Seismic Swarm S20170810.1 Near Nikolski, Alaska: Geological Context and Event Analysis

Swarm S20170810.1 occurred approximately 72 km south of Nikolski, Alaska, in the central Aleutian Islands. The sequence began at 04:29 on 9 August 2017 and concluded at 12:17 on 11 August 2017, spanning 55 hours and 48 minutes. During this period, 32 earthquakes were recorded, with magnitudes ranging from 1.6 to 5.5 and focal depths between 3 km and 48 km.

The swarm initiated with a magnitude 3.9 event at 26 km depth, followed within two hours by two magnitude 5.5 earthquakes at 29 km depth. Subsequent activity included a magnitude 4.8 event on 9 August at 24 km depth and a magnitude 3.4 event on 11 August at 34 km depth. Most events clustered between 20 km and 40 km depth, consistent with intermediate-depth seismicity in subduction environments. Smaller events (magnitudes below 3.0) dominated the later stages, indicating a typical decay in energy release.

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 setting produces frequent earthquakes and volcanic activity along the arc. The region’s geology features a complex assemblage of accreted terranes, active volcanoes, and fault systems that accommodate oblique convergence. Historical records document major earthquakes, including the 1957 magnitude 8.6 Andreanof Islands event and the 1965 magnitude 8.7 Rat Islands earthquake, both occurring along nearby segments of the megathrust.

Since 2000, eight earthquake swarms have been documented in the broader area, occurring in 2006 (one swarm), 2007 (one swarm), 2010 (four swarms), and 2015 (two swarms). These episodes reflect episodic stress release within the subducting slab or overlying crust, often without a single dominant mainshock. Swarm S20170810.1 aligns with this pattern, exhibiting clustered activity over a short timeframe rather than a classic foreshock–mainshock–aftershock sequence.

The Aleutian Islands remain one of the most seismically active regions on Earth. Ongoing monitoring by regional networks continues to track such swarms to improve understanding of subduction dynamics and associated hazards.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
SeismoSight internal swarm classification data