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Geologists get a better look at longer Teton Fault

April 24, 2019 GMT

Thousands of backcountry skiers slid over the most southern extent of the Teton Fault this winter, though the imperceptible geologic feature was likely not on their radar.

That’s not to say skiers don’t care about rocks. Geologists just recently learned that the north-to-south fault bisects Old Pass Road, cutting through just a couple hundred yards uphill of the trailhead.

“It’s right next to Wilson, running intermittently along the eastern side of Phillips Ridge,” said Mark Zellman, a contracted geoscientist who helped the Wyoming State Geological Survey update its imaging of the Teton Fault map. “That was not previously mapped at all.”

The Teton Fault, studied for decades, runs for over 40 miles along the base of the Tetons, pushing the young granite mountains skyward every few thousand years when a major earthquake erupts. When that happens, Jackson Hole simultaneously slumps.

The Teton Fault’s northern terminus is just beyond Steamboat Mountain, north of Jackson Lake’s man-made narrow tip. Until now, nobody had ever attempted to detect the southern terminus.

Moose resident and University of Utah Professor Emeritus Bob Smith was involved in the last update of the Teton Fault map, an effort that dates to the 1990s, he said.

“We didn’t miss it, it’s just that we were contracted to do Grand Teton National Park,” Smith said. “So we went down to do as far as Phillips Bench, and that’s about it.

“The fault is harder to see down there,” he said, “because the displacement is smaller.”

Zellman, the lead author of the latest Teton Fault map, confirmed the contention. U.S. Geological Survey research geologist Chris DuRoss and Idaho State University’s Glenn Thackray were co-authors.

“It does cross the road that goes up to Teton Pass, but it’s not very clear,” Zellman said. “It’s harder to see in there than it is along the range front in other spots.”

Where it’s most prominent, in the heart of the Tetons, the fault can be obvious. From the Cathedral Group turnout off Jenny Lake Road, for example, the fault appears as a short, steep hillside running for hundreds of yards at the base of Rockchuck Peak.

Mapping the Teton Fault with the best precision yet relied on flying a plane that carried a sensor that uses a laser-beam technology called “light detection and ranging,” or LiDAR. Data from a 2015 flight became available courtesy of Grand Teton National Park, Zellman said.

“We started evaluating that data,” he said, “and realized very quickly that it was showing details in the fault that weren’t currently represented in the existing maps.”

Few Rocky Mountain faults move more quickly than the Teton Fault, and concurrent with the new mapping geoscientists have been trying to get a better grip on its historic activity and its present-day hazard. A 5-year-old update to the USGS’s seismic hazard map suggests that northwest Wyoming is as seismically hazardous as anywhere in the United States, due in no small part to the Teton Fault.

For years geophysicists were aware of two major eruptions over the last 8,000 or so years, but in 2017 map co-author DuRoss detected a third when they excavated into where Teton Fault cuts across Jackson Hole Mountain Resort. The newest major earthquake was detected deeper and thus is older, DuRoss said, and confirms that the Teton Fault has ruptured along the Earth’s surface three times between about 10,000 and 5,000 years ago — and never since then.

Historic earthquakes that have been detected during excavations reached or exceeded magnitude 7, a big enough event today to badly damage buildings, send a tsunami across Jackson Lake and cause widespread avalanching in the winter.

Researchers have also recently refined their understanding of Teton Fault’s “slip rate,” which is essentially how much it’s moving. The past estimate was somewhere between a 0.2 millimeters and 2 millimeters a year, and newer analyses confirm that the middle ground from this range is correct.

“Our data is suggesting that it’s in the ballpark of 0.8 to 1.1 millimeters a year,” DuRoss said

Although the fault is now mapped more precisely than ever, the scientific investigators aren’t about to rest. This summer a USGS team will dig into the fault’s newfound southern reaches on Teton Pass, duplicating a north-end excavation at Steamboat Mountain done in 2017.

“The purpose is to resolve the timing of larger earthquakes on the fault so we can use that information to determine whether it breaks all at once,” DuRoss said. “Do all 70 kilometers break at once? Or is it possible to have one here and then another there?

“The more fault that breaks,” he said, “the larger the magnitude of the earthquake.”

Editor’s note: This story has been modified to correct the title of Chris DuRoss.