliest episode in SeismoSight’s classification of sequences in the region since 1 January 2000. The consistent depth range and moderate magnitudes align with typical intermediate-depth seismicity driven by slab dehydration and stress transfer within the subducting plate.

Such swarms provide valuable data on stress accumulation along the Aleutian megathrust and surrounding crustal structures. Continued monitoring supports improved understanding of precursory patterns that may precede larger tectonic events in this high-hazard arc.

References

SeismoSight internal swarm catalogue S20110902.1
USGS Earthquake Catalog (regional tectonics background)
Alaska Earthquake Center (Aleutian arc seismicity overview)