Seismic Swarm S20150904.1: Analysis of Earthquake Activity Near Atka, Alaska

A seismic swarm designated S20150904.1 occurred 109 km south-southwest of Atka, Alaska, in the central Aleutian Islands. The sequence began at 12:15 UTC on 4 September 2015 and concluded at 21:20 UTC on 5 September 2015, spanning 33 hours and 4 minutes. During this period, 45 earthquakes were recorded, with magnitudes ranging from 1.4 to 4.7 and focal depths between 4 km and 39 km.

The swarm initiated with two magnitude 4.7 events at depths of 39 km and 18 km, respectively, within the first hour. Subsequent activity included additional events of magnitude 4.0 at 12:21 and 13:49 UTC, as well as a magnitude 4.1 event at 16:50 UTC. Depths showed notable variation, with many events clustered between 20 km and 30 km, while shallower activity (under 10 km) appeared intermittently. The sequence exhibited higher frequency in the initial 12 hours, followed by a gradual decline, with the final recorded event at magnitude 1.6 on 5 September.

This swarm reflects typical tectonic behavior in a subduction zone setting. The Aleutian Islands form part of the Pacific Ring of Fire, where the Pacific Plate converges with and subducts beneath the North American Plate along the Aleutian Trench at rates of approximately 6–7 cm per year. Such convergence generates frequent seismic activity, including both individual large earthquakes and episodic swarms driven by stress transfer along the megathrust interface and overlying crustal faults. Depths recorded in the swarm align with the expected range for intermediate-depth events in this arc system.

Historically, the region near Atka has experienced recurrent swarm activity. Since 1 January 2000, nine swarms have been documented in the vicinity. These occurred in 2008 (one swarm), 2010 (one swarm), 2013 (six swarms), and 2014 (one swarm). The 2015 event continues this pattern of clustered seismicity, which may indicate localized stress adjustments without immediate progression to a major rupture.

No significant volcanic signals were associated with this swarm, though the broader Atka volcanic complex lies nearby and remains monitored for potential interactions between tectonics and magmatism. The Aleutian arc hosts several active volcanoes, underscoring the interplay between subduction-driven earthquakes and volcanic processes.

Seismic swarms in this setting provide valuable data for understanding plate boundary dynamics and refining hazard assessments. Continued monitoring by regional networks supports improved forecasting of future activity in this high-seismicity zone.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
SeismoSight Internal Swarm Classification Database