Seismic Swarm S20130904.1: Analysis of Activity Near Atka, Alaska

Seismic swarm S20130904.1 occurred 95 km south-southwest of Atka, Alaska, in the central Aleutian Islands. The sequence began at 02:43 UTC on 4 September 2013 and concluded at 13:34 UTC on 5 September 2013, spanning 34 hours and 50 minutes. During this interval, 57 earthquakes were recorded.

The swarm exhibited a typical pattern for Aleutian sequences, with the majority of events clustered in the first 12 hours. Magnitudes ranged from 1.5 to 4.8, with four events exceeding magnitude 4.0. The largest shock reached 4.8 at 03:43 on 4 September at a depth of 10 km. Depths varied between 1 km and 39 km, indicating activity within both the overriding plate and the subducting slab interface.

Notable events included a magnitude 4.7 doublet at 08:16 on 4 September, separated by only six seconds, followed by a magnitude 4.1 at 08:22. Later activity tapered, with the final event of magnitude 1.5 recorded at 13:34 on 5 September. Shallow events (depths under 10 km) were infrequent and generally small, while mid-crustal depths around 25 km hosted many of the moderate shocks.

The Aleutian arc forms where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin produces frequent earthquakes along the megathrust, within the downgoing slab, and in the overlying volcanic arc. Atka Island lies near the intersection of the arc and several transverse faults, contributing to distributed seismicity.

Since 2000, six swarms have been identified in the immediate region. Earlier episodes occurred in 2008 and 2010, while four took place in 2013, underscoring episodic clustering rather than steady background rates. Such swarms are commonly linked to fluid migration or stress transfer along the subduction interface and do not typically culminate in great earthquakes.

No surface rupture or volcanic unrest was associated with this sequence. The event distribution aligns with known Wadati-Benioff zone geometry beneath the central Aleutians, where intermediate-depth earthquakes reflect dehydration and phase changes within the subducting Pacific Plate.

References
USGS Earthquake Hazards Program – Aleutian Arc tectonics and historic seismicity.
Alaska Earthquake Center – Regional monitoring data and swarm documentation.
Global CMT Catalog – Focal mechanisms and depth constraints for Aleutian events.