University of Utah doctoral candidate Sin-Mei Wu created this cross section of the underground plumbing of Old Faithful, based on information gathered from 133 portable seismographs deployed for 2 weeks when the area was closed. She calculated that the geyser’s hydrothermal reservoir was as big as a football stadium and is comprised of a network of cracks and fractures through which water flows. It all is heated by magma 3- to 25-miles deep. (University of Utah/Sin-Mei Wu)

Scientists from the University of Utah have published a paper outlining how seismographs enabled them to map the underground plumbing of Old Faithful. Here’s an overview of their findings, keyed to their diagram, above.

1) Researchers at the University of Utah deployed 133 6-inch-high, portable seismographs for two weeks while the area was closed to record minute ground tremors. The devices recorded shaking caused by people, wind, vehicles, and underground boiling water. Their information produced an anatomy of the world’s most famous geyser and the surrounding shallow geologic formations.

2) Seismic waves travel differently through various geologic features, allowing researchers to determine the boundaries between distinct parts of Old Faithful’s underground world. Below this cross section, two magma reservoirs 3- to 25-miles deep heat water near the surface and create hot pots, fumaroles, and geysers.

3) Researchers determined the location, size and shape of what they believe is Old Faithful’s hydrothermal reservoir. Doctoral student Sin-Mei Wu, first author of “Anatomy of Old Faithful from subsurface seismic imaging of the Yellowstone Upper Geyser Basin” published in Geophysical Research Letters, estimated the reservoir to be about 219 yards in diameter and 55 yards high.

4) The reservoir is not a hollow chamber, as one might assume. Instead, it is “a network of cracks and fractures through which water flows,” researchers said in a description. It’s about the size of the University of Utah’s Rice-Eccles Stadium (capacity 45,017) and holds more than 79 million gallons of water. “Although it’s a rough estimation, we were surprised that it was so large,” Wu said in a statement. Only about 8,000 gallons are ejected during an eruption. Take a video journey into Old Faithful’s vent.

 5) The reservoir lies southwest of Old Faithful’s vent, extending under part of Old Faithful Inn and a parking lot. The National Park Service wanted to know where hydrothermal features were relative to buildings and infrastructure, said Bob Smith, the university’s distinguished research professor of geology and geophysics. Smith said he believes the inn has negligible effect on the geyser in a landscape where a heavy blanket of snow can cause minute surface deformations that have been measured by an array of GPS stations.

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Angus M. Thuermer Jr. is the natural resources reporter for WyoFile. He is a veteran Wyoming reporter and editor with more than 35 years experience in Wyoming. Contact him at or (307)...

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