Seismic Swarm S20251206.1: Analysis of Activity Near Yakutat, Alaska

A significant seismic swarm designated S20251206.1 occurred 93 km north-northeast of Yakutat, Alaska, beginning at 20:41 UTC on 6 December 2025 and concluding at 18:50 UTC on 28 December 2025. Over 526 hours and 9 minutes, the sequence produced 2086 earthquakes. This event provides insight into the region's ongoing tectonic processes within the St. Elias orogen.

The swarm initiated with a magnitude 7.0 earthquake at 10 km depth, followed by a rapid succession of aftershocks. The first 100 events exhibited magnitudes ranging from 7.0 down to 2.1, with the majority clustered between 2.5 and 4.5. Depths were predominantly shallow, centered around 5 km, though several events occurred at 1–2 km and a few extended to 14 km. Early activity included multiple events above magnitude 5.0 within the first hour, transitioning to a sustained sequence of smaller quakes. This pattern reflects typical swarm behavior in convergent margins, where fluid migration or stress transfer sustains elevated seismicity without a single dominant mainshock-aftershock decay.

Yakutat lies at the boundary where the Yakutat terrane, a buoyant oceanic plateau, collides with the North American plate. This interaction drives the uplift of the St. Elias Mountains and produces frequent earthquakes along thrust and strike-slip faults. The Pacific Plate subducts northwestward beneath Alaska at rates exceeding 5 cm per year, contributing to the region's high seismic hazard. Historical records document recurrent moderate-to-large events, underscoring the area's status as one of North America's most active seismic zones.

Prior to S20251206.1, the only swarm recorded in the region since 1 January 2000 occurred in 2014. That earlier sequence similarly featured shallow crustal events linked to the same tectonic regime. Both swarms highlight episodic strain release along the eastern edge of the Yakutat collision zone.

Geological studies of the St. Elias orogen indicate that the current deformation accommodates convergence through a combination of thrust faulting and strike-slip motion. The shallow depths observed in S20251206.1 align with known seismogenic zones in the upper crust, where sedimentary and volcanic rocks overlie deeper metamorphic basement. Ongoing GPS measurements confirm continued northwestward motion of the Yakutat block, sustaining the potential for future seismic sequences.

This swarm underscores the importance of continuous monitoring in southeastern Alaska. Improved understanding of swarm dynamics aids in refining seismic hazard assessments for nearby communities and infrastructure.

References

USGS Earthquake Catalog Alaska Earthquake Center Annual Reports Geological Society of America Bulletin on St. Elias Tectonics