Seismic Swarm S20231004.1: Analysis of Recent Activity Near West Yellowstone

A seismic swarm designated S20231004.1 was recorded 26 km east-northeast of West Yellowstone, Montana, between 10:54 on 3 October 2023 and 01:44 on 5 October 2023. In 38 hours and 50 minutes the sequence produced 39 earthquakes, with magnitudes ranging from -0.8 to 2.3 and focal depths between 2 km and 8 km. The largest event, magnitude 2.3, occurred at 04:21 on 4 October at 7 km depth. Most events clustered between 2 km and 7 km, consistent with shallow crustal processes.

This swarm exemplifies the low-magnitude, high-frequency earthquake sequences typical of the Yellowstone region. Activity began with several events below magnitude 1.0, intensified during the evening of 3 October with a cluster of events between magnitude 0.9 and 1.8, and continued with sporadic larger shocks on 4 October before tapering off early on 5 October. No single mainshock dominated; instead, the energy release was distributed across numerous small events, a hallmark of swarm behavior driven by fluid migration or localized stress changes rather than tectonic fault rupture.

The swarm occurred within the tectonically active zone surrounding the Yellowstone Caldera, a Quaternary volcanic system 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 maintain elevated seismicity, with earthquake swarms occurring regularly as fluids interact with fractured rock. Historical records document 99 swarms in the area since 2000, with annual counts varying from one to ten. Notable years include 2000 (10 swarms), 2022 (9 swarms), and 2013, 2014, and 2021 (7 swarms each). The 2023 total reached two swarms prior to this event.

Regional monitoring by the U.S. Geological Survey and the University of Utah Seismograph Stations shows that such swarms rarely produce damaging ground motion. Magnitudes remain below 3.0 in the great majority of cases, and depths typically stay shallower than 10 km. The October 2023 sequence aligns with this pattern, posing no significant hazard beyond minor, localized shaking.

Continued observation of swarm characteristics helps refine models of volcanic-tectonic interaction at Yellowstone. The distributed timing and depth range observed here support interpretations involving hydrothermal fluid movement within the shallow crust, a mechanism repeatedly documented in the region’s seismic history.

References

USGS Earthquake Hazards Program – Yellowstone seismicity reports
University of Utah Seismograph Stations – Regional earthquake catalog
Smith RB, et al. (2009) – Geodynamics of the Yellowstone hotspot and caldera system