Seismic Swarm SVS20100612.1: Analysis of Activity East of West Yellowstone, Montana

A seismic swarm designated SVS20100612.1 occurred 56 km east-southeast of West Yellowstone, Montana, between 06:29 on 12 June 2010 and 04:58 on 13 June 2010. Over 22 hours and 29 minutes, the sequence produced 28 earthquakes. This event aligns with the well-documented pattern of swarm seismicity in the Yellowstone region, where clusters of small-magnitude events frequently occur without a single dominant mainshock.

The earthquakes exhibited low to moderate magnitudes, ranging from -0.2 to 2.2, with the largest event recorded at 12:06:57 on 12 June. Depths were predominantly shallow, clustered between 1 km and 5 km, although two events reached 9 km and 10 km. Such shallow focal depths are characteristic of the brittle upper crust beneath the Yellowstone Plateau, where hydrothermal fluids and minor magmatic movements can trigger rapid sequences of failure along pre-existing fractures.

Event Characteristics

The swarm initiated with a magnitude 1.0 event at 2 km depth, followed rapidly by additional small events. Activity peaked midday on 12 June, including the magnitude 2.2 shock at 4 km depth. Later events remained small and shallow, with the final magnitude 1.4 earthquake occurring at 2 km depth early on 13 June. The tight spatial and temporal clustering, combined with the absence of a clear aftershock decay pattern, confirms the swarm classification.

Regional Geological Context

The swarm location lies within the eastern margin of the Yellowstone Caldera, a 640,000-year-old volcanic system driven by a mantle hotspot. The area features extensive rhyolitic lava flows, caldera-fill tuffs, and an active hydrothermal system that circulates heated groundwater through fractured volcanic rocks. Ongoing uplift and subsidence of the caldera floor, measured by GPS and InSAR, reflect pressure changes in a shallow crustal magma reservoir and deeper hydrothermal zones. These processes generate frequent earthquake swarms as fluids migrate and reduce effective stress on faults.

Seismic monitoring has recorded persistent background seismicity across the Yellowstone Plateau for decades. Swarms often concentrate along caldera ring faults or in areas of known hydrothermal alteration, where fluid overpressure facilitates episodic slip.

Historical Swarm Activity

Since 1 January 2000, sixteen swarms have been identified in the broader region. Yearly counts include one swarm each in 2000, 2001, 2004, and 2008; six swarms in 2002; two in 2006; and four in 2009. This recurrence underscores the persistent tectonic and volcanic stresses that characterize the Yellowstone volcanic field.

Implications

Swarm SVS20100612.1 represents a typical expression of Yellowstone’s dynamic crustal environment. Continued geophysical surveillance remains essential for distinguishing routine hydrothermal or magmatic unrest from any escalation that could signal larger volcanic or tectonic events.

References

United States Geological Survey, Yellowstone Volcano Observatory reports (2010–2023).
Yellowstone National Park geologic maps and seismic catalogs, National Park Service.
Smith, R. B., et al., “Geodynamics of the Yellowstone hotspot and its volcanic history,” Geological Society of America Special Papers.