Seismic Swarm S20050211.1: Analysis of Earthquake Activity Near Yakutat, Alaska

Seismic swarm S20050211.1 occurred 64 km NNE of Yakutat, Alaska, beginning at 04:15 on 11 February 2005 and concluding at 11:13 on 13 February 2005. Over this period of 54 hours and 57 minutes, the swarm comprised 44 earthquakes. Depths ranged primarily between 6 km and 17 km, with the majority clustered around 10–14 km. Magnitudes remained modest overall, though two events exceeded magnitude 5.0.

The sequence initiated with a magnitude 2.1 event at 10 km depth. Activity intensified later on 11 February, featuring a magnitude 5.4 earthquake at 15 km depth at 21:00:23, followed within minutes by several smaller events between magnitudes 1.7 and 2.7. A magnitude 5.0 event at 11 km depth occurred at 21:29:33. Subsequent activity included a magnitude 3.6 shock at 13 km depth at 21:59:44 and a magnitude 3.0 event at 7 km depth at 21:33:16. On 12 February, events stayed below magnitude 3.0, with the largest reaching 2.7 at 12 km depth. The final recorded event on 13 February was a magnitude 1.6 shock at 15 km depth.

This temporal clustering, with the bulk of events on the first day and a rapid decline thereafter, is characteristic of swarm behavior rather than a typical mainshock-aftershock sequence. Depths indicate shallow crustal sources consistent with regional fault systems.

Yakutat lies within the tectonically complex transition zone where the Yakutat terrane collides with the North American plate. This interaction produces a combination of thrust faulting and strike-slip motion along the Fairweather Fault system and associated structures. The area forms part of the St. Elias orogen, where ongoing convergence accommodates several centimeters of relative plate motion annually. Historical records document major earthquakes in the Yakutat region, including the 1899 sequence of events with magnitudes estimated near 8.0 that generated significant coastal uplift and tsunamis.

Modern seismic monitoring by the Alaska Earthquake Center and the U.S. Geological Survey confirms persistent low-to-moderate seismicity in this sector. Updated catalogs through 2023 show recurrent swarms and isolated events along the same structural trends, reflecting continued strain accumulation at the plate boundary. Depths observed in S20050211.1 align with the typical brittle-ductile transition zone in this part of southern Alaska.

The swarm provides insight into episodic strain release along secondary faults within the broader Yakutat collision zone. Such activity helps delineate active structures that may participate in future larger earthquakes. Continued instrumentation and real-time analysis remain essential for refining hazard assessments in this high-seismicity corridor.

References
Alaska Earthquake Center, University of Alaska Fairbanks.
U.S. Geological Survey Earthquake Hazards Program.
Plafker, G., et al. (various publications on Yakutat terrane tectonics).