Seismic Swarm SVS20181231.1: Analysis of December 2018 Activity in Wyoming

A notable seismic swarm, designated SVS20181231.1, occurred in Wyoming on 31 December 2018. The sequence began at 07:10 UTC and concluded at 11:16 UTC, spanning 4 hours and 5 minutes. During this interval, 57 earthquakes were recorded, with magnitudes ranging from -0.1 to 2.7 and focal depths primarily between 1 and 6 km. One event reached a depth of 16 km. The largest shocks measured 2.7, occurring at 07:25:45 and 10:21:22 UTC.

This swarm exhibited classic characteristics of clustered microseismicity, with events concentrated in the initial 30 minutes followed by sporadic activity. Such patterns often reflect fluid migration or stress redistribution along fault networks rather than a single mainshock-aftershock sequence. Magnitudes remained low overall, with only six events exceeding magnitude 2.0, indicating limited potential for felt shaking or structural damage.

Wyoming occupies a tectonically active portion of the western United States within the Intermountain Seismic Belt. This zone experiences distributed extensional deformation linked to the broader Basin and Range province and the influence of the Yellowstone hotspot. Historical seismicity includes both isolated events and episodic swarms driven by volcanic, hydrothermal, or tectonic processes. The region's crust features numerous normal faults that accommodate ongoing extension at rates of several millimeters per year.

Since 1 January 2000, 36 seismic swarms have been documented in Wyoming. Annual counts vary, with peaks in 2002 (6 swarms) and 2009 (5 swarms). The years 2000, 2013, and 2014 each recorded 4 swarms, while multiple years registered only one. The 2018 total reached 2, including SVS20181231.1. These statistics underscore the recurrent nature of swarm activity in the state, providing context for interpreting individual sequences like the December 2018 event.

Seismic monitoring in Wyoming relies on regional networks operated by the U.S. Geological Survey and university partners. Data from such networks enable precise location and magnitude determination, supporting hazard assessment in this intraplate setting. Although individual swarms rarely produce significant ground motion, their cumulative occurrence informs long-term probabilistic models for the Intermountain region.

References

U.S. Geological Survey Earthquake Hazards Program reports on Intermountain Seismic Belt tectonics.
Wyoming Geological Survey summaries of regional fault systems and historical seismicity.
SeismoSight internal swarm classification records for SVS20181231.1 parameters and historical counts.