Seismic Swarm SVS20060318.1: Analysis of Activity Near West Yellowstone, Montana

On 18 March 2006, a seismic swarm designated SVS20060318.1 was recorded 61 km east-southeast of West Yellowstone, Montana. The sequence began at 15:35 UTC and concluded at 03:46 UTC on 19 March, encompassing 28 earthquakes over 12 hours and 11 minutes. All events were of low magnitude, consistent with typical swarm behavior in the region.

The earthquakes exhibited magnitudes between 0.4 and 2.3, with focal depths ranging from 1 km to 14 km. The largest event, magnitude 2.3, occurred at 15:57 on 18 March at a depth of 3 km. Other notable shocks included two magnitude 2.2 events at 16:05 (3 km depth) and 18:41 (6 km depth). Depths clustered predominantly between 2 km and 6 km, reflecting shallow crustal processes.

This swarm aligns with the tectonic setting of the Yellowstone volcanic field, situated atop a continental hotspot. The area experiences ongoing seismicity driven by crustal extension within the Intermountain Seismic Belt, combined with magmatic and hydrothermal influences from the underlying mantle plume. The Yellowstone Caldera, formed by massive eruptions approximately 2.1 million, 1.3 million, and 631,000 years ago, remains a locus for such activity. Shallow seismicity often indicates fluid migration or stress adjustments in fractured volcanic rock.

Historical records since 2000 indicate nine prior swarms in the vicinity, distributed as follows: one in 2000, one in 2001, six in 2002, and one in 2004. These episodes underscore the recurrent nature of swarm activity, which differs from mainshock-aftershock sequences by lacking a dominant event and instead featuring numerous events of similar size over short periods.

Insights from the 2006 swarm data reveal a rapid onset with multiple events within the first hour, followed by a gradual decline. Depths remained consistently shallow, supporting interpretations of upper-crustal deformation rather than deeper magmatic movement. Such patterns aid in monitoring potential precursors to larger unrest at Yellowstone.

References

USGS Earthquake Catalog
Yellowstone Volcano Observatory reports
Intermountain Seismic Belt geological summaries