Seismic Swarm S20180725.1: Earthquake Activity Near Mammoth, Wyoming

Seismic swarm S20180725.1 was recorded 23 km SSW of Mammoth, Wyoming, beginning at 12:47 on 24 July 2018 and concluding at 08:50 on 27 July 2018. Over 68 hours and 3 minutes, the sequence produced 52 earthquakes. This event occurred within the tectonically active Yellowstone Plateau, a region shaped by Quaternary volcanism, rhyolitic caldera formation, and ongoing hydrothermal circulation.

The Yellowstone volcanic system lies above a mantle hotspot that has driven episodic caldera-forming eruptions over the past 2.1 million years. The most recent major event, approximately 631,000 years ago, created the present Yellowstone caldera. Post-caldera activity includes the extrusion of rhyolite flows and the development of an extensive hydrothermal network. Earthquake swarms in this setting commonly arise from fluid migration, thermal contraction, and minor stress perturbations rather than direct magma intrusion.

Within the swarm, event magnitudes ranged from 0.2 to 2.2, with the majority between 0.5 and 2.0. Focal depths clustered between 2 km and 9 km, consistent with the brittle-ductile transition zone above the shallow crustal magma reservoir. The largest events (magnitudes 2.0–2.2) occurred early in the sequence on 24 and 25 July, followed by a gradual decline in both rate and maximum magnitude. Depths remained stable, indicating no significant vertical migration of the causative process.

Swarm activity is characteristic of the Yellowstone region. Since 1 January 2000, 70 distinct swarms have been documented. Annual counts vary: 2000 recorded 10 swarms, 2008 and 2013–2014 each recorded 7, while several years produced only one. The 2018 season included two swarms, of which S20180725.1 was the second. These recurrent sequences reflect the interplay between regional extension along the Intermountain Seismic Belt and localized pressurization of the hydrothermal system.

No surface deformation or changes in hydrothermal discharge were associated with this swarm. The modest energy release and limited spatial extent align with background swarm behavior observed throughout the Yellowstone Plateau. Continued seismic monitoring remains essential for distinguishing routine hydrothermal swarms from those that may precede larger tectonic or volcanic unrest.

References

SeismoSight internal swarm catalog (S20180725.1 parameters and historical counts since 2000).
U.S. Geological Survey, Yellowstone Volcano Observatory geologic summaries (Yellowstone caldera history and swarm context).