Seismic Swarm S20130906.1: Analysis of Activity South of Atka, Alaska

Seismic swarm S20130906.1 occurred approximately 89 km south of Atka, Alaska, in the central Aleutian Islands. The sequence began at 07:26 UTC on 5 September 2013 and concluded at 16:09 UTC on 7 September 2013, spanning 56 hours and 43 minutes. During this interval, 36 earthquakes were recorded, with magnitudes ranging from 1.4 to 4.9 and focal depths between 3 km and 43 km.

The swarm exhibited a clustered temporal distribution, with the majority of events concentrated in the first 24 hours. Notable events included a magnitude 4.9 earthquake at 26 km depth on 5 September at 07:44 UTC, followed by a magnitude 4.6 event at 3 km depth later that morning. Subsequent activity featured additional events of magnitude 4.6 and 4.0 on 6 September, culminating in a magnitude 4.8 shock at 26 km depth on 7 September. Depths showed significant variation, indicating activity across both shallow crustal levels and intermediate depths consistent with the regional tectonic setting.

This sequence aligns with patterns observed in prior swarms in the area since 2000, when five such episodes have been documented. Earlier swarms occurred in 2008 (two events) and 2013 (three events total, including the present sequence). These episodes reflect episodic strain release without a dominant mainshock-aftershock signature typical of larger tectonic events.

The location lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic framework generates the Aleutian volcanic arc and produces frequent seismicity, including both shallow thrust events and deeper intraslab earthquakes. The central Aleutians near Atka feature active volcanoes such as Korovin and Kliuchef, underscoring the coupled volcanic-seismic character of the region. Historical records document major earthquakes, including the 1957 magnitude 8.6 event and the 2006 magnitude 8.3 earthquake, both occurring along nearby segments of the megathrust.

Seismic swarms in this setting often arise from fluid migration, stress triggering, or localized slip along faults within the overriding plate or subducting slab. Depths exceeding 20 km in several events of swarm S20130906.1 are consistent with intraslab deformation, while shallower activity may relate to upper-plate faulting. No surface rupture or significant volcanic unrest was associated with this swarm.

Ongoing monitoring by regional networks continues to track background seismicity in the Aleutians, where annual earthquake rates remain elevated compared with most continental interiors. Updated catalogs confirm that subduction-related processes sustain the potential for both moderate swarms and infrequent great earthquakes along the arc.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks (aeic.alaska.edu)
USGS Tectonic Summary for the Aleutian Islands (pubs.usgs.gov)