Big Quake Could Roil Lake Tahoe

Floods, wildfires, mudslides, and earthquakes are among the natural hazards that threaten life and property in California. Now, people visiting idyllic Lake Tahoe may face a different peril: giant waves. A new study suggests that an earthquake below the lake could swamp the shores with a devastating tsunami up to 10 meters high, although it may not happen for centuries.

The deep waters of popular Lake Tahoe nestle within a dramatic nook of the California-Nevada border, where tectonic plates shearing past each other have built the Sierra Nevada. In 1998, researchers with the U.S. Geological Survey mapped the lake's floor with sonar, exposing new details about a pair of active faults. And last year, geologists documented that two earthquakes, with probable magnitudes of 7.2 and 7.5, struck within the last 2000 years along a fault just 20 kilometers east of the lake. If a similar quake ruptured within Lake Tahoe, parts of the lake bottom could drop a few meters within seconds.

To study the effects of such sudden underwater motions, a team led by graduate student Gene Ichinose of the University of Nevada, Reno, worked with tsunami expert Kenji Satake of the Geological Survey of Japan in Tsukuba. Computer models showed that a pulse of water would race outward from the disturbance and build to heights of 3 to 10 meters along the coastline. Worse, the lake's steep walls might trap much of the wave's energy, creating a large "seiche" that would slosh back and forth for hours like water in a bathtub, the team reports in the 15 April issue of Geophysical Research Letters.

Ichinose estimates the chances of a magnitude 7 quake under Lake Tahoe as 3% to 4% within the next 50 years. However, the team doesn't yet know how frequently the faults break or how far they typically move. Researchers from the Scripps Institution of Oceanography in San Diego are now using pulses of sound to probe for evidence of recent ruptures beneath the lake floor.

Public awareness of the risk is crucial, especially among those who live near the lake or visit low-lying resorts on the shore, says geologist Alan Ramelli of the Nevada Bureau of Mines and Geology in Reno. "A lot of people at Tahoe don't even realize they have an earthquake hazard," much less the potential for high waves lasting for hours, he says. His colleagues hope to search for telltale deposits of sediments around Lake Tahoe that would pinpoint whether--and when--such tsunamis have risen in the past.

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