Earthquake Swarm S20220918.2: Seismic Activity Near Mammoth, Wyoming

Earthquake swarm S20220918.2 was recorded 23 km SSW of Mammoth, Wyoming, within the tectonically active Yellowstone region. The sequence began at 09:12 on 17 September 2022 and concluded at 05:26 on 24 September 2022, spanning 164 hours and 14 minutes. A total of 389 earthquakes were detected during this period. Analysis of the first 100 events reveals predominantly low-magnitude seismicity. Magnitudes ranged from -0.4 to 3.9, with the majority below 2.0. Depths clustered between 2 km and 10 km, consistent with shallow crustal processes. Notable events included a magnitude 2.3 quake at 23:13 on 17 September (depth 8 km) and the sequence's largest shock of magnitude 3.9 at 12:55 on 18 September (depth 10 km). Negative magnitudes and events near 0.0 indicate the swarm's microseismic character, typical of fluid-driven or stress-release episodes. The Yellowstone Plateau hosts frequent earthquake swarms due to its position above a mantle hotspot and underlying silicic magma reservoir. Regional geology features extensive rhyolitic volcanism, hydrothermal systems, and normal faulting along the Intermountain Seismic Belt. Historical records since 2000 document 91 swarms in the area, with annual counts varying from one to ten. Peak activity occurred in 2000 (10 swarms), 2008 and 2013–2014 (seven each), and 2021 (seven). This background underscores the region's persistent, episodic seismicity driven by magmatic and hydrothermal interactions rather than large tectonic events. Swarm S20220918.2 aligns with established patterns of short-duration, high-event-count sequences that rarely produce felt shaking beyond the immediate vicinity. Depths and magnitudes observed in the initial 100 events support interpretations of distributed brittle failure within the shallow crust above the magma body.

References

SeismoSight internal swarm catalog S20220918.2.
USGS Earthquake Hazards Program, Yellowstone seismicity summaries.