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Yellowstone's many faults (don't blame the volcano for everything, though!)

The faults in Yellowstone National Park vary greatly in age and how they formed, and they can be divided into two groups: relatively young faults that have been active in the last 1.6 million years of Earth's history, and older faults that are no longer active. Earthquakes are often associated with many of the younger faults in the park.

Map showing three types of young faults in Yellowstone National Park
Map showing three types of young faults in Yellowstone National Park. 1) Resurgent dome faults. 2) Volcanism and caldera faults. 3) Basin and Range faults. Courtesy of the Wyoming State Geological Survey. (Public domain.)

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Seth Wittke, hazards manager and James Mauch, hazards geologist, at the Wyoming State Geological Survey.

Yellowstone National Park (YNP) is home to one of the largest volcanic systems on Earth; evidence of volcanic eruptions associated with the Yellowstone volcano, and the eastward track of the hot spot that created it, stretch from Idaho into northwest Wyoming. However, the eruption history of the volcano is only part of the geologic story. The park also contains evidence of faults and other geologic processes which can form faults at the earth's surface.

The faults in YNP vary greatly in age and how they formed, and they can be divided into two groups: relatively young faults that have been active in the last 1.6 million years of Earth's history, and older faults that are no longer active. Earthquakes are often associated with many of the younger faults in the park. Young faults in Yellowstone are generally related to one of three geologic origins: (1) caldera formation and associated volcanism during one of Yellowstone's three large eruptions, (2) ground deformation from the ongoing movement of magma beneath Yellowstone, and (3) tectonic extension of the Basin and Range Province.

Yellowstone's NE caldera rim faults and Mirror Lake....
An aerial view of faults associated with the NE rim of Yellowstone Caldera. The view is looking southeast. Lines mark top of fault scarps associated with the collapse of Yellowstone Caldera during enormous eruption about 640,000 years ago. Mirror Lake fills a depression due to uneven settling of the ground along one of the faults, called a sagpond by seismologists. The low area between the outermost caldera rim fault and the next fault is a graben. The two faults that straddle Mirror are good examples of antithetic faults, which are normal faults that are of the opposite orientation to the major fault with which they are associated (the outermost caldera rim fault). In this view, the right side (west) of the outermost fault has dropped downward; in the other faults, the left sides (east) dropped downward.The snow covered Absaroka Mountains are visible in the far distance; Yellowstone Lake is in upper right. Access to this remote part of Yellowstone National Park requires special permission from the park.

Faults related to the Yellowstone volcanic system are found primarily in the center of the park. Caldera boundary faults, such as those at the margin of the Henrys Fork and Yellowstone calderas, were created as their respective calderas collapsed after an eruption. As lava withdrew from beneath the caldera, the regional surface level dropped and an extensional environment was created, which caused other faults to form outside the caldera margins. The Mirror Lake Plateau is a good example of these types of faults.

Faults also form as a result of the ongoing and episodic rise and fall of the ground in Yellowstone as magma moves through the subsurface. For example, the Mallard Lake and Sour Creek resurgent dome faults, near Old Faithful and Fishing Bridge, respectively, are related to post-eruption deformation inside the Yellowstone caldera. Though largely imperceptible to park visitors, scientists can monitor inflation and deflation of the resurgent domes at the millimeter scale using high-precision GPS.

Other faults in YNP are related to the Basin and Range Province, a region in the western United States characterized by alternating valleys and fault-bounded mountain ranges. The Basin and Range Province began forming at this latitude about 15 million years ago as the earth's crust was stretched in an east-west direction. The eastern most boundary of the Basin and Range Province reaches western Wyoming, including YNP and the Teton mountain range, and contains faults that are still active and capable of large earthquakes. Whereas other faults in the park are considered to be capable of medium sized earthquakes (M5 or M6), Basin and Range faults could produce larger events (in the range of M7).

An example of a large Basin and Range Province earthquake is the 1959 Hebgen Lake earthquake, which registered a magnitude of 7.3 and had an epicenter about 15 miles northwest of West Yellowstone, Montana. This event caused the Red Canyon and Hebgen faults to rupture and produced damage across a large area. A landslide triggered by the earthquake dammed the Madison River and formed Earthquake Lake.

Other Basin and Range faults within the borders of YNP include the East Mount Sheridan, Eagle Bay, and Upper Yellowstone faults. Scientists consider these faults to be capable of producing large earthquakes, potentially similar to the Hebgen Lake earthquake. Park visitors can see the scarp (surface expression) of the East Mount Sheridan fault by hiking about one mile up the Mount Sheridan trail from Heart Lake.

With so many active processes occurring, it's no wonder that Yellowstone is one of the most seismically active areas in the United States! This is why seismic hazards are such an important issue, not just in Yellowstone, but throughout the western United States.

Faulting on the East Sheridan fault has oversteepened the lower part of Factory Hill
Faulting on the East Sheridan fault has oversteepened the lower part of Factory Hill and formed the Heart Lake basin. Photo taken from Pierce and others, 2007 (USGS Professional Paper 1717A) (Public domain.)

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