Earthquake Swarm SVS20230417.1: Seismic Activity in Wyoming

An earthquake swarm designated SVS20230417.1 occurred in Wyoming between 13:33 on 16 April 2023 and 09:47 on 18 April 2023. Over 44 hours and 14 minutes, the sequence produced 38 events with magnitudes ranging from 0.1 to 2.0 and focal depths between 1 and 4 km. The events clustered tightly in time, with peak activity on 16 and 17 April, including multiple events exceeding magnitude 1.5. Such swarms lack a dominant mainshock and instead feature numerous events of similar size, a pattern consistent with fluid migration or localized stress adjustments in the crust.

The swarm began with an initial magnitude 0.9 event at 4 km depth, followed rapidly by a magnitude 1.5 shock. Subsequent activity included several magnitude 1.9–2.0 events at shallow depths of 3–4 km. Later stages showed a decline in both frequency and magnitude, concluding with isolated events near magnitude 1.2. Depths remained predominantly shallow throughout, indicating activity within the upper crust.

Wyoming lies within the Intermountain Seismic Belt, a region of distributed extensional tectonics stretching from Montana through Utah. The state's seismicity arises from Basin and Range-style normal faulting and interactions with the Yellowstone hotspot to the northwest. Historical records document recurrent swarms, with 53 documented episodes since 1 January 2000. Annual counts peaked in 2002 and 2006 (six each), followed by elevated activity in 2013 and 2021 (five each). The 2023 swarm marks the first recorded instance that year, continuing a pattern of episodic, low-magnitude clustering rather than large single ruptures.

Geological context indicates that shallow swarm activity often correlates with hydrothermal or magmatic influences near the Snake River Plain transition zone, though precise source mechanisms require further network analysis. Depths of 1–4 km align with brittle failure above the brittle-ductile transition in this tectonic setting. No damage or felt reports were associated with the low-energy events.

Continued monitoring remains essential given Wyoming's position in an active seismic province. Future swarms may follow similar temporal distributions, underscoring the value of dense seismic networks for early detection.

References
USGS Earthquake Hazards Program – Wyoming seismicity summaries
Wyoming State Geological Survey – Regional tectonic framework reports
SeismoSight internal swarm classification database