Seismic Swarm S20130905.1: Analysis of Earthquake Activity Near Atka, Alaska

A seismic swarm designated S20130905.1 occurred 93 km SSW of Atka, Alaska, from 02:41 on 4 September 2013 to 09:34 on 10 September 2013. Over 150 hours and 53 minutes, the swarm comprised 96 earthquakes, with magnitudes ranging from 1.5 to 4.8 and depths primarily between 0 and 40 km.

The sequence began with a magnitude 3.5 event at 25 km depth, followed rapidly by stronger shocks including magnitudes 4.1, 4.6, and 4.1 within the first hour. Subsequent activity featured clusters of moderate events, such as a magnitude 4.8 at 30 km depth on 4 September and multiple magnitude 4+ events on 5 September at depths from 2 to 26 km. Later days showed declining frequency and intensity, with the final recorded event a magnitude 1.5 at 20 km depth on 10 September. Depths varied widely, indicating activity across both crustal and deeper fault zones typical of subduction settings.

This swarm aligns with the tectonic framework of the Aleutian Islands, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. The resulting compressional stress produces frequent earthquakes and volcanic activity throughout the arc. Atka lies within this highly active margin, characterized by intermediate-depth seismicity and shallow crustal events associated with volcanic systems.

Historical records indicate four seismic swarms in the region since 2000, with two occurring in 2008 and two in 2013. Such episodic clustering reflects periodic stress release along the subduction interface rather than isolated mainshock-aftershock sequences.

The Aleutian volcanic arc has experienced significant seismic and eruptive events throughout recorded history, underscoring the dynamic nature of the plate boundary. Ongoing monitoring by regional networks supports improved understanding of swarm patterns and their relation to long-term tectonic deformation.

References

USGS Earthquake Catalog Alaska Earthquake Center reports