Seismic Swarm S20001210.1 Near Mammoth, Wyoming: Geological Context and Event Analysis

Seismic swarm S20001210.1 occurred 23 km south of Mammoth, Wyoming, within the Yellowstone volcanic region. The sequence began at 09:37 on 9 December 2000 and concluded at 10:41 on 10 December 2000, spanning 25 hours and 3 minutes. During this period, 29 earthquakes were recorded, with magnitudes ranging from 0.2 to 1.5 and focal depths between 2 km and 13 km.

The events clustered in time, showing an initial low-magnitude phase followed by a peak in activity between 04:37 and 07:49 on 10 December. Notable shocks included a magnitude 1.5 event at 13 km depth and several magnitude 1.4 events at shallower levels around 4–6 km. Depths remained predominantly in the upper crust, consistent with hydrothermal and magmatic influences in the area.

This swarm represents the earliest recorded under the SeismoSight classification system, which has identified seven such swarms since 1 January 2000. The 2000 sequence initiated the dataset and exhibited typical characteristics of Yellowstone seismicity, where fluid migration along fractures generates rapid sequences of small events without a dominant mainshock.

The Mammoth area lies on the northern margin of the Yellowstone Plateau, part of an active volcanic field driven by a mantle hotspot. The region features the 640,000-year-old Yellowstone Caldera, extensive rhyolitic lava flows, and a shallow crustal magma reservoir. Ongoing deformation, including uplift and subsidence, results from pressure changes in the magmatic-hydrothermal system. Earthquake swarms here frequently arise from the movement of hot fluids or gases rather than tectonic fault slip alone.

Yellowstone’s seismic history includes persistent low-level activity punctuated by swarms, reflecting the interplay between the hotspot and the overlying crust. Monitoring by the Yellowstone Volcano Observatory documents that most events remain below magnitude 2.0, with swarms serving as indicators of subsurface fluid dynamics. Depths in the 2–13 km range align with the brittle-ductile transition zone influenced by elevated geothermal gradients.

Analysis of S20001210.1 reveals a compact spatiotemporal distribution, with events migrating slightly in depth during the peak hours. Such patterns suggest episodic permeability changes within fractured volcanic rock, allowing pressurized fluids to trigger successive failures. Magnitudes stayed modest, underscoring the swarm’s non-destructive nature while providing valuable data on local stress conditions.

Continued observation of similar sequences contributes to refined models of Yellowstone’s volcanic plumbing system. The seven swarms documented since 2000 illustrate recurring behavior tied to the region’s dynamic geology.

References

SeismoSight internal swarm classification records
USGS Earthquake Hazards Program – Yellowstone region data
Yellowstone Volcano Observatory geological summaries