Seismic Swarm S20191209.1 Near Huslia, Alaska
Seismic swarm S20191209.1 was recorded northwest of Huslia in interior Alaska. The sequence began at 05:31 on 8 December 2019 and concluded at 12:37 on 20 December 2019. Over 295 hours and 5 minutes, 132 earthquakes were detected at a location 67 km northwest of Huslia.
Analysis of the first 100 events shows magnitudes between 0.8 and 3.3, with the majority falling between 1.0 and 1.6. Depths remained shallow, generally 0–15 km, consistent with crustal seismicity in the region. The largest event reached magnitude 3.3 at 21:17 on 10 December 2019 at 10 km depth. Several other notable shocks included magnitudes of 3.0 on 17 December and 2.6 on 11 December. Activity clustered in the initial days, with a gradual decline toward the end of the swarm period.
The Huslia area lies within the Yukon-Koyukuk geologic province of central Alaska. This region features Mesozoic sedimentary and volcanic rocks overlying older basement, shaped by accretionary processes along the North American margin. Modern seismicity arises from intraplate deformation linked to the distant Pacific plate subduction and strike-slip motion along regional fault systems. Shallow focal depths recorded in the swarm align with known patterns of brittle failure in the upper crust.
Since 1 January 2000, five earthquake swarms have occurred in the broader area. Swarm S20191209.1 represents the first such sequence documented in 2019. These episodic clusters differ from typical mainshock-aftershock sequences by lacking a dominant primary event and instead exhibiting diffuse, temporally concentrated activity.
Interior Alaska experiences moderate background seismicity, with historical records noting events capable of producing light shaking over wide areas. The 2019 swarm remained below thresholds for significant damage or felt reports in nearby communities. Ongoing monitoring by regional networks supports continued assessment of such activity.
References
SeismoSight internal swarm classification records
USGS Earthquake Catalog (Alaska region)
Alaska Earthquake Center annual summaries