Seismic Swarm S20140813.1: Analysis of Activity Near Nikolski, Alaska

The earthquake swarm designated S20140813.1 occurred approximately 46 km west-southwest of Nikolski on Umnak Island in the Aleutian Islands of Alaska. This sequence began at 18:27 on 12 August 2014 and concluded at 09:11 on 15 August 2014, spanning 62 hours and 43 minutes. During this period, 53 earthquakes were recorded, with magnitudes ranging from 1.6 to 4.5 and focal depths primarily between 1 km and 21 km.

The swarm's temporal distribution showed an initial cluster of events on 12 August, followed by heightened activity on 13 August that included several events above magnitude 3.0. Notable shocks included a magnitude 4.2 earthquake at 13:19 on 13 August at 21 km depth and a magnitude 4.5 event at 13:57 the same day at 10 km depth. Additional peaks occurred with a magnitude 4.1 shock at 16:00 on 13 August at 15 km depth and a magnitude 3.6 event at 03:31 on 15 August at 4 km depth. Most events remained shallow, consistent with activity along crustal faults in the region.

Nikolski lies 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. This setting produces frequent seismic events, including both individual large earthquakes and clustered swarms. The area's volcanic arc geology, featuring stratovolcanoes and associated fault systems, further contributes to localized seismicity at shallow to intermediate depths.

Historical records since 1 January 2000 indicate six swarms in the immediate vicinity. Earlier episodes occurred in 2007 (one swarm) and 2010 (four swarms), with the 2014 sequence representing the fifth and final documented swarm in this interval. These recurring clusters suggest episodic strain release along segments of the subduction interface and overlying crustal structures.

Such swarms typically reflect fluid migration or aseismic slip triggering successive failures rather than a single mainshock-aftershock sequence. The 2014 events exhibited a maximum magnitude of 4.5 and an absence of events exceeding magnitude 5.0, aligning with patterns observed in prior swarms in this sector of the Aleutians.

References

SeismoSight internal classification data for swarm S20140813.1
USGS Earthquake Hazards Program regional tectonic summaries for the Aleutian Islands