Seismic Swarm S20240218.1 Near Mammoth, Wyoming: Analysis and Regional Context

A seismic swarm designated S20240218.1 occurred 25 km south-southwest of Mammoth, Wyoming, between 21:29 on 18 February 2024 and 07:17 on 19 February 2024. The event sequence lasted 9 hours and 48 minutes and comprised 50 earthquakes. Magnitudes ranged from -0.3 to 2.0, with the majority below 1.0 and depths concentrated between 2 km and 8 km. The largest events included two magnitude-2.0 shocks and one magnitude-1.9 event, all recorded within the first 12 hours.

The swarm exhibited a typical pattern of clustered, low-magnitude activity with no single dominant mainshock. Early events clustered at shallow depths of 2–3 km, while later phases showed slightly deeper foci around 6–8 km. This vertical distribution is consistent with fluid migration along pre-existing fractures in the brittle upper crust.

The Mammoth region lies on the northwestern margin of the Yellowstone Plateau, an area shaped by three major caldera-forming eruptions over the past 2.1 million years. The most recent, approximately 631,000 years ago, created the present Yellowstone Caldera. Ongoing magmatic heat sustains one of the world’s largest hydrothermal systems, driving periodic earthquake swarms through pressurization and movement of hydrothermal fluids and gases. Since 2000, 102 swarms have been documented in the broader Yellowstone region, with notable concentrations in 2000 (10 swarms), 2022 (9 swarms), and multiple years recording 5–7 swarms. Swarm frequency has remained relatively steady, reflecting the persistent influence of the underlying magmatic system rather than any short-term escalation.

Seismic monitoring in Yellowstone is conducted continuously by the Yellowstone Volcano Observatory, which integrates data from the University of Utah Seismograph Stations and the U.S. Geological Survey. Historical records show that swarms in this setting rarely culminate in volcanic unrest; instead, they typically dissipate within hours to days as fluid pressures equalize. The February 2024 swarm followed this established pattern, with activity declining sharply after the initial 10-hour window and no detectable surface deformation or gas-emission anomalies reported.

In summary, swarm S20240218.1 represents a routine manifestation of hydrothermal and tectonic processes within the Yellowstone volcanic field. Continued surveillance remains essential for distinguishing background swarm activity from any potential precursory signals, though current observations align with long-term regional norms.

References

• U.S. Geological Survey, Yellowstone Volcano Observatory monthly reports (2024)
• University of Utah Seismograph Stations, Yellowstone seismic catalog (2000–2024)
• Yellowstone Caldera geology summaries, USGS Professional Paper series