Seismic Swarm SVS20190723.1: Analysis of Activity Near West Yellowstone, Montana

The region 29 km east-southeast of West Yellowstone, Montana, lies within the seismically active Yellowstone volcanic system. This area experiences frequent earthquake swarms driven by magmatic and hydrothermal processes beneath the Yellowstone caldera. The caldera formed approximately 631,000 years ago during the most recent supereruption and remains a dynamic geological feature characterized by ongoing uplift, subsidence, and fluid migration.

Earthquake swarms in this zone typically consist of numerous small-magnitude events clustered in time and space, often without a single dominant mainshock. Such activity reflects stress changes induced by the movement of magma or pressurized hydrothermal fluids. Historical records since 2000 indicate 62 swarms have occurred in the broader area, with notable yearly counts including 9 in 2002, 8 in 2000, and 7 in both 2006 and 2018. These events underscore the persistent tectonic and volcanic influences shaping the landscape.

Swarm SVS20190723.1 began at 13:47 on 22 July 2019 and concluded at 21:41 on 25 July 2019, spanning 79 hours and 54 minutes. During this period, 61 earthquakes were recorded. Event magnitudes ranged from 0.0 to 2.8, with the largest shock of magnitude 2.8 occurring on 23 July at 06:40:29 at a depth of 7 km. Depths throughout the swarm varied between 2 km and 12 km, with the majority concentrated around 5–8 km. Early activity on 22 July featured events of magnitudes 0.6 to 1.4, followed by a peak in frequency and intensity on 23 July that included multiple events above magnitude 1.0. Activity tapered on 24 and 25 July, with the final recorded event at magnitude 0.5.

This swarm aligns with established patterns of low-magnitude, shallow-to-mid-crustal seismicity in the Yellowstone region. The distribution of depths suggests involvement of both shallow hydrothermal systems and deeper magmatic influences. No significant surface deformation or volcanic unrest was associated with this specific sequence, consistent with many prior swarms that dissipate without escalation.

Ongoing monitoring by regional seismic networks continues to track such episodes, contributing to refined models of Yellowstone’s subsurface dynamics. The 2019 event adds to the documented history of swarm behavior that has persisted across multiple decades.

References

• SeismoSight internal swarm classification data for SVS20190723.1 and historical statistics (2000–2019)
• U.S. Geological Survey, Yellowstone Volcano Observatory reports on regional seismicity and caldera geology