Seismic Swarm SVS20110414.1 Near West Yellowstone, Montana

On April 14, 2011, a seismic swarm designated SVS20110414.1 was recorded 59 km east-southeast of West Yellowstone, Montana. The sequence began at 10:54 and concluded at 18:39, encompassing 24 earthquakes over 7 hours and 44 minutes. Magnitudes ranged from 0.2 to 2.0, with focal depths primarily between 4 km and 9 km. The largest event reached magnitude 2.0 at 11:15:33, while several smaller events registered negative or near-zero values consistent with microseismicity.

This swarm occurred within the tectonically active Yellowstone volcanic region. The area lies along the eastern margin of the Yellowstone Caldera, a large Quaternary volcanic system formed by repeated rhyolitic eruptions. Seismicity here arises from a combination of regional Basin and Range extension, magmatic fluid migration, and hydrothermal processes within the shallow crust. Depths observed in the swarm align with the brittle-ductile transition zone typical of the caldera’s western and southern flanks.

Earthquake swarms are a recurring feature of Yellowstone geology. Since 2000, fourteen such swarms have been documented in the broader region. Earlier episodes occurred in 2002 (five swarms), 2004 (one), 2006 (two), 2008 (one), 2009 (four), and 2010 (one). These events generally consist of numerous small-magnitude earthquakes without a single dominant mainshock, distinguishing them from typical tectonic aftershock sequences. The 2011 swarm fits this pattern, with activity concentrated over a brief interval and no reported surface deformation or volcanic unrest.

The April 14 sequence began with low-magnitude events near 8 km depth, followed by a rapid increase in rate and a slight shallowing of foci. Peak activity clustered between 11:15 and 11:27, after which rates declined steadily until cessation. Such temporal clustering is characteristic of fluid-driven swarms in volcanic settings, where pressurized hydrothermal fluids or magmatic gases transiently reduce effective normal stress on pre-existing fractures.

No damage or felt reports were associated with this swarm, consistent with its modest magnitudes and remote location. Routine monitoring by regional seismic networks continues to track similar activity as part of long-term hazard assessment for the Yellowstone area. Continued observation helps refine models of crustal stress and fluid movement beneath the caldera.

References

USGS Earthquake Hazards Program – Yellowstone Seismic Data
Yellowstone Volcano Observatory Annual Reports (2011)
Smith, R. B., et al., “Yellowstone Seismic and Volcanic Activity,” Journal of Volcanology and Geothermal Research (updated summaries through 2023)