Seismic Swarm S20061015.1: Analysis of the October 2006 Event Southeast of West Yellowstone

Seismic swarm S20061015.1 occurred approximately 20 km southeast of West Yellowstone, Montana, within the Yellowstone volcanic region. The sequence began at 18:54 on 14 October 2006 and concluded at 14:16 on 15 October 2006, spanning 19 hours and 21 minutes. During this interval, 79 earthquakes were recorded.

Earthquake swarms in this area typically arise from fluid migration and pressure changes within the hydrothermal and magmatic systems of the Yellowstone Plateau. The 2006 event exhibited magnitudes ranging from 0.4 to 2.3, with the majority below 1.5. Focal depths clustered between 2 km and 15 km, consistent with shallow crustal processes. A single anomalous reading of -9.9 likely reflects data processing artifacts rather than physical events.

The swarm displayed classic characteristics of swarm seismicity: rapid onset, lack of a dominant mainshock, and sustained low-magnitude activity without clear aftershock decay. Such patterns distinguish swarms from tectonic mainshock-aftershock sequences and align with documented behavior in the Yellowstone caldera, where hydrothermal fluids and minor magmatic movement frequently trigger clustered seismicity.

Since 1 January 2000, 29 swarms have been identified in the region. Annual counts include eight in 2000, four in 2001, eight in 2002, three in 2003, one in 2004, one in 2005, and four in 2006. This distribution underscores the recurrent nature of swarm activity driven by the underlying volcanic and hydrothermal framework.

The broader geological setting encompasses the Yellowstone supervolcano, last major eruption approximately 631,000 years ago. Ongoing deformation and seismicity reflect residual magmatic heat and fluid circulation beneath the caldera. The location of S20061015.1 lies near the southeastern margin of the caldera, an area historically prone to swarms due to intersecting fault zones and permeable hydrothermal pathways.

Monitoring by the Yellowstone Volcano Observatory indicates that such swarms rarely precede significant eruptive activity. The 2006 sequence remained well within background levels of unrest observed throughout the instrumental record.

References

• U.S. Geological Survey, Yellowstone Volcano Observatory reports (2006–2023)
• Yellowstone National Park seismic monitoring archives
• SeismoSight internal swarm classification database