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Town backs housing but won't mandate
Bureau says shock would be a magnitude 9, but officials won't release report.

By Angus M. Thuermer Jr.

A new study by the Bureau of Reclamation says the Teton Fault could produce ground shaking under the Jackson Lake Dam equivalent to a magnitude 9 earthquake lasting three minutes.

The fault is considered capable of producing only a 7.5 magnitude quake. But the forces from such a temblor would be amplified and prolonged by a basin of alluvial fill ­ essentially historic river and lake silt ­ that lies under the dam, according to the study titled "Final Report, Jackson Lake Dam Ground Motion Evaluation."

The dam is seven miles east of the fault in Grand Teton National Park. The new figures of magnitude and shaking duration ­ the study was dated June 13 but obtained by the News&Guide only recently ­ astounded geologists and public officials.

Valley resident and geologist Wally Ulrich called magnitude 9 "beyond comprehension."

"Every geologist I've spoken to in the last two weeks just gulps at that figure," he said. "It's unbelievable how anyone could look at that and not wonder about the safety of that dam."

A bureau official said an accompanying engineering report shows that the dam meets acceptable levels of safety. But he refused to release the report, saying terrorists could use the information to launch an attack.

"It's information that could be useful to terrorists if it got into the wrong hands," Bruce Muller, chief of the bureau's dam safety office, said in a telephone interview from Denver on Tuesday. Muller said the engineering report shows that dam reconstruction in the 1980s was sufficient to withstand the force and duration revealed in the latest study.

"Both of those statements were taken into account in the engineering analysis that was done," Muller said. "The analysis says the dam meets our guidelines. We consider it to be safe. We consider it to not need any modifications."

At the Wyoming Geologic Survey, the state agency charged with evaluating earthquake hazards, two officials said they want more information. The new data "sort of kicks it up a notch," state hazards geologist Jim Case said of the seriousness of the dam safety issue. "Why aren't they freely releasing the engineering report?

"You want to make sure the engineering evaluation was thorough," Case said. "That's why I think Teton County would like to have their own review."

State Geologist Lance Cook also is wary. "We have no idea what their assumptions are that went into the modeling," he said.

Bill Paddleford, chairman of the Teton County Board of Commissioners, said the study increases his anxiety about the dam. He is already in a spat with the Bureau of Reclamation over its removal of a 20-station seismic network the county and U.S. Geologic Survey intended to upgrade. The network was providing information on the fault and its relation to the dam. Removal of old gear could greatly increase replacement costs, Paddleford said.

Paddleford plans to travel to Washington, D.C., to meet with Bennett William Raley, assistant secretary of the interior for water and science, over the issues. Paddleford said the predicted forces are "astronomical."

"If you use a little bit of common sense, [they] certainly wouldn't do that dam any good," he said.

The commissioner said he can't simply take the bureau at its word.

"It isn't a question of whether I believe," he said of the bureau report. "I have to put my mind at rest because of my responsibility to Teton County."

Muller, the bureau's dam safety chief, said the studies show the dam might be damaged by a quake but not destroyed.

"I think you could expect some sort of damage to occur," he said. "Any facility that experiences a magnitude 7.5 [quake] acting as a 9 would expect to receive some sort of damage. But we wouldn't expect it to be detrimental damage. We wouldn't expect it to result in an uncontrolled release of the reservoir."

The dam is composed of a concrete section containing outlet works and spillway, plus an earth dike extending almost a mile north. It rises 49 feet above the bed of the Snake River and holds 847,000 acre feet of water in Grand Teton National Park. Its failure likely would inundate large portions of Jackson Hole, excluding the town of Jackson but covering most of the west bank of the Snake from Moose to Wilson and south to Munger Mountain. People would die, and damage could exceed $600 million, geologists say.

Muller said the forces potentially involved at the dam won't be catastrophic. He called three minutes of shaking "a long duration." And he said he does not know of a past quake in the lower 48 states as forceful as what would be felt under the dam.

"I am not aware personally of any recorded quake of that duration of shaking," Muller said. Other than in Alaska, "I'm not aware that there's been a magnitude 9 in this country before."

Muller said there were enough modifications made to the dam in a program that ended in 1988 to convince his agency it is adequate. Among the improvements were compaction of the alluvial fill that serves as the dam's foundation, and reconstruction of the dike.

"There is an unmeasured margin of safety that comes about from all those features," he said. "Our normal term is 'multiple redundant measures.'"

Muller said one factor in computing risk is how often the lake is full. In calculating safety, the agency takes into account that the reservoir is not full sometimes, and hence the dam is less vulnerable to failure.

But Muller said the bureau doesn't think the dam would fail even in the event of a 7.5 quake with a full reservoir. "We don't believe it would," he said.

Muller said residents of Jackson Hole should "trust the bureau, talk to the state engineer, see if he has concerns."

Dave Benner, dam safety engineer at the Wyoming state engineer's office, is apparently the only person outside the bureau who has received the engineering study.

"Their studies show the dam as repaired is safe." Benner said Tuesday. "If they hadn't done the repairs, it wouldn't be. What they did was adequate at the time and still is."

The study uses a relatively new method, risk analysis, to calculate safety, he said. "That's part of the probability thing," he said. "You assume the different levels, then add all those things together to get the total risk involved.

"They took into account the different levels and the amount of time the [lake] level would be at that high point," Benner said. "Potentially you could have all of those things come together and have that perfect storm. It's a very rare event."

The ground motion study, available to the public, was peer reviewed, Benner said.

"The risk analysis was done by bureau," he said. "They didn't have anybody review this risk analysis but had outside people help with the procedures involved."

While Benner said he believes in the study, he admitted some of it is complex. "The earthquake part and the engineering are above my head," he said.

Nevertheless, he is satisfied, he said. "We met with them back in May and they met all of our requirements."

What would he tell people in Jackson Hole?

"If they have that big earthquake, the dam is going to be safe," Benner said. "I don't know about the buildings they're going to be in. Multi-storied buildings may be under-designed."

Benner said he understands the security worries of the bureau, but he will work with the county and other state agencies who are seeking to review the engineering study.

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