Seismic Swarm Analysis: January 2000 Event Near West Yellowstone, Montana

The earthquake swarm designated S20000125.1 occurred 6 km north-northeast of West Yellowstone, Montana, beginning at 14:57 on 25 January 2000 and concluding at 23:53 on 26 January 2000. Over 32 hours and 55 minutes, the sequence produced 67 recorded events. Magnitudes ranged predominantly from negative values to 1.9, with the majority clustered between 0.0 and 1.3. Focal depths varied from 5 km to 15 km, indicating activity within the shallow crustal layers typical of the region.

This swarm unfolded in the northeastern sector of the Yellowstone volcanic field, an area shaped by repeated caldera-forming eruptions over the past 2.1 million years. The most recent major event, the Lava Creek eruption approximately 631,000 years ago, formed the present Yellowstone Caldera. Ongoing magmatic and hydrothermal processes drive persistent seismicity, with fluids migrating through fractured rock triggering swarms rather than isolated mainshock-aftershock sequences. Depths recorded during the 2000 event align with the brittle-ductile transition zone influenced by elevated geothermal gradients.

Seismicity in this portion of Montana and Wyoming has been documented since the late 19th century, with instrumental monitoring beginning in the 1970s through the Yellowstone Seismic Network. Historical swarms, such as those in 1985 and 1999, similarly featured hundreds of small-magnitude events concentrated over days to weeks. These episodes commonly occur along north-south trending faults outside the caldera rim, reflecting regional extensional tectonics superimposed on volcanic influences. The 2000 swarm’s timing and location fit established patterns of non-eruptive unrest.

Event timing shows an initial burst followed by sustained activity through the evening of 25 January, tapering overnight and into 26 January. The largest events reached magnitude 1.9 at depths near 10 km, while many smaller shocks clustered at 10–12 km. Such distributions suggest distributed slip on multiple small faults facilitated by pressurized hydrothermal fluids. No surface rupture or felt reports exceeding intensity III were associated with this sequence, consistent with the low energy release.

Monitoring by the U.S. Geological Survey and the Yellowstone Volcano Observatory confirms that swarms of this scale remain routine and do not indicate imminent volcanic activity. Current deformation data from GPS and InSAR show steady background inflation and deflation cycles driven by the underlying magma reservoir at depths exceeding 5 km. The 2000 swarm contributed to the long-term catalog used to refine probabilistic hazard assessments for the greater Yellowstone area.

References

U.S. Geological Survey, Yellowstone Volcano Observatory.
Yellowstone National Park Seismic Monitoring Reports (2000–2023).
Smith, R. B., et al., “Yellowstone Seismic and Volcanic Activity,” Journal of Volcanology and Geothermal Research.