Seismic Swarm S20140624.2 Near Attu Station, Alaska: Geological Context and Event Analysis

The seismic swarm designated S20140624.2 occurred approximately 240 km east-southeast of Attu Station in Alaska's western Aleutian Islands. The sequence began at 03:14 UTC on 24 June 2014 and concluded at 22:07 UTC the same day, spanning 18 hours and 53 minutes. During this period, 33 earthquakes were recorded, with magnitudes ranging from 1.9 to 4.7 and focal depths between 0 and 31 km.

This activity took place within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–8 cm per year. The western Aleutians feature a complex transition from subduction to strike-slip faulting near the Commander Islands, contributing to elevated seismicity. Attu Station lies near the edge of this transition, where the subduction angle steepens and the slab experiences significant bending stresses.

The swarm's temporal distribution showed clustering in the first several hours, with the largest events occurring early: a magnitude 4.6 earthquake at 03:19:29 (depth 9 km) followed by a magnitude 4.7 at 03:24:41 (depth 6 km). Subsequent notable events included magnitudes 4.3 (03:27:51, 31 km), 4.0 (03:55:33, 15 km), and 4.1 (04:45:44, 4 km). Later activity consisted primarily of events below magnitude 3.2, tapering off by evening.

Focal depths varied widely, indicating involvement of both shallow crustal faults and intermediate-depth structures within the subducting slab. Shallower events (under 10 km) likely reflect brittle failure in the overriding plate or upper slab, while deeper events (20–31 km) align with intraslab seismicity common in the Aleutian arc.

Historically, the region experiences infrequent swarms. Since 1 January 2000, only one prior swarm has been documented in the immediate vicinity, occurring in 2006. This rarity underscores the episodic nature of clustered seismicity amid the background of regular subduction-related earthquakes.

The Aleutian Islands arc has a long record of significant seismic events, including great megathrust earthquakes that have repeatedly affected the central and eastern portions of the chain. Western segments, however, tend toward more moderate and distributed activity due to the obliquity of convergence. No damage or tsunami reports were associated with swarm S20140624.2, consistent with its moderate magnitudes and offshore location.

Ongoing monitoring by regional seismic networks continues to track background rates in this remote area, where volcanic and tectonic processes interact. Such swarms provide valuable data on stress transfer and fault interactions within the subduction environment.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (Aleutian region tectonics summaries)
Alaska Earthquake Center regional reports