Seismic Swarm S20130713.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20130713.1 occurred approximately 21 km east-northeast of West Yellowstone, Montana, beginning at 05:48 on 13 July 2013 and concluding at 01:31 on 14 July 2013. Over 19 hours and 43 minutes, the swarm comprised 30 earthquakes. This event took place in a tectonically active zone adjacent to Yellowstone National Park, where the interplay of volcanic processes and regional faulting routinely generates clustered seismicity.

The recorded events exhibited low magnitudes, predominantly between -0.5 and 1.7, with one anomalous reading of -9.9 likely representing an instrument artifact or data anomaly rather than a true event. Focal depths ranged from 2 km to 9 km, clustering most frequently around 6–8 km. The sequence initiated with several small shocks in the first hour, followed by intermittent activity that peaked with a magnitude 1.7 event at 06:26 on 13 July. Subsequent events remained modest, tapering off by the following morning. Such patterns are characteristic of swarm behavior, in which numerous small earthquakes occur without a dominant mainshock.

This swarm aligns with the broader seismicity of the Yellowstone region, situated on the eastern margin of the Snake River Plain and influenced by the Yellowstone hotspot. The underlying geology features a large caldera system formed by massive eruptions, the most recent of which occurred approximately 640,000 years ago. Ongoing crustal deformation, hydrothermal fluid movement, and slip along normal faults contribute to frequent earthquake activity. Depths in the 4–9 km range commonly reflect brittle failure within the upper crust above deeper magmatic or hydrothermal sources.

Historical records indicate that 52 swarms have been documented in the area since 1 January 2000. Yearly counts include 10 events in 2000, 5 in 2001, 6 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 6 in 2006, 3 in 2007, 7 in 2008, 5 in 2009, 1 in 2010, 1 in 2011, and 2 in 2013. These episodes underscore the persistent seismic restlessness of the region, often linked to subtle changes in subsurface pressure or fluid migration rather than large tectonic strain release.

Monitoring by the U.S. Geological Survey confirms that Yellowstone experiences hundreds to thousands of earthquakes annually, the majority too small to be felt. Swarms such as S20130713.1 typically pose minimal hazard but provide valuable data on the dynamics of the volcanic system. No significant surface deformation or hydrothermal changes were associated with this particular sequence.

References

U.S. Geological Survey, Yellowstone Volcano Observatory – Earthquake Monitoring Reports (updated through 2023 data releases).
SeismoSight internal swarm classification database for event parameters and historical counts.