Seismic Swarm S20130507.1: Analysis of Activity Near West Yellowstone

Seismic swarm S20130507.1 occurred 14 km southeast of West Yellowstone, Montana, from 13:22 on 7 May 2013 to 12:04 on 8 May 2013. The sequence lasted 22 hours and 41 minutes and included 34 earthquakes, with magnitudes ranging from 0.4 to 2.9 and focal depths primarily between 4 and 14 km.

The swarm began with a magnitude 2.9 event at 8 km depth. Subsequent activity featured events clustered between 6 and 10 km depth, including a magnitude 2.5 shock at 8 km. Later phases showed slightly deeper occurrences, such as a magnitude 0.7 event at 14 km. Magnitudes remained modest throughout, with most events below 1.5, consistent with swarm behavior where energy release occurs through numerous small shocks rather than a dominant mainshock-aftershock pattern.

The region lies within the Yellowstone volcanic field, influenced by the Yellowstone hotspot and the Intermountain Seismic Belt. This tectonic setting produces frequent seismicity from a combination of magmatic fluid movement, hydrothermal processes, and regional extension. Depths observed in the swarm align with the brittle-ductile transition zone typical of the area, where earthquakes commonly occur between 5 and 15 km.

Historical records indicate persistent swarm activity. Since 2000, 51 swarms have been documented in the vicinity, with annual counts varying from one to ten. Notable years include 2000 with 10 swarms and 2008 with 7, demonstrating episodic clustering rather than uniform occurrence. Such patterns reflect the dynamic volcanic-tectonic environment of the Yellowstone caldera and its surroundings.

Insights from the 2013 swarm underscore the value of dense monitoring networks for resolving fine-scale migration of hypocenters. The tight spatiotemporal clustering and limited magnitude range suggest a localized trigger, possibly related to fluid pressure changes at depth. Continued observation remains essential for distinguishing background volcanic unrest from potential precursors to larger events.

References

USGS Earthquake Hazards Program – Yellowstone seismicity reports
Montana Bureau of Mines and Geology – Regional seismic bulletins
Smith, R. B., et al. (2009). Geodynamics of the Yellowstone hotspot. Journal of Volcanology and Geothermal Research.