Seismic Swarm S20090525.1 Near West Yellowstone, Montana
Seismic swarm S20090525.1 occurred 23 km east-southeast of West Yellowstone, Montana, on 25 May 2009. The sequence began at 09:20 and concluded at 19:23, encompassing 70 earthquakes over 10 hours and 2 minutes. Magnitudes ranged from 0.1 to 2.9, with the largest events reaching 2.9 at depths of 7 km and 9 km. Depths across the swarm varied between 2 km and 12 km, clustering predominantly around 7–10 km.
The events displayed typical swarm characteristics, featuring numerous small-magnitude earthquakes without a dominant mainshock. Early activity included events of 1.9 and 2.4 within the first 20 minutes, followed by a peak period around 10:28–10:41 when multiple shocks of 2.2–2.9 occurred. Later phases showed declining frequency but sustained low-magnitude activity until the final recorded event of 0.5 at 19:23.
This swarm aligns with patterns observed in the Yellowstone region, where the area lies within the Yellowstone Plateau volcanic field. The plateau formed through repeated caldera-forming eruptions, the most recent approximately 631,000 years ago. Ongoing activity stems from the underlying Yellowstone hotspot, which drives crustal extension, magmatic intrusions, and hydrothermal fluid circulation. These processes generate frequent earthquake swarms as pressurized fluids migrate through fractured rock.
Historical records indicate 41 swarms in the region since 1 January 2000. Annual counts include 9 swarms in 2000, 5 in 2001, 8 in 2002, 3 in 2003, 1 in 2004, 6 in 2006, 2 in 2007, and 7 in 2008. Such sequences reflect the dynamic interplay between tectonic stress and volcanic-hydrothermal systems rather than precursory signals of large eruptions.
Monitoring data from this swarm contribute to understanding short-term crustal deformation in the area. Depths predominantly in the upper crust correspond to the brittle-ductile transition zone influenced by elevated geothermal gradients. No surface deformation or significant ground shaking was associated with the 2009 sequence beyond the immediate epicentral zone.
References
SeismoSight internal swarm classification S20090525.1
USGS Earthquake Catalog (Yellowstone region historical data)
Yellowstone Volcano Observatory geological summaries