Seismic Swarm SVS20130623.1: Analysis of Activity Near West Yellowstone

On 23 June 2013, a seismic swarm designated SVS20130623.1 occurred 64 km east-southeast of West Yellowstone, Montana. The sequence began at 07:59 and concluded at 13:59, encompassing 29 earthquakes over five hours and 59 minutes. Magnitudes ranged from -0.6 to 1.8, with focal depths primarily between 1 km and 13 km. Events clustered in two main phases, with a notable increase in activity after 12:31, including several events exceeding magnitude 1.0 at depths of 8–11 km.

This swarm exemplifies typical swarm behavior, in which earthquakes occur in rapid succession without a dominant mainshock followed by aftershocks. Such patterns frequently arise from fluid migration or stress changes within fractured rock rather than from a single large rupture. Depths recorded during the sequence align with shallow crustal processes common in tectonically and volcanically active zones.

The swarm location lies within the broader Yellowstone Plateau, an area shaped by the Yellowstone hotspot. The region hosts the Yellowstone Caldera, formed by three major caldera-forming eruptions over the past 2.1 million years, the most recent approximately 631,000 years ago. Ongoing magmatic and hydrothermal activity drives ground deformation, heat flow, and frequent seismicity. Earthquake swarms have been documented in the area for decades, often linked to movement of hydrothermal fluids or minor adjustments along existing faults.

Historical records indicate 15 swarms in the vicinity since 1 January 2000. Prior episodes occurred in 2002 (5 swarms), 2004 (1), 2006 (1), 2008 (1), 2009 (4), 2010 (1), 2011 (1), and 2013 (1). These recurrent swarms underscore the persistent seismic character of the Yellowstone volcanic field, where small-magnitude events serve as sensitive indicators of subsurface conditions.

Monitoring by the Yellowstone Volcano Observatory integrates seismic, geodetic, and gas data to distinguish background activity from signs of magmatic unrest. The 2013 swarm remained within normal variability and produced no measurable surface deformation or changes in hydrothermal features. Continued observation supports refined models of stress transfer and fluid dynamics in caldera settings.

References

• Yellowstone Volcano Observatory, U.S. Geological Survey
• SeismoSight internal swarm classification records