Seismic Swarm PS20150905.1 Near Atka, Alaska

The Aleutian Islands form a tectonically active segment of the Pacific Ring of Fire, where the Pacific Plate subducts northwestward beneath the North American Plate. This convergent margin produces frequent earthquakes and sustains the Aleutian volcanic arc. Atka lies on Atka Island within this setting, approximately 92 km north of the swarm centroid. Subduction occurs at rates of 6–7 cm per year, generating both shallow crustal events and deeper intraslab seismicity.

Seismic swarm PS20150905.1 was recorded between 12:42 UTC on 4 September 2015 and 07:08 UTC on 5 September 2015. Five earthquakes with magnitudes 5.0–5.5 occurred during this 18-hour-26-minute interval. The sequence began with a magnitude-5.0 event at 44 km depth. Subsequent events included a magnitude-5.1 shock at 20 km, a magnitude-5.2 event at 48 km, a magnitude-5.5 earthquake at 20 km, and a final magnitude-5.0 shock at 20 km. The majority of activity clustered at shallow to intermediate depths, consistent with the regional subduction geometry.

Historical records since 2000 indicate 17 swarms in the broader Atka region. Notable swarm years include 2008 (four events), 2010 (two), 2011 (one), 2013 (eight), 2014 (one), and the 2015 sequence described here. A magnitude-7.0 earthquake occurred on 30 August 2013, centered 101 km southwest of Atka and 63 km from the 2015 swarm centroid, underscoring the area’s capacity for larger events.

The 2015 swarm fits established patterns of clustered, moderate-magnitude activity along the Aleutian subduction interface. Such sequences commonly reflect stress redistribution within the downgoing slab or along the plate boundary without producing surface rupture. Depths ranging from 20 km to 48 km align with the expected geometry of the Wadati-Benioff zone beneath the central Aleutians.

Continued monitoring remains essential given the region’s high seismic productivity. The 2015 swarm, while moderate in scale, illustrates the persistent tectonic loading that characterizes the Aleutian margin.

References

USGS Earthquake Catalog (events 2000–2023)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification PS20150905.1