Seismic Swarm S20140722.1: Analysis of Activity Near Atka, Alaska

Seismic swarm S20140722.1 occurred in the central Aleutian Islands, with its epicenter located 119 km south-southwest of Atka, Alaska. The sequence began at 19:36 UTC on 21 July 2014 and concluded at 22:14 UTC on 23 July 2014, spanning 50 hours and 37 minutes. During this period, 39 earthquakes were recorded, with magnitudes ranging from 1.5 to 5.1 and focal depths between 5 km and 45 km.

This swarm unfolded within the tectonically active 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. The resulting compressional stresses generate frequent seismicity along the megathrust interface and within the overriding plate. The region surrounding Atka Island forms part of the Aleutian volcanic arc, characterized by stratovolcanoes such as Korovin and Kliuchef, which are directly linked to ongoing subduction-driven magmatism.

The sequence initiated with two moderate events: a magnitude 4.7 earthquake at 45 km depth followed one minute later by a magnitude 5.1 event at 25 km depth. Subsequent activity included multiple events above magnitude 4.0, notably a magnitude 4.6 at 39 km and a magnitude 4.7 at 33 km, all within the first several hours. Shallower events, some as shallow as 5–8 km, occurred intermittently, suggesting possible involvement of crustal faulting above the main subduction interface. The swarm exhibited a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock-aftershock sequence.

The Aleutian Islands have a well-documented history of seismic swarms driven by both tectonic and volcanic processes. Since 2000, seven such swarms have been identified in the broader region, with notable occurrences in 2008 and 2010, followed by five in 2013. These episodes reflect episodic stress release along the subduction margin, often concentrated near volcanic centers or along transverse faults.

No surface rupture or significant tsunami was associated with this swarm, consistent with its moderate magnitudes and offshore location. Monitoring by regional seismic networks continues to track background seismicity in the arc, providing context for future events.

References

USGS Earthquake Hazards Program
Alaska Earthquake Center
Global Volcanism Program, Smithsonian Institution