Seismic Swarm S20090417.1 Near West Yellowstone, Montana

Seismic swarm S20090417.1 occurred in the Yellowstone region, beginning at 08:03 on 16 April 2009 and concluding at 02:18 on 22 April 2009. The events were centered 5 km NNW of West Yellowstone, Montana, within the tectonically active Yellowstone Plateau. Over 138 hours and 15 minutes, 136 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.1 to 2.1, with the majority below 1.0. Depths clustered between 8 km and 12 km, though some reached as shallow as 2 km or as deep as 17 km. Early events on 16 April showed magnitudes near 0.0 to 0.4 at depths of 9–11 km. Activity intensified on 17–18 April, including events of 1.7 and 2.1 magnitude at approximately 10–11 km depth. Negative magnitude readings reflect the high sensitivity of local monitoring networks to microseismicity. This swarm aligns with the broader pattern of seismic activity in the area. Since 1 January 2000, 35 swarms have been documented, distributed across years as follows: six in 2000, four in 2001, five in 2002, three in 2003, two in 2004, one in 2005, five in 2006, three in 2007, and six in 2008. Such recurrent swarms are characteristic of the region. The Yellowstone Plateau lies above a large volcanic system featuring a shallow crustal magma reservoir and extensive hydrothermal circulation. Earthquake swarms here commonly arise from fluid migration and pressure changes within fractured rock rather than direct magma ascent. Depths observed in S20090417.1 correspond to the brittle-ductile transition zone influenced by elevated geothermal gradients. Historical monitoring indicates that similar swarms have occurred throughout the instrumental record, often lasting days to weeks without leading to larger tectonic events. No surface deformation or significant ground changes were associated with this particular swarm based on available records. The events remained imperceptible to the public given their small magnitudes.

References

USGS Earthquake Hazards Program – Yellowstone Seismic Data
Yellowstone Volcano Observatory Annual Reports (2009)
Smith et al., Geophysical framework of the Yellowstone Plateau (updated regional summaries)