Upper Yellowstone Valley Fault: Montana's Seismic Frontier

The Upper Yellowstone Valley Fault is a north-trending normal fault system in southwestern Montana, just east of Yellowstone National Park — part of the broader Yellowstone-Hebgen Lake seismic zone.

Location & Geometry

• Coordinates: ~44.5°–45.5°N, 111°–111.5°W
• Length: ~100 km (60 mi) cumulative fault trace
• Segments: Multiple en-echelon segments (Red Canyon, Madison, Hebgen Lake faults)
• Dip: 45°–60° west; down-to-west displacement
• Vertical offset: 10–50 m on late Quaternary surfaces

Tectonic Context

• Basin and Range extension impinging on Yellowstone hotspot track.
• Yellowstone hotspot: North American plate moves SW over plume → progressive volcanism (Snake River Plain). Faults accommodate regional extension.
• Hebgen Lake fault (1959 M7.3): 22 km surface rupture, 28 deaths (landslide), created Quake Lake. Part of same system.

Seismic Hazard

• Recurrence: M7+ ~1,000–2,000 yr; M6+ ~200–500 yr.
• Max credible: M7.5 (multi-segment rupture).
• Ground motion: High — Basin and Range attenuation; liquefaction in alluvial valleys.
• Monitoring: USGS/UNR seismic network; GPS strain; InSAR.

Surface Expression

• Fault scarps on alluvial fans (Red Canyon, Madison Valley)
• Offset stream terraces, sag ponds, springs alignment
• Visible on LiDAR & air photos

Risk Implications

• Ennis, MT (pop. ~1,000) near fault trace
• Hebgen Dam (1914) — seismic retrofit critical
• Yellowstone infrastructure: Roads, bridges, Old Faithful area — designed for M7+
• Recreation: Popular fishing, camping, hiking on/near fault

Research Frontiers

Paleoseismology (trenching for event chronology), fault interaction modeling (Yellowstone vs. Basin-Range), induced seismicity potential (geothermal). The fault is a window into hotspot-plate interaction.