Seismic Swarm S20070620.1: Analysis of Activity Near West Yellowstone

On 20 June 2007, a compact earthquake swarm designated S20070620.1 was recorded 24 km east-northeast of West Yellowstone, Montana. The sequence began at 12:43 UTC and concluded at 14:33 UTC, spanning 1 hour and 50 minutes. During this interval, 26 events were detected, with magnitudes ranging from -0.4 to 1.3 and focal depths predominantly between 2 km and 8 km.

The swarm exhibited a rapid onset followed by a gradual decline in activity. Early events clustered tightly around 2 km depth and remained below magnitude 0.5. A modest increase occurred near 12:46 UTC with a magnitude 1.1 event at 4 km depth. Later phases included isolated larger shocks, such as the magnitude 1.3 event at 13:15 UTC recorded at 8 km depth. The final events after 14:23 UTC returned to shallow depths near 2 km and magnitudes below 0.5. This pattern reflects typical swarm behavior driven by fluid migration rather than mainshock-aftershock sequences.

The location lies within the Yellowstone volcanic field, part of the eastern Snake River Plain and the North American plate boundary zone. The region overlies a continental hotspot responsible for three major caldera-forming eruptions, the most recent approximately 631,000 years ago. Ongoing crustal deformation arises from both magmatic intrusions and hydrothermal circulation within the shallow brittle crust. Earthquake swarms in this setting commonly result from pressurized fluids reducing effective normal stress on pre-existing fractures.

Historical records maintained by SeismoSight document 32 swarms in the area since 1 January 2000. Annual counts show notable variability: 10 swarms in 2000, 5 in 2001, 6 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, and 5 in 2006. These episodes underscore the persistent seismic unrest characteristic of the Yellowstone caldera margin.

Such swarms contribute to monitoring efforts that track potential changes in magmatic or hydrothermal systems. Although individual events remain small, their spatial and temporal clustering provides valuable data on subsurface stress conditions and fluid pathways.

References

United States Geological Survey Earthquake Hazards Program
Yellowstone Volcano Observatory Annual Reports
SeismoSight Internal Swarm Catalog