Seismic Swarm SVS20201202.1 Near Mammoth, Wyoming: Geological Context and Event Analysis

The Yellowstone volcanic region in northwestern Wyoming remains one of the most seismically active areas in the United States. Situated above a continental hotspot, the area experiences frequent small-magnitude earthquakes driven by hydrothermal fluid circulation, faulting, and magmatic processes within the shallow crust. The December 2020 swarm SVS20201202.1 occurred approximately 66 km south-southeast of Mammoth, placing it within the central volcanic plateau where brittle failure commonly occurs at depths of 1–9 km.

This swarm initiated at 17:33 UTC on 1 December 2020 and concluded at 02:11 UTC on 4 December 2020, spanning 56 hours and 37 minutes. A total of 114 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from –0.7 to 2.6. The largest event reached magnitude 2.6 at 06:46:51 UTC on 2 December. Most events clustered between magnitudes 0.5 and 1.8, while negative magnitudes indicate microseismicity detectable only by dense instrumentation.

Depth distribution shows the majority of hypocenters between 2 km and 7 km, consistent with the brittle–ductile transition zone above the Yellowstone magma reservoir. Early activity on 2 December featured rapid succession, with multiple events occurring within seconds, followed by a gradual decline in rate through the afternoon and evening. Later events on 3 December exhibited slightly greater average depths, suggesting possible downward migration of stress or fluid pressure.

Since 1 January 2000, the region has recorded 22 earthquake swarms. Yearly totals include five swarms in 2002, four in 2009, two each in 2013 and 2018, and single swarms in 2004, 2006, 2008, 2010, 2011, 2015, 2016, 2019, and 2020. These recurrent swarms reflect the dynamic interplay between tectonic stress and the underlying magmatic-hydrothermal system, a pattern documented across decades of monitoring by the U.S. Geological Survey and University of Utah Seismograph Stations.

The SVS20201202.1 swarm fits within this established framework of episodic unrest. Its shallow depths and modest magnitudes align with historical patterns observed throughout the Yellowstone caldera and its margins. Continued seismic monitoring remains essential for distinguishing routine hydrothermal swarm behavior from any longer-term changes in the volcanic system.