Seismic Swarm S20201021.2: Analysis of Activity Near Sand Point, Alaska

The seismic swarm designated S20201021.2 was recorded 97 km south-southeast of Sand Point, Alaska. Activity began at 03:15 on 20 October 2020 and concluded at 06:46 on 22 October 2020, spanning 51 hours and 30 minutes during which 40 earthquakes were detected.

The sequence featured a range of magnitudes, with the largest event reaching 4.8 at a depth of 28 km on 20 October at 08:37. Most events clustered between magnitudes 2.0 and 3.3, occurring at depths predominantly between 20 and 25 km. Shallower events, such as one at 0 km depth, were also noted, alongside a few deeper occurrences up to 32 km. The temporal distribution showed peak activity in the initial hours, followed by a gradual decline, with the final recorded event at magnitude 2.5 on 22 October.

This swarm unfolded in the tectonically dynamic Aleutian Islands region, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. The subduction process drives frequent seismicity through thrust faulting and associated deformation. Historical records indicate that since 1 January 2000, four swarms have occurred in the area, with S20201021.2 representing the first such sequence. The swarm followed closely after the magnitude 7.6 Sand Point earthquake of 19 October 2020, whose epicenter lay approximately 14 km from the swarm centroid, suggesting a possible link to post-mainshock stress redistribution.

Geological context reveals that the region's crust accommodates ongoing convergence at rates exceeding 6 cm per year, fostering both large megathrust events and smaller clustered sequences. Depths observed in the swarm align with typical intermediate-depth activity within the subducting slab and overlying plate interface. Such patterns contribute to the broader understanding of aftershock productivity and swarm behavior in subduction settings.

Data for this analysis derive from the SeismoSight internal classification system. Additional geological framework draws from established records of Aleutian subduction zone dynamics maintained by the U.S. Geological Survey.