Seismic Swarm S20260115.3: Analysis of Activity 113 km North of Yakutat, Alaska

A seismic swarm designated S20260115.3 was recorded 113 km north of Yakutat, Alaska, from 00:22 on 15 January 2026 to 10:03 on 16 January 2026. Over 33 hours and 41 minutes, the sequence produced 34 earthquakes. Magnitudes ranged from 1.4 to 3.8, with the majority falling between 1.8 and 2.3. Depths remained shallow, concentrated between 1 km and 19 km, consistent with activity along upper-crustal structures.

The swarm began with modest events near magnitude 2.0–2.3 at depths of 7–10 km. Activity continued at a steady rate through 15 January, featuring repeated events of similar size and depth. The sequence intensified on 16 January, culminating in the largest shock of magnitude 3.8 at 3 km depth, followed by a magnitude 3.1 event at 5 km depth that marked the swarm’s conclusion.

This pattern reflects typical swarm behavior in the region, where clustered seismicity occurs without a single dominant mainshock. Shallow focal depths indicate slip on near-surface faults associated with regional compression. No events exceeded magnitude 4.0, limiting the likelihood of significant surface effects.

The Yakutat area occupies a tectonically complex zone at the transition between the Pacific Plate and the North American Plate. North of Yakutat, ongoing collision of the Yakutat terrane drives oblique convergence and crustal shortening within the Saint Elias orogen. This setting produces both strike-slip and thrust mechanisms, with frequent small-magnitude earthquakes distributed across a network of active faults. Historical records document large events, including the 1899 sequence that reached magnitudes near 8.0, underscoring the region’s capacity for substantial seismic release.

Since 1 January 2000, five swarms have been identified in the immediate vicinity. Earlier episodes occurred in 2025 (four swarms) and 2026 (one swarm prior to S20260115.3). These recurrent clusters highlight persistent low-level strain accumulation along the same structural trends.

Continued monitoring by regional networks remains essential for tracking any evolution in swarm characteristics or migration of activity. The present sequence aligns with established patterns of background seismicity in this highly active margin.

References
Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity reports.
USGS Earthquake Hazards Program – Tectonic framework of southern Alaska.
SeismoSight internal swarm classification database.