Seismic Swarm S20241209.1: Analysis of Activity Southwest of Adak, Alaska

Seismic swarm S20241209.1 occurred approximately 123 km south-southwest of Adak in the Aleutian Islands, Alaska. The sequence began at 20:03 UTC on 8 December 2024 and concluded at 10:19 UTC on 12 December 2024, spanning 86 hours and 16 minutes. During this period, 84 earthquakes were recorded, with the majority exhibiting focal depths near 10 km.

The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity and volcanism characteristic of the Pacific Ring of Fire. Adak itself occupies a position on the Andreanof Islands segment of the arc, where historical megathrust events have exceeded magnitude 8. Depths of most swarm events align with the upper portion of the subducting slab or overlying crust, consistent with typical intraslab or interface activity in the region.

The swarm featured several notable events. An initial cluster on 8 December included a magnitude 4.4 earthquake at 20:09 UTC followed by a magnitude 5.8 event at 21:02 UTC. Activity intensified overnight, culminating in a magnitude 6.1 earthquake at 00:38 UTC on 9 December. Subsequent strong shocks on 9 December reached magnitudes 5.7, 5.6, 5.2, and 5.1, with the largest aftershocks clustered between 03:26 and 05:56 UTC. Later events through 12 December remained below magnitude 4.0, marking a gradual decline in energy release.

Event depths remained predominantly shallow, between 9 and 25 km, with isolated occurrences extending to 40 km. This distribution suggests involvement of both crustal and shallow slab structures without significant migration to greater depths.

Historical records indicate four comparable swarms in the broader region since 2000. These occurred in 2008 (one swarm), 2017 (two swarms), and one additional swarm earlier in 2024. Such episodic clustering is not uncommon in subduction zones and may reflect stress transfer along the plate interface or fluid migration within the overriding crust.

The swarm’s temporal evolution—marked by an initial high-magnitude burst followed by sustained moderate activity—provides insight into typical swarm dynamics in the central Aleutians. Continued monitoring remains essential given the area’s capacity for larger tectonic events.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification S20241209.1