Analysis of Earthquake Swarm S20180216.1 Near West Yellowstone

Seismic swarm S20180216.1 was recorded 12 km northeast of West Yellowstone, Montana, beginning at 06:40 on 15 February 2018 and concluding at 08:10 on 26 February 2018. Over 265 hours and 29 minutes, the sequence produced 728 earthquakes. This event occurred within the Yellowstone volcanic system, where tectonic extension along the Intermountain Seismic Belt interacts with hydrothermal and magmatic processes beneath the Yellowstone Caldera.

The first 100 events, spanning 15–18 February, exhibited magnitudes from 0.0 to 2.6 and focal depths predominantly between 4 and 8 km, with occasional outliers reaching 11 km. Early activity included a magnitude 2.5 event at 8 km depth on 15 February at 18:41, followed by numerous smaller shocks clustered around 6–7 km. By 17 February, magnitudes increased slightly, with events of 2.0–2.6 recorded at depths of 6–9 km. The pattern shows a dense concentration of low-magnitude events consistent with fluid migration or stress redistribution in fractured volcanic rock.

Yellowstone’s geology features a large caldera formed by massive eruptions 2.1 million, 1.3 million, and 640,000 years ago. Ongoing uplift and subsidence driven by a crustal magma reservoir produce frequent earthquake swarms. Depths of 2–11 km align with the brittle-ductile transition zone above shallow hydrothermal systems and deeper magmatic sources. Similar swarms have been documented throughout the region’s instrumental record.

Since 1 January 2000, 73 swarms have been identified in the area. Annual counts include 10 in 2000, 7 each in 2008, 2013, and 2014, and 5 each in 2001, 2009, and 2017. These episodes typically involve hundreds of events over days to weeks and rarely culminate in damaging mainshocks. Swarm S20180216.1 fits this established pattern of distributed microseismicity.

The sequence underscores the dynamic nature of the Yellowstone Plateau, where monitoring by regional seismic networks continues to track activity linked to the underlying volcanic system. Such data contribute to refined models of crustal deformation and hazard assessment in this geologically active setting.

References

• SeismoSight internal swarm classification records
• USGS Yellowstone Volcano Observatory geological summaries