Seismic Swarm S20000728.1: Analysis of Yellowstone Activity in July–August 2000

Seismic swarm S20000728.1 was recorded between 10:26 on 27 July 2000 and 22:23 on 1 August 2000, approximately 20 km south-southwest of Mammoth, Wyoming. Over 131 hours and 57 minutes, the sequence comprised 144 earthquakes. This event marks the first swarm documented in the region since systematic recording began in 2000.

Analysis of the initial 100 events reveals a typical swarm pattern dominated by low-magnitude activity. Magnitudes ranged from −9.9 to 2.7, with the majority falling below 1.0. Depths were consistently shallow, concentrated between 1 km and 7 km. Notable events included a magnitude 2.6 quake at 6 km depth on 28 July at 05:04, a magnitude 2.7 event at 7 km on 29 July at 05:29, and several magnitude 2.4–2.5 shocks clustered around 05:00–05:16 on 28–29 July. The data show episodic bursts of activity separated by quieter intervals, consistent with fluid-driven seismicity rather than tectonic mainshock-aftershock sequences.

The Mammoth area lies on the northern margin of the Yellowstone Caldera, a large Quaternary volcanic system formed by three major caldera-forming eruptions. The most recent, approximately 631,000 years ago, created the present 45-by-75 km caldera. Ongoing magmatic and hydrothermal processes sustain elevated seismicity throughout the region. Earthquake swarms occur frequently as pressurized hydrothermal fluids or minor magma movement fracture brittle rock at shallow depths.

Since 1 January 2000, five swarms have been identified in the area, with S20000728.1 representing the earliest. These sequences contribute to the long-term monitoring record used to assess volcanic unrest. Depths recorded during the swarm align with the typical 1–10 km range observed in Yellowstone’s upper crustal hydrothermal system.

No significant surface deformation or gas emissions were associated with this particular swarm, underscoring its classification as a background hydrothermal event within the active caldera. Continued instrumentation by regional seismic networks supports real-time tracking of similar activity.

References
USGS Yellowstone Volcano Observatory – Caldera geology and swarm characteristics
Wyoming Geological Survey – Regional tectonic and volcanic history
SeismoSight internal swarm catalog S20000728.1