Seismic Activity in Yellowstone: The July-August 2008 Earthquake Swarm Southeast of West Yellowstone

The Yellowstone region in Montana and Wyoming sits atop an active volcanic hotspot, where the North American tectonic plate moves over a mantle plume. This setting produces the Yellowstone Caldera, formed by three massive eruptions over the past 2.1 million years, the most recent around 631,000 years ago. Ongoing geothermal features such as geysers and hot springs result from shallow magmatic heat, while frequent earthquake swarms reflect adjustments in the crust driven by fluid migration, hydrothermal processes, and regional extension.

Swarm S20080729.1 occurred 16 km southeast of West Yellowstone, Montana, beginning at 17:30 on 28 July 2008 and concluding at 11:36 on 2 August 2008. Over 114 hours and 5 minutes, the sequence produced 189 earthquakes. Analysis of the first 100 events shows magnitudes ranging from -0.6 to 2.1, with the majority between 0.5 and 1.5. Focal depths clustered between 5 and 12 km, though a few reached 18-19 km. Activity initiated with events near 9 km depth and remained shallow overall, consistent with upper-crustal processes typical of the area.

The temporal pattern featured an initial burst on 28 July, followed by sustained elevated rates through 29 and 30 July before tapering. No single mainshock dominated; instead, the sequence displayed the classic swarm characteristics of numerous events of similar size without clear foreshock-aftershock decay.

Since 1 January 2000, 42 swarms have been recorded in the broader Yellowstone region. Annual counts include 10 in 2000, 5 in 2001, 8 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 6 in 2006, 3 in 2007, and 4 in 2008 up to the time of this swarm. These episodes underscore the persistent, swarm-dominated seismicity of the volcanic plateau rather than isolated large-magnitude events.

Such swarms rarely trigger significant ground deformation or eruptive activity but provide valuable data on subsurface fluid dynamics and stress conditions within the caldera system. Continued monitoring by the U.S. Geological Survey and university networks remains essential for understanding long-term volcanic and tectonic behavior.

References
U.S. Geological Survey Earthquake Hazards Program – Yellowstone Seismicity Reports
Yellowstone Volcano Observatory – Swarm Catalog and Historical Statistics (2000–2008)
SeismoSight internal swarm classification S20080729.1