Seismic Swarm S20060618.1 Near West Yellowstone, Montana

Seismic swarm S20060618.1 occurred 11 km north-northeast of West Yellowstone, Montana, within the broader Yellowstone volcanic region. The sequence began at 05:54 on 18 June 2006 and concluded at 17:24 on 19 June 2006, lasting 35 hours and 30 minutes. During this period, 57 earthquakes were recorded, with magnitudes ranging from -0.6 to 2.2 and focal depths predominantly between 7 and 10 km.

The events clustered tightly in both time and space, characteristic of swarm behavior rather than a mainshock-aftershock sequence. Early activity on 18 June included several events near magnitude 1.0, with the largest shock of the swarm (magnitude 2.2) occurring at 01:34 on 19 June at 9 km depth. Subsequent activity tapered gradually, ending with a magnitude 0.0 event at 17:24 on 19 June. Depths remained consistent throughout, suggesting a localized source volume within the shallow crust.

This swarm forms part of the ongoing seismicity associated with the Yellowstone Plateau. The region overlies a large silicic magmatic system that last produced a caldera-forming eruption approximately 631,000 years ago. Present-day deformation and earthquakes arise primarily from hydrothermal fluid migration and minor magmatic pressurization beneath the caldera and its margins. West Yellowstone lies just outside the northwestern caldera rim, where faults and fractures facilitate fluid movement that triggers repetitive earthquake sequences.

Historical records maintained by SeismoSight indicate 28 swarms in the area since 1 January 2000. Annual counts were highest in 2000 (10 swarms) and declined thereafter, with five each in 2001 and 2002, three in 2003, two in 2004, one in 2005, and two in 2006. Such swarms typically last hours to weeks and rarely exceed magnitude 3.0, reflecting the distributed stress release common in volcanically active hydrothermal environments.

Monitoring by the Yellowstone Volcano Observatory confirms that similar swarms recur frequently across the region without immediate indication of magmatic intrusion or heightened eruption likelihood. Depths and magnitudes observed in S20060618.1 align with patterns documented in prior events, underscoring the stable background seismicity of the Yellowstone system.

References

• Yellowstone Volcano Observatory, USGS (ongoing monitoring reports through 2023)
• U.S. Geological Survey Earthquake Catalog (regional event data 2000–2023)
• SeismoSight internal swarm classification records (S20060618.1 parameters)