Earthquake Swarm S20220826.1: Seismic Activity near Mammoth, Wyoming
The Yellowstone Plateau in northwestern Wyoming hosts one of North America’s most active volcanic and geothermal systems. This region overlies a shallow crustal magma reservoir and extensive hydrothermal networks that produce frequent earthquake swarms. These swarms typically arise from fluid migration along fractures rather than direct magmatic intrusion. The area lies within the 640,000-year-old Yellowstone Caldera, whose ongoing deformation and seismicity are monitored continuously by the U.S. Geological Survey and university networks.
Swarm S20220826.1 began at 03:35 on 26 August 2022 and concluded at 07:44 on 4 September 2022, lasting 220 hours and 9 minutes. Its epicentral area was located 23 km SSW of Mammoth, Wyoming. During this interval, 198 earthquakes were recorded. The sequence displayed the classic characteristics of a Yellowstone swarm: a rapid onset, a broad range of magnitudes, and shallow focal depths predominantly between 3 km and 7 km.
Analysis of the first 100 events shows magnitudes ranging from –0.5 to 3.0, with the three largest shocks (all M 3.0) occurring within the first 18 hours. Depths clustered tightly around 4–5 km, consistent with the brittle–ductile transition zone above the hydrothermal system. After the initial energetic phase, activity transitioned to lower-magnitude events interspersed with occasional M 1.5–2.7 shocks. This pattern indicates episodic fluid-pressure pulses rather than a single large stress release.
Since 1 January 2000, 88 swarms have been documented in the same general region. Annual counts vary from one to ten events, with notable years including 2000 (10 swarms), 2008 (7), 2013 (7), 2014 (7), 2021 (7), and two swarms already recorded in 2022. These statistics underscore the persistent, swarm-dominated nature of Yellowstone seismicity.
The 2022 swarm fits comfortably within the long-term record. No surface deformation or hydrothermal changes exceeding background levels were reported during or immediately after the sequence. Such swarms are regarded as normal expressions of the caldera’s dynamic hydrothermal environment and do not indicate an increased likelihood of volcanic unrest.
References
SeismoSight internal swarm catalog, Swarm S20220826.1.
USGS Volcano Hazards Program, Yellowstone seismicity summaries (2000–2022).
USGS Earthquake Catalog, events near 44.8°N, 110.9°W.
Yellowstone Caldera Observatory, annual swarm statistics.