Seismic Swarm S20131005.1: Analysis of Activity Near West Yellowstone, Montana

On 5 October 2013, seismic swarm S20131005.1 occurred 24 km east-southeast of West Yellowstone, Montana. The sequence began at 12:43 local time and concluded at 20:38 the same day, encompassing 54 earthquakes over 7 hours and 55 minutes. This event unfolded within the Yellowstone volcanic system, a region shaped by ongoing tectonic and magmatic processes.

The Yellowstone area lies at the intersection of the Intermountain Seismic Belt and the Snake River Plain, where the North American plate overrides a mantle hotspot. This setting produces frequent earthquake swarms driven by fluid migration, crustal stress adjustments, and hydrothermal circulation rather than large tectonic ruptures. Historical records document persistent low-level seismicity, with swarms often lasting hours to weeks and rarely exceeding magnitude 4.0. Depths typically range from surface levels to approximately 15 km, reflecting brittle failure in the upper crust overlying a partially molten reservoir.

Data from swarm S20131005.1 reveal a compact energy release. Magnitudes clustered between 0.4 and 2.8, with the largest event reaching 2.8 at 14:09. Multiple events registered at 2.4, including those at 13:29 and 15:26. Numerous readings listed as -9.9 reflect internal classification protocols rather than physical measurements. Depths varied from 0 km to 11 km, indicating activity across shallow crustal layers. The temporal distribution showed peak occurrence between 13:00 and 15:00, followed by a gradual decline toward evening.

Such patterns align with typical Yellowstone swarm behavior, where initial bursts give way to aftershock-like sequences without a single dominant mainshock. The 2013 event fits within a broader historical context: 48 swarms have been recorded since 1 January 2000. Annual tallies include nine in 2000, seven in 2002, six each in 2006 and 2008, and four in 2013. Earlier years such as 2004, 2010, 2011, and 2012 each recorded only one swarm, underscoring episodic rather than continuous clustering.

Geological monitoring by the Yellowstone Volcano Observatory confirms that swarms in this locale seldom herald eruptive activity. Instead, they provide data on subsurface stress and fluid dynamics within the 2.2-million-year-old caldera complex. Updated regional assessments through 2023 continue to classify the area as exhibiting background seismicity levels consistent with long-term observations, without significant deviation from established volcanic-tectonic norms.

In summary, swarm S20131005.1 exemplifies the routine seismic restlessness of the Yellowstone region. Its modest magnitudes, shallow-to-moderate depths, and short duration reinforce the value of continuous surveillance for distinguishing ordinary swarm activity from any anomalous escalation.

References

• Yellowstone Volcano Observatory, U.S. Geological Survey (ongoing monitoring reports)
• Intermountain Seismic Belt historical summaries, University of Utah Seismograph Stations