Seismic Insights into Swarm S20120912.1 Near Nikolski, Alaska

An earthquake swarm designated S20120912.1 was recorded southeast of Nikolski, Alaska, on Umnak Island in the central Aleutian arc. The sequence began at 16:42 on 11 September 2012 and concluded at 04:22 on 12 September 2012, spanning 11 hours and 40 minutes. During this interval, 25 earthquakes were registered, with magnitudes ranging from 1.7 to 4.1 and focal depths between 5 km and 50 km.

The events clustered in two main phases. The initial phase, lasting roughly the first two hours, included the largest shock of magnitude 4.1 at 19 km depth, followed by events of 3.3, 3.1, and 2.8. Depths in this phase were predominantly 19–41 km. A later phase after 21:00 featured additional events of magnitude 3.2 and 3.4, with one notably shallow event at 5 km depth. Subsequent activity tapered to smaller magnitudes below 2.5 as the swarm ended.

This swarm occurred within the tectonically active Aleutian subduction zone, where the Pacific plate converges with the North American plate at rates of approximately 6–7 cm per year. The central Aleutians host frequent seismic swarms linked to both megathrust slip and upper-plate volcanic processes. Umnak Island lies near several stratovolcanoes, including Mount Vsevidof and Mount Recheshnoi, whose magmatic systems can influence local seismicity through fluid migration or stress changes.

Historical records indicate limited swarm activity in the region since 2000. Only two prior swarms have been identified: one in 2007 and another in 2009. The 2012 sequence therefore represents a relatively infrequent cluster, consistent with episodic release of accumulated strain along the arc.

Swarm characteristics, such as the absence of a single dominant mainshock and the tight spatiotemporal clustering, align with patterns observed in subduction-zone environments where small stress perturbations trigger multiple similar-sized events. Depths extending to 50 km suggest involvement of both crustal and upper-mantle structures typical of the Aleutian slab interface.

• SeismoSight internal catalog for swarm parameters and timeline.
• USGS Earthquake Hazards Program for regional tectonic framework and Aleutian arc convergence rates.