Analysis of Earthquake Swarm S20220930.1 Near Mammoth, Wyoming

Earthquake swarm S20220930.1 was recorded 22 km SSW of Mammoth, Wyoming, beginning at 04:46 on 29 September 2022 and concluding at 09:37 on 2 October 2022. Over the 76-hour-and-50-minute duration, 53 earthquakes were detected. Magnitudes ranged from -0.7 to 2.6, with the largest event occurring at 23:59 on 29 September at a depth of 8 km. Depths throughout the swarm varied between 2 km and 10 km, consistent with shallow crustal activity.

The sequence exhibited typical swarm behavior, featuring numerous small events clustered in time without a distinct mainshock-aftershock pattern. Early activity on 29 September included events of magnitude 1.1 and 1.4, followed by the peak magnitude 2.6 event late that evening. Subsequent days showed continued low-magnitude seismicity, with notable clusters around 10:36 on 30 September (magnitudes 2.0 and 1.4) and isolated events extending into early October. Depths remained predominantly in the 4–7 km range, suggesting a compact source volume.

This swarm occurred within the Yellowstone volcanic plateau, a region shaped by three major caldera-forming eruptions over the past 2.1 million years. The most recent, approximately 631,000 years ago, formed the present Yellowstone Caldera. Ongoing magmatic and hydrothermal processes drive frequent seismicity, as fluids and gases migrate through fractured rock. Earthquake swarms are a hallmark of the area, often linked to these subsurface movements rather than tectonic fault rupture alone.

Since 2000, 90 swarms have been documented in the region. Annual counts show variability, with peaks of 10 swarms in 2000, 7 each in 2008, 2013, 2014, and 2021. Lower activity years, such as single swarms in 2003, 2004, 2005, 2010, 2011, and 2016, alternate with moderate periods. The five swarms recorded in 2022, including S20220930.1, align with this established pattern of episodic unrest.

Such events provide valuable data on the dynamic state of the Yellowstone system. Monitoring by the U.S. Geological Survey and academic partners continues to track these patterns to assess any potential for escalation, though swarms of this scale remain common and low-hazard.

References

U.S. Geological Survey, Yellowstone Volcano Observatory reports (2022–2023 updates).
Smith, R. B., et al., "Yellowstone Seismic Activity and Volcanic Processes," Geological Society of America (updated syntheses).
SeismoSight internal swarm classification database.