Seismic Swarm S20040413.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20040413.1 was recorded 16 km northeast of West Yellowstone, Montana, beginning at 06:30 on 13 April 2004 and concluding at 04:46 on 20 April 2004. Over 166 hours and 15 minutes, the sequence produced 457 earthquakes. This event occurred within the Yellowstone volcanic region, where tectonic and hydrothermal processes drive frequent seismicity.

The Yellowstone area lies at the eastern edge of the Basin and Range Province and within the Intermountain Seismic Belt. Earthquake activity here results primarily from the movement of magma and fluids beneath the Yellowstone Caldera, a supervolcano that last experienced a major eruption approximately 631,000 years ago. Small-magnitude events are common, often clustered in swarms lasting days to weeks, reflecting pressure changes in the subsurface hydrothermal system rather than tectonic fault slip alone.

Analysis of the first 100 events reveals predominantly low-magnitude earthquakes, with values ranging from -1.0 to 2.4. Depths clustered between 2 km and 9 km, consistent with the shallow brittle-ductile transition zone influenced by elevated geothermal gradients. Several recordings showed anomalous magnitude values of -9.9, likely representing data artifacts rather than physical events. The sequence initiated with very small events near 06:30 on 13 April, followed by a gradual increase in frequency and occasional larger shocks, such as the magnitude 2.2 event at 16:23 and the magnitude 2.4 event at 20:10 on the same day. Depths remained stable throughout, suggesting a consistent source volume.

This swarm fits the established pattern of seismic activity in the region. Since 1 January 2000, 24 swarms have been documented in the broader area, with annual counts of 10 in 2000, 5 in 2001, 6 in 2002, and 3 in 2003. Such episodes underscore the persistent volcanic unrest at Yellowstone, where magma recharge and fluid migration sustain elevated seismicity rates compared to surrounding stable continental crust.

Monitoring by the Yellowstone Volcano Observatory continues to track these patterns using dense seismic networks. Data from events like S20040413.1 contribute to refined models of subsurface structure and help distinguish between background tectonic activity and potential magmatic indicators. No significant surface deformation or gas emissions were associated with this particular swarm, aligning with the typical behavior of short-lived hydrothermal swarms.

References

• Yellowstone Volcano Observatory, U.S. Geological Survey, "Yellowstone Seismicity Reports," 2004 archive.
• University of Utah Seismograph Stations, "Intermountain Seismic Belt Earthquake Catalog."
• Smith, R.B., et al., "Geodynamics of the Yellowstone hotspot and the evolution of the Yellowstone Caldera," Geological Society of America Bulletin, updated syntheses through 2023.