Seismic Swarm S20221013.1: Analysis of Activity Near Mammoth, Wyoming
An earthquake swarm designated S20221013.1 was recorded 20 km SSW of Mammoth, Wyoming, between 05:21 on 13 October 2022 and 12:02 on 21 October 2022. The sequence lasted 198 hours and 41 minutes and included 111 events. This activity occurred within the Yellowstone volcanic region, where the interplay of tectonic extension, caldera dynamics, and hydrothermal circulation produces frequent earthquake swarms.
The first 100 events exhibited magnitudes ranging from -0.5 to 2.8, with the majority falling between 0.0 and 1.5. Depths concentrated between 4 km and 8 km, though several events reached 11–12 km. Early peaks included a magnitude 2.8 event at 11 km depth on 13 October at 11:11 and a magnitude 2.4 event at 9 km on the same day. Subsequent larger events comprised a magnitude 2.2 at 11 km on 15 October and a magnitude 2.3 at 12 km on 16 October. Most events remained below magnitude 1.0, consistent with swarm behavior driven by fluid migration rather than mainshock-aftershock sequences.
The broader historical record for the area documents 93 swarms since 1 January 2000. Annual counts show variability, with peaks of 10 swarms in 2000, 7 each in 2008, 2013, 2014, 2021, and 2022. Lower activity occurred in years such as 2003 and 2010, each recording only one swarm. This pattern underscores the persistent seismicity of the region.
Geologically, the swarm site lies on the northwestern margin of the Yellowstone Plateau, underlain by Quaternary rhyolitic flows and tuffs associated with the 640,000-year-old Yellowstone Caldera. Regional extension along the Intermountain Seismic Belt interacts with pressurized hydrothermal systems, generating swarms through brittle failure induced by fluid pressure changes. Depths observed in S20221013.1 align with the brittle-ductile transition zone typical of the area, where temperatures rise rapidly with depth due to elevated heat flow exceeding 2 W/m².
No surface rupture or significant ground deformation was associated with this swarm, and maximum magnitudes remained well below levels that produce felt shaking beyond a few kilometers. Such characteristics are typical of Yellowstone swarms, which rarely exceed magnitude 3.5 yet provide important indicators of subsurface fluid movement.
- SeismoSight internal swarm catalog (S20221013.1 parameters and event list).
- USGS Earthquake Catalog for regional context and magnitude validation.
- Yellowstone Volcano Observatory geological summaries on caldera structure and hydrothermal seismicity.