Seismic Swarm S20110628.1: Fox Islands, Aleutian Islands, Alaska

The seismic swarm designated S20110628.1 occurred in the Fox Islands region of the Aleutian Islands, Alaska, from 23:29 on 27 June 2011 to 21:45 on 30 June 2011. Over approximately 70 hours and 15 minutes, 33 earthquakes were recorded. Magnitudes ranged from 2.9 to 4.5, with the largest event reaching 4.5 at a depth of 42 km on 28 June. Depths varied between 20 km and 69 km, clustering primarily in the 40–55 km range, consistent with activity along the subducting slab.

The sequence began with a 4.2-magnitude event at 50 km depth, followed by a series of smaller shocks. Notable events included multiple magnitude 4+ earthquakes within the first 24 hours, such as a 4.3 at 43 km and a 4.1 at 55 km. Activity tapered after 29 June, concluding with a 3.1-magnitude event at 41 km depth. No single dominant mainshock was evident, characteristic of swarm behavior where events occur in rapid succession without clear foreshock-aftershock patterns.

The Fox Islands lie within the central Aleutian arc, a tectonically active segment of the Pacific Ring of Fire. Here, the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year along the Aleutian Trench. This convergent margin produces frequent seismicity, including both shallow crustal events and deeper intraslab earthquakes. The arc hosts several active volcanoes, such as those on Unimak and Akutan islands, where magma movement can occasionally influence seismic swarms, though the 2011 event aligns more closely with tectonic stress release.

Historically, the Aleutian Islands have experienced some of the largest earthquakes on record, including the 1957 magnitude 8.6 event and the 1965 magnitude 8.7 Rat Islands quake, both generating significant tsunamis. Since 2000, regional monitoring has documented persistent background seismicity, with this swarm representing the sole cluster of its type in the catalog through the present. Such swarms may reflect episodic slip or fluid migration along the plate interface, contributing to long-term strain accumulation that could precede larger megathrust ruptures.

Analysis of the depth distribution indicates most activity occurred within the Wadati-Benioff zone, where the downgoing slab experiences internal deformation. The absence of events shallower than 20 km suggests limited involvement of the overriding plate. Continued monitoring by networks such as the Alaska Earthquake Center provides updated data on similar sequences, aiding in refining models of subduction dynamics in this high-hazard region.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Alaska Earthquake Center, University of Alaska Fairbanks
• SeismoSight internal swarm classification records