Seismic Swarm S20100719.1 Near Nikolski, Alaska: Geological Context and Event Analysis

The Aleutian Islands form a classic volcanic arc resulting from the oblique subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent earthquakes and volcanism along the entire chain, including Umnak Island where Nikolski is located. The region lies within the tectonically active Aleutian subduction zone, part of the Pacific Ring of Fire, where the downgoing slab generates both interplate thrust events and intraslab seismicity at shallow to intermediate depths. Swarm S20100719.1 was recorded 48 km SSW of Nikolski. It initiated at 16:41 on 18 July 2010 and concluded at 03:05 on 5 August 2010, spanning 418 hours and 24 minutes during which 390 earthquakes were detected. Such swarms are characteristic of the Aleutian arc and may reflect fluid migration, stress triggering, or minor magmatic processes without culminating in a mainshock-aftershock sequence. Analysis of the first 100 events reveals predominantly low-to-moderate magnitudes and shallow focal depths. Magnitudes ranged from 1.9 to 4.7, with the majority between 2.0 and 3.0. The largest event reached magnitude 4.7 at 30 km depth on 19 July 2010 at 13:43:22. Depths were concentrated between 0 and 13 km for most events, consistent with upper-crustal brittle failure in the overriding plate. Temporal clustering was evident on 19–20 July, when over 60 events occurred within roughly 48 hours, followed by a gradual decline in rate. Historical records indicate that earthquake swarms in this sector of the Aleutians remain infrequent. Since 1 January 2000, only three swarms have been documented in the Nikolski area: one each in 2006, 2007, and 2010. This low recurrence underscores the episodic nature of swarm activity within an otherwise steadily deforming subduction margin. The 2010 swarm did not produce reported damage or trigger volcanic unrest at nearby volcanoes such as Mount Vsevidof or Mount Recheshnoi. Continued monitoring by regional seismic networks remains essential for distinguishing background subduction-related seismicity from potential precursory signals.

References

USGS Earthquake Catalog
Alaska Earthquake Center annual reports
SeismoSight internal swarm classification database