Seismic Swarm S20140802.1: Analysis of Earthquake Activity Near Nikolski, Alaska

Seismic swarm S20140802.1 was recorded 41 km west-southwest of Nikolski, Alaska, on Umnak Island in the Aleutian archipelago. The sequence began at 17:26 on 1 August 2014 and concluded at 23:08 on 4 August 2014, encompassing 36 earthquakes over 77 hours and 41 minutes. Magnitudes ranged from 1.6 to 3.3, with the majority occurring at shallow depths between 0 and 14 km. The largest event reached magnitude 3.3 at 5 km depth shortly after initiation, followed by additional events of magnitude 3.2 at similar depths later on 1 August. Activity showed clustering in the first 24 hours, with 12 events exceeding magnitude 2.0, then gradually declined through 2–4 August.

The swarm exhibited typical characteristics of low-magnitude, shallow seismicity without a dominant mainshock. Depths predominantly fell between 1 and 10 km, consistent with crustal processes in an active tectonic setting. No events exceeded magnitude 4.0, and the sequence remained confined to a compact spatial area.

Nikolski lies within the 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 tectonic regime produces frequent earthquakes and volcanism along the 2,500 km island arc. Umnak Island hosts several volcanoes, including Mount Recheshnoi and Mount Vsevidof, reflecting ongoing magmatic and hydrothermal activity that can influence local seismicity. Historical records document numerous earthquake swarms in the central Aleutians, often linked to fluid migration or stress adjustments along the plate interface and overlying crust.

Since 2000, seven swarms have occurred in the broader region, with prior episodes in 2007 (one swarm) and 2010 (six swarms). These events underscore the persistent seismic productivity of the area, though individual swarms vary in duration and event count. The 2014 sequence aligns with this pattern of episodic, moderate activity rather than large-magnitude mainshock-aftershock sequences.

Regional monitoring by the Alaska Earthquake Center and USGS confirms the Aleutians as one of the most seismically active zones globally, with the potential for both moderate swarms and great earthquakes exceeding magnitude 8. Updated tectonic models emphasize the role of slab dehydration and crustal faulting in generating shallow swarms like S20140802.1.

References

USGS Earthquake Hazards Program – Aleutian Arc Tectonics
Alaska Earthquake Center – Regional Seismicity Reports
NOAA – Aleutian Islands Geological Overview