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Lake Tahoe’s turbulent history explored

MARY THOMPSON, Sun News Service

Using 2-week-old technology, scientists at Lake Tahoe are confirming what they’ve always suspected – the jewel of the Sierra suffered from a violent past.

“Tahoe’s had a pretty catastrophic history. You know that when you have faults that are 40 to 50 feet deep,” said researcher Gordon Seitz, Ph.D. “People have speculated this in the past and this is a way to verify the information.”

Building on a sonar mapping technique used in 1998 by the U.S. Geological Survey to chart Lake Tahoe’s floor, scientists from the Scripps Institute of Oceanography and the Lawrence Livermore National Laboratory have developed a method that allows them to probe below the lake bottom and into the sedimentary layers that make up the lake’s floor.

The newly developed sounding equipment can “see” through about 150 feet of sediment, showing with high resolution on a computer screen the different densities of deposits in a layer-cake transection.

Those layers, the scientists say, tell a rich story of Tahoe’s seismic activity.

By finding the fault lines and then measuring how much sand and silt has filled in the crevices, the team will be able to determine when the faults were most recently active. They will also be able to hypothesize when the ground might shake again.

“You can’t predict an earthquake but it’s sort of statistical,” said Scripps Institute scientist Graham Kent. “If you know an area is prone to having an earthquake every 1,000 years and there hasn’t been one in the last 750, you can guess the time might be coming.”

He said in Tahoe’s case a magnitude 7.4 earthquake originating from a fault near Genoa (Nev.) is possible – so is a 30-foot tsunami.

“A tsunami could cause quite a bit of damage considering that on an average summer day Tahoe’s beaches have about 17,500 visitors,” Kent said. “But it’s not known when we last had a magnitude7 earthquake here. This new technology, along with coring, may help address some of these issues.”

It may also help water quality scientists determine where sediment, the greatest threat to Tahoe’s remarkable and threatened clarity, is flowing into the lake at a higher rate.

Lahontan Regional Water Quality Control Board staffers joined the scientists Wednesday on the University of California, Davis research vessel for a tour of the scientists’ work. As the area’s water quality regulator, Lahontan is interested in new information that may help in its efforts to protect Lake Tahoe from turning into a murky, green lake.

“People are trying to understand how erosion works and in Tahoe’s case, how it affects lake clarity,” Kent said. “This technology is brand new and it provides good insight to what the Upper Truckee river can put out.”

Data collection will allow the scientists to compare sedimentation rates from the 19th century Comstock era, when Tahoe’s forests were clear cut, with current rates and ancient rates. They could also examine how the Tahoe Keys development, which wiped out a significant portion of Tahoe’s largest wetlands, affected silt deposits in the lake.

“It allows people to look back a few hundred or few thousand years,” Seitz said.

After two weeks of charting sediment patterns around the basin, the scientists’ field work is complete.

Seitz said the next step will be taking core samples from layers for Carbon-14 testing. The testing will allow scientists to tag a timeline to seismic and sedimentation events, piecing together another part of Tahoe’s history.

Some seismic mapping was done at Lake Tahoe in the 1970s but the details of the maps were poor, Seitz added.


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