Earthquake Swarm S20160328.1: Seismic Activity 97 km SSE of Nikolski, Alaska

An earthquake swarm designated S20160328.1 occurred in the Aleutian Islands region of Alaska, centered approximately 97 km south-southeast of Nikolski on Umnak Island. The sequence began at 18:04 UTC on 27 March 2016 and concluded at 13:51 UTC on 29 March 2016, spanning 43 hours and 46 minutes. During this period, 33 earthquakes were recorded, with magnitudes ranging from 2.4 to 4.8 and focal depths between 8 km and 41 km.

The swarm exhibited a typical pattern of clustered seismic events without a single dominant mainshock. Initial activity included events of magnitude 3.8 at 20 km depth and 4.5 at 10 km depth within the first minutes. Peak magnitude reached 4.8 at 12 km depth shortly after onset. Subsequent events maintained moderate intensities, with several additional shocks of magnitude 4.0–4.5 occurring through 28 and 29 March. Depths showed variability, clustering most frequently around 10–25 km, consistent with crustal and upper-mantle processes in the subduction environment.

This region 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. The resulting megathrust interface generates frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. Nikolski and surrounding islands sit along the volcanic arc associated with this subduction, part of the Pacific Ring of Fire. Historical records document repeated earthquake swarms in the vicinity, with four documented episodes since 2000: one each in 2006, 2007, 2009, and 2015. These swarms reflect episodic stress release along fault segments influenced by plate coupling and fluid migration.

Geological mapping of the area reveals a complex assemblage of volcanic and sedimentary rocks overlying the subduction-related basement. Umnak Island itself hosts active volcanoes such as Mount Recheshnoi and Okmok Caldera, underscoring the interplay between magmatism and seismicity. Depths recorded during the 2016 swarm align with expected ranges for both interplate thrust events and intraslab normal or strike-slip faulting.

Analysis of swarm statistics indicates a relatively short duration and moderate energy release compared with larger regional sequences. No significant surface rupture or tsunami generation was associated with these events, consistent with their maximum magnitude remaining below 5.0. Continued monitoring by regional seismic networks remains essential given the area's high seismic hazard rating.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (historical swarm catalog)
• USGS Earthquake Hazards Program (Aleutian subduction zone parameters)
• SeismoSight internal swarm classification database (S20160328.1 event list)