Seismic Swarm S20080311.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20080311.1 occurred approximately 20 km east-southeast of West Yellowstone, Montana, within the tectonically active Yellowstone volcanic region. The sequence began at 02:22 on 11 March 2008 and concluded at 15:38 on 13 March 2008, spanning 61 hours and 15 minutes. During this period, 69 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.5 and focal depths primarily between 2 km and 10 km.

This swarm exemplifies typical seismic behavior in the Yellowstone area, where clusters of small-magnitude events occur without a dominant mainshock. The events clustered in two main phases: an initial intense period on 11 March featuring 30 events, followed by continued activity on 12 and 13 March. Notable larger events included a magnitude 2.5 earthquake at 10:57 on 11 March (depth 8 km), another magnitude 2.5 at 09:09 on 12 March (depth 8 km), and a magnitude 2.3 at 07:26 on 12 March (depth 7 km). Shallower events, such as those at 2 km depth, suggest possible involvement of near-surface hydrothermal processes alongside deeper magmatic influences.

Geologically, the swarm location lies within the Yellowstone Plateau, part of the eastern Snake River Plain and the Intermountain Seismic Belt. The region overlies a continental hotspot responsible for voluminous volcanism over the past 17 million years. The Yellowstone Caldera, formed by three major caldera-forming eruptions at approximately 2.1, 1.3, and 0.63 million years ago, continues to exhibit unrest through uplift, subsidence, and frequent seismicity driven by magma movement and fluid circulation in the crust. Earthquake swarms here are common, often linked to the migration of hydrothermal fluids or minor magmatic intrusions rather than tectonic faulting alone.

Historical records indicate persistent swarm activity in the broader region. Since 1 January 2000, 38 swarms have been documented, with annual counts as follows: 10 in 2000, 4 in 2001, 8 in 2002, 3 in 2003, 1 in 2004, 6 in 2006, 3 in 2007, and 3 in 2008 up to the time of this event. Such patterns underscore the dynamic nature of the Yellowstone volcanic system, where swarms serve as indicators of subsurface pressure changes.

Monitoring of this swarm contributes to understanding potential precursors to larger volcanic or hydrothermal events. Depths averaging around 7 km align with the brittle-ductile transition zone influenced by elevated geothermal gradients. Continued observation remains essential for assessing hazards in this high-visitation area adjacent to Yellowstone National Park.

References

USGS Earthquake Hazards Program – Yellowstone Seismic Data
Yellowstone Volcano Observatory Annual Reports (2000–2008)
Smith, R.B., et al., 2009, Geodynamics of the Yellowstone hotspot and its volcanic history.