Seismic Swarm S20030131.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20030131.1 occurred in the tectonically active Yellowstone region, centered 17 km south-southeast of West Yellowstone, Montana. The sequence began at 07:56 on 31 January 2003 and concluded at 23:26 on 1 February 2003, spanning 39 hours and 29 minutes. During this interval, 36 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.6 and focal depths consistently between 10 and 14 km.

The events clustered tightly in both space and time, a hallmark of swarm behavior rather than a mainshock-aftershock sequence. Early activity on 31 January featured low-magnitude events (0.3–1.5) at depths of 13–14 km. Midday recordings included a 1.7 event at 11 km depth, followed by a peak magnitude of 2.6 at 11 km depth later that afternoon. Subsequent events maintained similar depth ranges, with magnitudes generally below 1.5. The final recorded event on 1 February registered magnitude 0.4 at 13 km depth. All events remained shallow, consistent with crustal processes influenced by regional volcanic and hydrothermal systems.

This swarm fits within the broader seismicity patterns of the Yellowstone volcanic field. The area overlies a large silicic magmatic system responsible for the Yellowstone Caldera, formed by massive eruptions approximately 2.1 million, 1.3 million, and 0.63 million years ago. Ongoing deformation arises from magma movement, fluid migration, and tectonic extension along the Intermountain Seismic Belt. Earthquake swarms here frequently reflect pressure changes in the shallow crust without leading to eruptive activity.

Historical records indicate elevated swarm frequency in the region since 2000. Sixteen swarms have been documented through early 2003, distributed as eight in 2000, four in 2001, three in 2002, and one in 2003. These episodes underscore persistent unrest driven by the underlying magmatic system.

The S20030131.1 swarm exemplifies typical low-magnitude, shallow seismicity that contributes to monitoring efforts in this high-hazard volcanic setting. Continued observation aids in distinguishing background unrest from potential precursors to larger events.

References

SeismoSight internal swarm classification records for S20030131.1.
USGS Earthquake Hazards Program summaries for the Yellowstone region.
Geological Society of America publications on Yellowstone Caldera evolution and seismicity.