Seismic Swarm S20251208.3: Analysis of Activity Near Yakutat, Alaska

A seismic swarm designated S20251208.3 was recorded 103 km north of Yakutat, Alaska, beginning at 19:38 on 7 December 2025 and concluding at 06:43 on 10 December 2025. Over 59 hours and 5 minutes, 139 earthquakes were detected. Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 13 km, with the majority clustered at 5 km or less. Magnitudes ranged from 2.1 to 5.8, indicating low-to-moderate energy release consistent with swarm behavior rather than a single mainshock-aftershock sequence. The largest event reached magnitude 5.8 at 10 km depth on 8 December 2025 at 00:56:50 UTC. Subsequent activity included multiple events above magnitude 3.0, such as a 4.1 at 5 km depth on 9 December and a 4.0 at 5 km on the same day. Depths remained consistently crustal, suggesting activation along shallow fault structures within the overriding plate. This swarm occurred one day after the magnitude 7.0 Hubbard Glacier Earthquake on 6 December 2025, whose epicenter lay only 11 km from the swarm centroid. The temporal and spatial proximity points to possible stress triggering, where the mainshock may have altered local fault conditions and promoted the subsequent swarm. Since 1 January 2000, only one prior swarm has been documented in the immediate region, also occurring in 2025. This scarcity underscores the episodic nature of swarm activity amid the area's background seismicity. The Yakutat region lies within the St. Elias orogen, where the Yakutat microplate collides with and partially subducts beneath the North American plate. This tectonic interaction drives rapid uplift of the Chugach–St. Elias Mountains and produces a network of thrust and strike-slip faults. Shallow seismicity is common due to the thin, fractured crust and high strain rates. Historical records show the area capable of generating great earthquakes, with the recent Hubbard Glacier event exemplifying ongoing plate-boundary deformation. References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (earthquake.alaska.edu)
SeismoSight internal swarm classification S20251208.3