Seismic Swarm S20210628.1 Near West Yellowstone, Montana

Seismic swarm S20210628.1 was recorded 18 km east-northeast of West Yellowstone, Montana, within the tectonically active Yellowstone region. The sequence began at 17:58 on 27 June 2021 and concluded at 10:59 on 3 July 2021, spanning 137 hours and encompassing 230 earthquakes. This activity aligns with the broader geological setting of the Yellowstone Plateau, where the underlying hotspot drives frequent seismicity through a combination of volcanic, hydrothermal, and tectonic processes.

The Yellowstone area sits atop a large volcanic system characterized by a resurgent caldera formed during massive eruptions over the past 2.1 million years. The most recent caldera-forming event occurred approximately 631,000 years ago, creating the present-day Yellowstone Caldera. Ongoing magma movement at depths of 5–15 km, coupled with fluid migration in the shallow hydrothermal system, produces episodic earthquake swarms. These swarms typically feature numerous small-magnitude events clustered in time and space, distinguishing them from mainshock-aftershock sequences. Depths recorded in swarm S20210628.1 ranged predominantly between 2 km and 10 km, consistent with shallow crustal processes in the region.

Analysis of the first 100 events reveals a rapid onset of activity on 27 June, with initial magnitudes between −0.2 and 0.5 at depths of 4–8 km. Activity intensified on 28 June, including a magnitude 3.1 event at 10 km depth at 14:02:29, followed by several events exceeding magnitude 2.0 within the same hour. Subsequent events through 29 June maintained magnitudes mostly below 1.5, with depths stabilizing around 5–8 km. Negative magnitudes indicate microseismicity detectable only by sensitive instrumentation, underscoring the swarm’s low-energy character. The temporal clustering, with multiple events occurring within minutes during peak periods, reflects fluid-driven triggering mechanisms common in Yellowstone.

Historical records document 74 swarms in the region since 1 January 2000. Yearly counts show variability: 9 swarms in 2000, 4 in 2001, 5 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 4 in 2006, 3 in 2007, 6 in 2008, 5 in 2009, 1 each in 2010 and 2011, 7 in 2013, 6 in 2014, 3 in 2015, 1 in 2016, 4 in 2017, 2 each in 2018 and 2019, 4 in 2020, and 1 in 2021. This pattern illustrates the persistent nature of swarm activity driven by the Yellowstone hotspot.

Such swarms contribute to ongoing monitoring efforts by agencies tracking volcanic and seismic hazards. While individual swarms rarely produce damaging ground shaking, their cumulative data refine models of subsurface fluid dynamics and magma movement.

References

U.S. Geological Survey, Yellowstone Volcano Observatory reports on regional seismicity.
Smith, R. B., and Siegel, L. J., Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks, Oxford University Press.
Farrell, J., et al., studies on Yellowstone earthquake swarms published in Geophysical Research Letters.