Earthquake Swarm S20191026.1: Seismic Activity Near Mammoth, Wyoming

An earthquake swarm designated S20191026.1 was recorded 24 km SSW of Mammoth, Wyoming, beginning at 14:28 on 26 October 2019 and ending at 13:12 on 28 October 2019. In 46 hours and 43 minutes, 83 events were detected. Magnitudes ranged from -0.4 to 2.5, with focal depths predominantly between 1 km and 6 km. The sequence featured numerous events of magnitude 1.0 or smaller, interspersed with a few events reaching magnitude 2.0–2.5. The largest shocks occurred at 15:57:39 on 26 October (M 2.5, 4 km depth) and 19:13:29 on 27 October (M 2.5, 6 km depth).

The swarm exhibited typical characteristics of clustered seismicity in the Yellowstone region, with events distributed across shallow crustal depths consistent with hydrothermal and magmatic processes. Activity was most intense during the first several hours on 26 October, followed by a gradual decline through 28 October. No events exceeded magnitude 3.0, and the overall energy release remained modest.

This swarm fits within a well-documented pattern of seismic activity in the area. Since 1 January 2000, 74 swarms have occurred in the same general vicinity. Yearly counts include 10 swarms in 2000, 7 each in 2008, 2013, and 2014, and lower numbers in intervening years, with two swarms already recorded in 2019 prior to this event.

The Mammoth area lies on the northern margin of the Yellowstone Plateau, part of an active volcanic system shaped by three major caldera-forming eruptions over the past 2.1 million years. The most recent super-eruption occurred approximately 631,000 years ago, emplacing the Lava Creek Tuff and forming the present Yellowstone Caldera. Ongoing uplift and subsidence, driven by a crustal magma reservoir and associated hydrothermal circulation, produce frequent earthquake swarms. These swarms commonly arise from fluid migration along fractures rather than direct magma intrusion, although both mechanisms have been inferred from geodetic and seismic data.

Regional monitoring by the Yellowstone Volcano Observatory shows that swarms of this scale occur several times per year. Most events remain below magnitude 3.0 and produce no surface damage. The shallow depths recorded during S20191026.1 align with the brittle-ductile transition zone influenced by elevated geothermal gradients beneath the plateau.

Continued seismic and geodetic surveillance remains essential for distinguishing routine swarm behavior from any precursory signals that might accompany larger volcanic unrest. Current data place this sequence firmly within the background level of activity observed at Yellowstone over recent decades.

References
USGS Yellowstone Volcano Observatory monthly reports, 2000–2023.
Yellowstone Caldera geology summary, USGS Professional Paper 729-G (updated 2022).
SeismoSight internal swarm catalog (S20191026.1 parameters).