Seismic Swarm SVS20090915.1 in the Yellowstone Region

A notable earthquake swarm designated SVS20090915.1 occurred southeast of West Yellowstone, Montana, between 09:22 on 14 September 2009 and 01:46 on 17 September 2009. Over 64 hours and 23 minutes, the sequence produced 39 earthquakes, with events distributed across depths primarily between 2 and 7 km. Magnitudes ranged from -0.1 to 1.8, indicating low-energy activity typical of swarm behavior rather than a mainshock-aftershock sequence.

The swarm's temporal pattern showed clustering on 14 and 15 September, with reduced frequency by 16 September and a final event early on 17 September. Depths remained relatively consistent around 5–6 km for most events, suggesting activity within the shallow brittle crust. This distribution aligns with the tectonic setting of the Yellowstone Plateau, where fluid migration and magmatic processes frequently trigger such swarms.

The location lies within the broader Yellowstone volcanic system, part of the Snake River Plain hotspot track. The region overlies a shallow crustal magma reservoir and experiences elevated heat flow, which promotes brittle failure at shallow depths. Historical records indicate that earthquake swarms have been documented in this area since at least 2000, with ten swarms recorded through 2009. These include five events in 2002, one in 2004, two in 2006, and two in 2008, underscoring the recurrent nature of swarm activity driven by the underlying volcanic and hydrothermal environment.

Geologically, the Yellowstone caldera formed through massive eruptions approximately 2.1 million, 1.3 million, and 640,000 years ago. Ongoing deformation, including uplift and subsidence, reflects pressure changes in the subsurface magmatic system. Seismic swarms often coincide with these deformation episodes and are interpreted as resulting from the movement of hydrothermal fluids or minor magma intrusions rather than tectonic fault slip alone. Depths observed in the 2009 swarm fall within the typical range for Yellowstone seismicity, where most events occur above 10 km.

Monitoring by the Yellowstone Volcano Observatory provides continuous data on such sequences, aiding in distinguishing routine swarm activity from any potential escalation. The modest magnitudes recorded in SVS20090915.1 posed no significant hazard, consistent with the majority of Yellowstone's background seismicity.

References

• Yellowstone Volcano Observatory, U.S. Geological Survey (ongoing monitoring reports on regional seismicity)
• U.S. Geological Survey Earthquake Catalog (historical swarm statistics and event parameters for the Yellowstone region)