Seismic Swarm SVS20000309.1: Analysis of Activity East of West Yellowstone

On 9 March 2000, a seismic swarm designated SVS20000309.1 was recorded 29 km east of West Yellowstone, Montana. The sequence began at 10:45 and concluded at 20:06, encompassing 28 earthquakes over 9 hours and 20 minutes. Magnitudes ranged primarily between 0.2 and 2.0, with focal depths concentrated between 4 km and 12 km. The largest event reached magnitude 2.0 at 14:31:34, while most activity clustered in the afternoon hours between 14:21 and 15:26.

The temporal distribution showed an initial event at magnitude 1.9, followed by a rapid succession of smaller shocks. Depths remained shallow throughout, consistent with upper-crustal processes. One anomalous reading of magnitude -9.9 was logged but does not alter the overall pattern of low-magnitude, shallow seismicity. The swarm exhibited no clear mainshock-aftershock sequence, instead displaying the characteristic overlapping occurrence typical of swarm behavior.

The epicentral area lies within the Intermountain Seismic Belt, immediately adjacent to the Yellowstone volcanic system. This region experiences ongoing deformation driven by the underlying Yellowstone hotspot, which has produced extensive rhyolitic volcanism and hydrothermal features over the past 2 million years. The local crust is characterized by fractured volcanic rocks and active fault zones that facilitate fluid migration and stress redistribution. Shallow seismicity in this setting commonly reflects interactions between tectonic extension and hydrothermal pressurization rather than direct magmatic intrusion.

Historical records maintained by SeismoSight indicate that only one swarm has occurred in the classified dataset since 1 January 2000, with the present event representing that sole occurrence. No additional swarms have been registered in the intervening period according to the same internal classification criteria.

Such swarms contribute to ongoing monitoring of the Yellowstone region, where background seismicity remains elevated relative to most continental interiors. Continued observation helps distinguish between routine hydrothermal or tectonic swarms and any potential precursory signals associated with deeper volcanic processes. The 2000 sequence, with its modest energy release and limited spatial footprint, aligns with the typical background level documented in the area.

References

SeismoSight internal swarm classification records
USGS Earthquake Hazards Program regional seismicity summaries
Yellowstone Volcano Observatory geological background reports