Seismic Swarm S20191109.1: Analysis of Activity Northwest of Yakutat, Alaska

The seismic swarm designated S20191109.1 occurred in a tectonically active region 114 km northwest of Yakutat, Alaska, a coastal community situated along the Gulf of Alaska margin. This area lies at the boundary between the Pacific and North American plates, where oblique convergence and strike-slip motion dominate. The Yakutat terrane, a buoyant crustal fragment, has been colliding with the North American margin since the Miocene, contributing to rapid uplift of the Saint Elias Mountains and elevated seismic hazard. The Fairweather Fault system, a major right-lateral transform boundary, passes near this location and accommodates much of the Pacific Plate's northwestward motion at rates exceeding 4 cm per year.

Between 08:24 UTC on 8 November 2019 and 22:23 UTC on 13 November 2019, the swarm produced 59 earthquakes over 133 hours and 58 minutes. Event magnitudes ranged from 0.7 to 1.7, with the majority occurring at very shallow depths near 0 km and a smaller number recorded between 5 km and 12 km. The sequence exhibited typical swarm characteristics: a lack of a single dominant mainshock, gradual onset, and fluctuating activity without clear aftershock decay. Such patterns often reflect localized stress perturbations, possibly driven by fluid migration or aseismic slip along pre-existing faults within the Yakutat terrane or adjacent accretionary complex.

Historical records since 2000 indicate that only three prior swarms have been identified in the immediate vicinity, occurring in 2008, 2014, and 2015. Each earlier episode was limited in duration and event count, suggesting that swarm activity in this sector remains infrequent relative to the high background seismicity of southern Alaska. The 2019 swarm aligns with the region's long-term tectonic regime, where distributed shallow seismicity is common due to the complex interaction of thrust and strike-slip structures.

No events in the sequence exceeded magnitude 2.0, indicating low potential for significant ground shaking or secondary hazards such as landslides in this remote offshore-adjacent setting. Continued monitoring by regional networks remains essential, as the broader Saint Elias and Fairweather systems have hosted great earthquakes in the past, including events exceeding magnitude 8.0. Updated assessments of strain accumulation along these faults incorporate both geodetic and seismic data to refine probabilistic hazard models for nearby communities.

References

Alaska Earthquake Center, University of Alaska Fairbanks.
US Geological Survey Earthquake Hazards Program.
SeismoSight internal swarm classification records.