Seismic Swarm S20140819.2: Analysis of Activity Near Nikolski, Alaska

Seismic swarm S20140819.2 occurred approximately 50 km southwest of Nikolski, Alaska, on Umnak Island in the Aleutian chain. The sequence began at 14:43 UTC on 18 August 2014 and concluded at 03:40 UTC on 20 August 2014, lasting 36 hours and 57 minutes. During this period, 43 earthquakes were recorded.

The events exhibited magnitudes between 1.5 and 4.7, with the largest shock reaching 4.7 at a depth of 5 km shortly after midnight on 19 August. Most events clustered at shallow depths of 3–8 km, though a few reached 12–13 km. Smaller events (magnitudes 1.5–2.5) dominated the sequence and occurred in rapid succession during the first 24 hours, followed by a gradual decline.

Nikolski lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate. Convergence rates average 6–7 cm per year, generating frequent shallow to intermediate-depth earthquakes and fueling the region’s active volcanoes. The local geology features volcanic arcs, fault systems, and sedimentary basins shaped by ongoing plate motion.

Earthquake swarms are recurrent in this setting, often linked to fluid migration or stress transfer along the megathrust interface. Historical records maintained by SeismoSight indicate seven swarms near Nikolski since 1 January 2000, occurring in 2007 (one swarm), 2010 (four swarms), and 2014 (two swarms). These episodes typically consist of numerous low-to-moderate magnitude events over hours to days without a single dominant mainshock.

The 2014 swarm’s shallow focal depths and tight spatial clustering align with typical Aleutian swarm characteristics, where brittle failure occurs within the overriding plate or along the plate interface. No damage or tsunami was reported, consistent with the modest magnitudes involved.

Continued monitoring by regional networks remains essential, given the Aleutian Arc’s capacity for larger subduction-zone events.

References
USGS Earthquake Hazards Program – Aleutian Arc tectonics
Alaska Earthquake Center – Regional seismicity reports
SeismoSight internal swarm classification database