Seismic Swarm S20030103.2: Analysis of Activity Near West Yellowstone, Montana

The seismic swarm designated S20030103.2 occurred 17 km south-southeast of West Yellowstone, Montana, within the Yellowstone volcanic region. It began at 07:00 on 3 January 2003 and concluded at 10:43 on 7 January 2003, spanning 99 hours and 43 minutes. During this interval, 75 earthquakes were recorded, with event depths primarily between 5 and 16 km and magnitudes ranging from -0.4 to 2.6. The largest event reached magnitude 2.6 at a depth of 10 km on 3 January.

This swarm exemplifies typical low-magnitude, clustered seismicity common in the area. Most events remained below magnitude 2.0, with only a handful exceeding 1.5. Activity peaked in the initial hours and gradually declined, consistent with fluid-driven processes rather than tectonic mainshock-aftershock sequences.

The swarm location lies on the Yellowstone Plateau, part of the active Yellowstone volcanic field. This region sits atop a mantle hotspot that has produced extensive rhyolitic volcanism over the past 2 million years, including three major caldera-forming eruptions. The current caldera, formed approximately 631,000 years ago, hosts an active magmatic system, extensive hydrothermal features, and elevated heat flow. Seismicity here arises from interactions between regional tectonic stresses of the Intermountain Seismic Belt, crustal deformation linked to the underlying magma reservoir, and migration of hydrothermal fluids through fractured rock.

Earthquake swarms have long characterized Yellowstone seismicity. Since 1 January 2000, nineteen such swarms have been documented in the broader area, with eight occurring in 2000, five in 2001, and six in 2002. These episodes reflect ongoing adjustments within the volcanic and hydrothermal environment rather than precursory signals of eruption.

Data from S20030103.2 align with the region's established pattern of shallow crustal events. Depths cluster around 10–14 km, corresponding to the brittle-ductile transition zone influenced by elevated temperatures. No surface deformation or significant changes in hydrothermal activity were associated with this particular swarm.

Ongoing monitoring by the Yellowstone Volcano Observatory continues to track such activity to refine understanding of subsurface processes. Swarms like S20030103.2 provide valuable records for assessing baseline seismic behavior in one of North America's most geologically dynamic landscapes.

References: SeismoSight internal classification records USGS Earthquake Hazards Program – Yellowstone region reports Yellowstone Volcano Observatory geological summaries