Seismic Swarm S20001124.1: Analysis of Activity South of Mammoth, Wyoming

A notable earthquake swarm designated S20001124.1 occurred in late November 2000, centered 24 km south of Mammoth, Wyoming, within the Yellowstone volcanic region. The swarm initiated at 21:59 on 23 November 2000 and concluded at 06:26 on 27 November 2000, spanning 80 hours and 26 minutes. During this interval, 120 earthquakes were recorded, providing valuable data on subsurface dynamics in a tectonically active area.

The events primarily clustered at shallow depths, with most occurring between 1 km and 5 km. Magnitudes remained generally low, though several exceeded 2.0. The largest event reached magnitude 4.2 at a depth of 8 km on 24 November at 04:20:04 UTC. Subsequent notable shocks included magnitudes of 3.5 at 7 km depth and 2.7 at 5 km depth. Analysis of the first 100 events reveals a pattern of frequent small-magnitude activity interspersed with occasional larger releases, consistent with fluid migration or stress adjustments along fault networks.

Geologically, the region forms part of the Yellowstone Plateau, underlain by a continental hotspot that has driven extensive volcanism over the past 17 million years. The current Yellowstone Caldera, formed approximately 631,000 years ago during the most recent supereruption, hosts active hydrothermal systems and ongoing crustal deformation. Earthquake swarms here often arise from interactions between magmatic intrusions, hydrothermal fluid circulation, and regional tectonic stresses associated with the Basin and Range province. Depths recorded in this swarm align with the brittle-ductile transition zone influenced by elevated geothermal gradients.

Historical records indicate this was the first swarm documented after 1 January 2000, with six additional swarms occurring in the region through subsequent years. Such episodes underscore the persistent seismic vitality of Yellowstone, where swarms serve as indicators of potential volcanic or hydrothermal unrest rather than isolated tectonic events.

The distribution of depths and magnitudes in S20001124.1 suggests a vertically confined source zone, likely linked to shallow crustal processes. Continued monitoring by networks such as the Yellowstone Volcano Observatory supports refined models of hazard assessment in this unique geological setting.

References

USGS Earthquake Hazards Program catalogs
Yellowstone Volcano Observatory annual reports
Geological Society of America publications on hotspot volcanism