Earthquake Swarm S20140717.1: Seismic Activity Near Yakutat, Alaska

The earthquake swarm designated S20140717.1 occurred 95 km NNW of Yakutat, Alaska, beginning at 13:09 on 16 July 2014 and concluding at 13:18 on 27 July 2014. Over 264 hours and 8 minutes, the sequence produced 524 earthquakes. This event cluster highlights the persistent tectonic stresses in southeastern Alaska, where the interaction between the Pacific and North American plates drives frequent seismicity. Yakutat lies within a complex tectonic zone influenced by the Yakutat terrane, a fragment of oceanic crust colliding with the North American margin. This collision contributes to rapid uplift in the adjacent St. Elias Mountains and activates major fault systems, including segments of the Fairweather fault. The region experiences both strike-slip and thrust faulting, with shallow crustal events common due to the shallow depth of the plate boundary interactions. Historical records show elevated seismic rates here compared to interior Alaska, consistent with ongoing convergence rates of several centimeters per year. Analysis of the first 100 events reveals a classic swarm pattern initiated by modest foreshocks. Early activity on 16 July included magnitudes between 0.9 and 3.1 at depths of 0–15 km. The sequence escalated with a magnitude 6.0 event on 17 July at 11:49:33, located at 10 km depth. This mainshock was followed immediately by a magnitude 4.8 aftershock at 11 km depth and numerous smaller events. Subsequent recordings show a rapid decline in magnitudes, with most events falling between 0.7 and 3.6 and depths predominantly 5–17 km. The distribution indicates a compact source volume, typical of fluid-influenced or stress-triggered sequences in this margin setting. Depths remained consistently shallow throughout the initial phase, rarely exceeding 19 km, aligning with the known brittle-ductile transition in the overriding plate. Magnitude progression after the peak event followed a standard decay, with events clustering tightly in time and space. Such patterns are characteristic of aftershock sequences in transform and convergent margins like those surrounding Yakutat. This swarm adds to the documented seismic history of the area, which includes larger events tied to the Queen Charlotte-Fairweather fault system. Continued monitoring supports improved hazard assessment for nearby communities and infrastructure exposed to both seismic shaking and secondary effects such as landslides in the rugged coastal terrain.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Alaska Earthquake Center (aeic.alaska.edu)
• SeismoSight internal swarm classification S20140717.1