Seismic Swarm S20090109.2 Near Silver Gate, Montana: Event Analysis and Regional Context

A seismic swarm designated S20090109.2 was recorded 41 km SSW of Silver Gate, Montana, beginning at 14:37 on 9 January 2009 and concluding at 06:20 on 11 January 2009. Over 39 hours and 43 minutes, the sequence comprised 35 earthquakes. Magnitudes ranged from 0.3 to 3.3, with the largest event occurring at 18:17 on 9 January. Depths were predominantly shallow, concentrated between 0 and 8 km, consistent with activity in the upper crust.

The swarm exhibited a typical pattern of clustered events, with higher activity on the first day followed by a gradual decline. Multiple events registered magnitudes of 1.0–2.1, while several entries indicated placeholder values of -9.9. No damage or felt reports were associated with the sequence.

The location lies immediately adjacent to the northeastern boundary of Yellowstone National Park, within the influence of the Yellowstone volcanic system. This region overlies the Yellowstone hotspot, a mantle plume responsible for extensive volcanism over the past 17 million years. The modern Yellowstone Caldera, formed approximately 631,000 years ago during the most recent supereruption, continues to exhibit ongoing deformation, hydrothermal activity, and seismicity driven by magmatic and hydrothermal fluids.

Earthquake swarms are common in this setting due to the interaction of regional tectonic stresses with local volcanic processes. The shallow focal depths observed in S20090109.2 align with the brittle upper crust above the partially molten magma reservoir, which resides at depths of roughly 5–15 km. Historical records indicate two prior swarms in the immediate area since 2000: one in 2004 and another in 2008. These episodes underscore the recurrent nature of swarm activity linked to the hotspot's influence.

Seismicity in the broader Yellowstone region is monitored continuously by the Yellowstone Volcano Observatory, which integrates data from regional networks to assess volcanic and tectonic hazards. While individual swarms rarely indicate imminent eruptive activity, they contribute to understanding fluid migration and stress changes within the crust. The 2009 sequence remained within background levels for the area and did not trigger elevated alert status.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program, Yellowstone region seismicity summaries (2009–2023 updates).
Yellowstone Volcano Observatory annual reports on caldera monitoring and swarm statistics.