Seismic Swarm S20080807.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20080807.1 occurred 24 km ENE of West Yellowstone, Montana, within the Yellowstone volcanic region. The event sequence began at 11:06 on 7 August 2008 and concluded at 17:35 the same day, registering 32 earthquakes over 6 hours and 28 minutes. All events remained small, with magnitudes between 0.0 and 2.1, and hypocentral depths ranging from 0 to 10 km. The largest event reached magnitude 2.1 at 14:54:14, at a depth of 7 km. Activity clustered in two main phases, with an initial burst of events between 11:06 and 12:08 followed by a secondary pulse around 14:53–15:08 and isolated later shocks until 17:35.

Earthquake swarms in this area reflect the dynamic geology of the Yellowstone Plateau, situated above a continental hotspot that drives crustal deformation and hydrothermal circulation. The region lies within the 640,000-year-old Yellowstone Caldera, where repeated magma intrusions and fluid migration produce frequent, low-magnitude seismic episodes rather than isolated mainshock-aftershock sequences. Depths recorded during the swarm align with the shallow brittle crust overlying the partially molten upper-crustal magma reservoir, typically 5–15 km beneath the surface.

Since 1 January 2000, 40 swarms have been documented in the broader Yellowstone area. Yearly counts show elevated activity in 2000 (10 swarms) and 2002 (6 swarms), followed by lower numbers in subsequent years, with five swarms recorded in 2008 itself. These episodes underscore the persistent, distributed nature of seismicity linked to volcanic and hydrothermal processes rather than tectonic plate-boundary faulting.

The 2008 swarm exhibited typical swarm characteristics: rapid onset, lack of a dominant mainshock, and quick decay within hours. Magnitudes remained below levels that would produce felt shaking outside the immediate vicinity, consistent with background swarm behavior observed throughout the caldera’s margins. Depths concentrated between 4 and 7 km for most events, indicating failure within the shallow hydrothermal and fractured volcanic rock layers.

Ongoing monitoring by the U.S. Geological Survey and the University of Utah Seismograph Stations continues to track such activity to assess any changes in the underlying magmatic system. No escalation to higher-magnitude events or surface deformation beyond normal background levels was associated with this particular swarm.

References

U.S. Geological Survey, Yellowstone Volcano Observatory reports (2008–2023 updates).
University of Utah Seismograph Stations, Yellowstone seismic catalog.
Smith, R. B., et al., “Yellowstone hotspot volcanism and seismicity,” Geological Society of America publications.