Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from James Mauch, geologist with the Wyoming State Geological Survey, and Yann Gavillot, geologist with the Montana Bureau of Mines and Geology.

Studying earthquakes and their associated hazards is an important part of the Yellowstone Volcano Observatory’s mission. Earthquakes are among Yellowstone’s geologic hazards most likely to occur over human timescales, and famous (and not-so-famous) strong earthquakes have occurred in the past several decades. Additionally, seismic events have set in motion other destructive geologic processes in Yellowstone’s past, such as landslides and possibly hydrothermal explosions, making earthquakes key to the big-picture understanding of hazards in the region.

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Modern-day earthquakes in Yellowstone are monitored through the Yellowstone Seismic Network, operated by the University of Utah Seismograph Stations. However, to understand long-term patterns of seismic activity stretching back into the distant past, scientists need to look for evidence of ancient earthquakes preserved in the landscape. Fault scarps—linear “steps” in the ground formed when past large earthquakes (generally magnitude 6.5 and higher) ruptured the Earth’s surface along geologic faults—are one such piece of evidence. By studying fault scarps, geologists can estimate hazard-related parameters that are difficult to determine from modern seismic monitoring alone, such as long-term fault slip rates and earthquake recurrence intervals.

The Yellowstone region is riddled with fault scarps, many of which have only recently been recognized thanks to the release of high-resolution lidar topographic data from the USGS 3DEP program. The East Gallatin-Reese Creek fault system (EGRCFS) is one of these locations with newly recognized scarps, and it is the subject of a collaborative project between the Wyoming State Geological Survey (WSGS) and the Montana Bureau of Mines and Geology (MBMG). The EGRCFS is a 40-kilometer-long (25 miles) east-dipping normal fault that forms the eastern front of the Gallatin Range in northwest Yellowstone National Park and crosses the Wyoming-Montana border. Driving the highway between Mammoth Hot Springs and Norris Junction, Yellowstone visitors can witness the geologic impact of the EGRCFS: the uplift of the Gallatin Range's scenic peaks and the subsidence of Gardners Hole.

The fault scarps along the EGRCFS cut through deposits from the Pinedale glaciation (the most recent ice age in the Rocky Mountains), indicating that surface-rupturing earthquakes have occurred since glaciers retreated from Yellowstone about 14,000 years ago. Geologists are collecting samples from these glacial deposits to determine their age with greater precision using cosmogenic radionuclide exposure dating, which measures the time a rock has been exposed to cosmic rays at Earth's surface. The team is also using lidar data and field mapping to document the distribution, geometry, and vertical offset of scarps along the EGRCFS.

Knowing the exposure age of the deposits and their vertical displacement across the fault scarp will provide important information on the faulting history of the EGRCFS, including the maximum age of the most recent surface-rupturing earthquakes, the rate at which the fault is slipping, and how these slip rates vary over time and along the length of the fault system. These data will be used as inputs to the USGS National Seismic Hazard Model and will provide land managers and scientists with information to help mitigate earthquake risk in the Yellowstone region.

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WSGS and MBMG geologists began this multi-year project in 2023, completing reconnaissance mapping and initial sample collection. Preliminary samples are currently at a laboratory for cosmogenic radionuclide analysis, and additional samples collected this summer will be processed at MBMG in the fall. As the project advances, results will help geologists to unravel the complexities of this fault system’s history and of earthquake hazards in and around Yellowstone National Park.