Seismic Swarm S20000526.1: The May 2000 Event Southwest of Mammoth, Wyoming

The Yellowstone region in northwestern Wyoming lies above a continental hotspot that has driven volcanic activity for millions of years. The most recent caldera-forming eruption occurred approximately 631,000 years ago, creating the Yellowstone Caldera. Ongoing crustal deformation, hydrothermal circulation, and magma movement sustain frequent earthquake swarms, which differ from typical mainshock-aftershock sequences by exhibiting numerous events of similar magnitude without a dominant shock. These swarms commonly occur at depths of 2–8 km and reflect fluid migration or stress changes within the volcanic system.

SeismoSight classified swarm S20000526.1 as the first of four swarms recorded in the area since 1 January 2000. The sequence began at 00:00 on 26 May 2000 and concluded at 23:24 the same day, lasting 23 hours and 24 minutes. The epicentral area was located 26 km south-southwest of Mammoth, Wyoming, inside the broader Yellowstone volcanic field. In that interval the network registered 25 earthquakes.

Event magnitudes ranged from 0.1 to 2.2, with one entry recorded at –9.9. Focal depths remained shallow, between 2 km and 6 km. The largest event (magnitude 2.2) occurred at 13:41:52 at 6 km depth. Most other shocks clustered between 0.1 and 1.2, consistent with the low-energy character typical of Yellowstone swarms. Activity showed two modest peaks: one near 13:41–13:45 and another near 16:10–17:05. The final recorded event took place at 23:24:14.

Because the swarm lasted less than one day and produced only small-magnitude events, it did not trigger notable surface deformation or changes in hydrothermal features. Such brief, low-energy swarms are common in the region and are monitored to track subtle shifts in subsurface pressure. The 2000 sequence established a baseline for subsequent swarms recorded by SeismoSight after that year.

Seismic monitoring in Yellowstone integrates data from the Yellowstone Volcano Observatory and regional networks to distinguish volcanic swarms from tectonic sequences. Depths and magnitudes reported for S20000526.1 align with the shallow crustal regime influenced by the hotspot. Continued observation helps refine models of fluid-driven seismicity beneath the caldera.

References USGS Volcano Hazards Program – Yellowstone Yellowstone Volcano Observatory Annual Reports 2000–2023 SeismoSight internal swarm catalogue (S20000526.1)