Seismic Swarm PS20150727.1: Analysis of Activity Near Nikolski, Alaska

On 27 July 2015, a seismic swarm designated PS20150727.1 was recorded approximately 98 km south-southwest of Nikolski, Alaska. The sequence began at 04:49 UTC and concluded at 08:27 UTC, encompassing eight earthquakes over a period of three hours and thirty-eight minutes. The largest event reached magnitude 6.9 at a depth of 29 km, followed closely by a magnitude 5.9 shock at 31 km depth. Subsequent events included magnitudes of 5.2 at 13 km, 5.7 at 27 km, 4.3 at 34 km, 5.5 at 35 km, 5.6 at 35 km, and a final magnitude 5.0 at 39 km depth. This tightly clustered activity reflects typical swarm behavior in subduction-zone settings, where multiple events occur without a single dominant mainshock-aftershock pattern.

The location lies within the central Aleutian Islands, part of the tectonically active Pacific Ring of Fire. Here, the Pacific Plate subducts northwestward beneath the North American Plate along the Aleutian Trench at rates of approximately 6–7 cm per year. This convergent margin produces frequent earthquakes and volcanic activity, with the region hosting numerous stratovolcanoes and a history of significant seismic releases. Historical records document major earthquakes in the Aleutians, including events exceeding magnitude 8 in the 20th century, underscoring the persistent tectonic stress accumulation and release along the megathrust interface.

Seismic swarms have been documented in this segment of the Aleutian arc since at least 2000. Between 1 January 2000 and the time of the 2015 swarm, nineteen such sequences were identified. The distribution across years shows notable concentrations in 2010 with seven swarms, followed by three each in 2007 and 2014. Single swarms occurred in 2006, 2008, 2009, 2011, 2012, and 2015. These episodes illustrate episodic clustering of seismicity that may relate to fluid migration or stress transfer within the subducting slab and overriding plate, although precise causative mechanisms require further geophysical investigation.

The 2015 swarm’s depth range, spanning 13–39 km, aligns with the expected seismogenic zone in this portion of the subduction interface. Shallower events near 13 km may involve the upper plate or shallow megathrust, while deeper shocks around 35–39 km likely occurred within the slab or at the plate interface. Such depth variation is consistent with the complex faulting geometry of the central Aleutians, where both thrust and normal-faulting mechanisms have been observed in prior sequences.

Continued monitoring by regional seismic networks remains essential for understanding swarm recurrence and potential links to larger tectonic events. The Aleutian subduction zone continues to exhibit elevated background seismicity, and sequences such as PS20150727.1 contribute valuable data for refining hazard assessments in remote island communities.

References USGS Earthquake Hazards Program catalog data Alaska Earthquake Center regional reports SeismoSight internal swarm classification records