Geologists Predict Possible Tsunami at Lake Tahoe

A group of geologists is predicting an unlikely scenario for the normally placid waters of Lake Tahoe. It’s possible, they say, that an earthquake could generate 30-foot-high tsunami waves and inundate many areas near the shore.

The study, presented this week at an annual meeting of seismologists, is based on a new analysis of two faults that lie directly beneath the lake. They are now thought capable of generating earthquakes as large as the 7.1-magnitude quake that hit Turkey last November.

The faults were long inaccessible to geologists because they are under more than 1,000 feet of water, but were accurately mapped recently with sonar.

Computer modeling by a group at the University of Nevada at Reno that is working with a Japanese tsunami expert showed ruptures along either fault could lift or drop the bottom the lake and possibly generate a tsunami. The tsunami in turn could trigger seiche waves--massive sloshing that could crisscross the lake, persist for hours and reach heights of 30 feet or more.

“It was surprising to me the wave heights were that high,” said Gene Ichinose, a graduate student in geophysics at the University of Nevada and lead author of the study. Though tsunamis are normally associated with oceans, Tahoe is the 10th-deepest lake in the world, more than 1,500 feet in places, so it contains enough water to generate large and dangerous waves.

While the study is causing some to give new thought to the hazards earthquakes pose to inland bodies of water, it should be no cause for panic, experts said.

“If you have a lake shore house, you shouldn’t sell it,” said David Schwartz, chief of the U.S. Geological Survey’s San Francisco Bay Area Earthquake Hazards Project. The study’s authors estimate the risk of a magnitude 7 quake under Lake Tahoe in the next 50 years to be between 3% and 4%, far less than perennial dangers from forest fires and floods in the region.

Advice to Duck, Cover, Then Sprint

Nevertheless, the study points out that the area may now face an additional hazard. “At this point, what we have in Lake Tahoe is a distinctly valid hypothesis, and it’s worrisome,” said Craig dePolo, a geologist with the Nevada Bureau of Mines and Geology who was not involved in the study but is an expert on Lake Tahoe’s geology.

“For now, people can treat it as an interesting story, but they should keep in mind if they feel a large earthquake in Lake Tahoe, they should first duck and cover and then sprint 30 feet in elevation.”

The closest analogy comes from a magnitude 7.5 earthquake that hit Hebgen Lake in Yellowstone National Park on Aug. 17, 1959.

Witnesses saw “a tsunami and waves that sloshed back and forth for hours that were five or six feet in height,” said Bob Smith, a professor of geophysics at the University of Utah who was a college student at the time of the quake. “It must have been quite a sight.”

The waves threatened but did not take out Hebgen Dam. Most damage at Hebgen Lake and nine deaths were attributed to an 85-million-ton landslide that poured onto a campground and into a river. The landslide itself triggered 30-foot waves, Smith said.

Another area Smith studies, the Great Salt Lake, could also be affected by earthquakes on the nearby Wasatch fault. Tsunamis and seiche waves are not a concern because the lake is relatively shallow, but flooding of sections of Salt Lake City caused by sinking of the lake bed after an earthquake remains a worry.

Fears of tsunamis and seiche waves at Lake Tahoe have been quietly raised before, because of worries that the waves could overtop and wash out Lake Tahoe Dam in Tahoe City. A U.S. Bureau of Reclamation study in the late 1980s predicted earthquake-generated waves of about 10 feet and led to improvements at the dam.

The new study is more precise because it uses a detailed USGS sonar map of the lake’s bottom created in 1998. That map shows that faults under Lake Tahoe have moved considerably in the past and therefore are capable of generating earthquakes of at least magnitude 7.

The map also showed evidence of a massive landslide that tumbled from the Homewood area across the lake on the Nevada side. “That would have been one heck of an event,” said John G. Anderson, who heads the University of Nevada’s seismology lab and was a coauthor of the paper, which is being published Saturday in the journal Geophysical Research Letters.

Fastest-Moving Fault in Region

Remains of that landslide and faults that crisscross the lake show a region rife with geological activity. One of Tahoe’s faults is the fastest-moving in the entire Basin and Range region, which includes Nevada and parts of Utah and California, Ichinose said.

“It’s certainly not a far-out idea they’re proposing. It’s certainly possible,” said Ken Hudnut, a geophysicist with the geological survey in Pasadena. Hudnut said he hopes any interest in Tahoe tsunamis will lead to greater awareness of the huge oceanic tsunami hazard faced by many coastal urban areas that ring the Pacific Ocean, including Southern California.

Anderson said the study could help steer future development of houses and roadways near the lake to safer, higher ground, but only after more studies confirm the findings. Ichinose is already at work modeling how long waves might last and hopes soon to predict which streets might be flooded.

Nevada Bureau of Mines and Geology’s dePolo, meanwhile, is hoping to unearth traces of any prehistoric tsunamis--errant sand deposits underneath today’s forests. While he knows they will be difficult, if not impossible, to find in Tahoe’s rapidly eroding high alpine environment, he says they would help accurately assess the risk of tsunamis in Tahoe and lead to better disaster preparedness.

There is an analogous story in the Pacific Northwest, where USGS geologist Brian Atwater, investigating Indian myths of massive earthquakes and tsunamis, found evidence for both. He even pinpointed the catastrophic event to Jan. 26, 1700, with the help of Japanese tsunami expert Kenji Satake, who is also a coauthor of the new Lake Tahoe study. The work in Seattle led to the development of a tsunami warning system in the Pacific Northwest, even though there have been no tsunamis there during recorded history.

When tsunamis cross the ocean, geologists can often give hours or even days of warning. But in Tahoe, models show, the large waves could be generated in seconds.

“If people feel a really strong earthquake, they don’t have a lot of time,” said Anderson, who stressed that people should be most worried about getting to higher ground if they experience very strong shaking that lasts more than 10 seconds. “It would be information they would not be able to miss.”

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The Big Wave

New computer models of the effects of earthquakes on faults underneath Lake Tahoe show that tsunami waves of more than 30 feet could be generated in near-shore areas by a large quake. Scientists also say high waves moving around the lake could pose a danger for hours after a quake. The danger of a large quake and tsunami within the next 50 years is relatively remote, but it exists.