Tsunami! Could it Happen Here?
by Melissa Siig
In the late 1880s, as the story goes, residents of Glenbrook were hit by a 100-foot wave that nearly knocked over a steamer. Rich Sweichert, a geology professor at the University of Nevada Reno, says that he can't vouch for the accuracy of this account. One thing he is certain of, however, is that over the last 100,000 years both earthquakes and tsunamis have helped to shape the Tahoe Basin. In the wake of the Asian tsunami disaster that killed 170,000 people in 11 countries last December, Tahoe's geological history now takes on new relevance. Is Lake Tahoe at risk of a tsunami similar to the one that struck the Indian Ocean? It depends on how you define risk.
Located between the Sierra Nevada and the Basin and Range provinces, the Lake Tahoe Basin is a tectonically active region, bounded by faults and constantly in flux. The Sierra Nevada block—which includes Sacramento and the San Joaquin mountains—is moving northwest at a rate of 14 millimeters a year. Concurrently, the continental crust is stretching, widening the state of Nevada by 275 km over the last 40,000 years. The rate of stretch increases as it moves westward, reaching its maximum at Tahoe.
"The Lake Tahoe Basin is where the action is," says Sweichert. In recent years, a few minor earthquakes have been recorded in the area. A 4.9 earthquake shook Incline Village in 1998, a 4.5 tremor hit six miles north of Kings Beach in 2004 and in August 2003 1,600 micro-quakes occurred in Carnelian Bay, "most too small to be felt," Sweichert says. "We don't understand what caused this type of earthquake swarm, which was too deep to be associated with the West Tahoe fault."
The effect of a strong quake—magnitude 7 or higher on the Richter scale—under The Lake would be enormous. Similar to what occurred in the Indian Ocean, a displacement of land under the water would create a large tsunami on the surface. A computer model done by UNR's Nevada Seismological Laboratory of the effect of a 7.0 tremor along the West Shore fault predicts waves over 30 feet high! The seiches—oscillation of waves within a closed body of water—would continue for hours after a quake. Depending on the epicenter of the tremor, exact locations around the Lakeshore would either be largely spared, or completely ravaged. Because of its steep shoreline, much of the East Shore would be out of harm's way. But low-lying areas of South Lake Tahoe, Kings Beach, Carnelian Bay and Tahoe City could be hit by multiple walls of water. The Lake's underwater topography has a huge impact on which areas experience the largest waves. For details on the UNR study, and animations of possible scenarios, visit http://www. seismo.unr.edu/htdocs/WGB/LakeTahoeTsunami.
While there is no evidence of past earthquake-induced tsunamis in Lake Tahoe, scientists have found evidence of a tsunami caused by landslides. According to Sweichert, 7,000 years ago Blackwood Canyon glacier's 200-foot-high moraines—accumulations of earth and stones deposited by a glacier—collapsed into The Lake at McKinney Bay, generating giant waves that carved out flat portions of Homewood and Tahoma.
How long after an earthquake would it take for the giant waves to hit shore? "You don't have much time," says UNR Geophysics Professor and laboratory Director John Anderson. "Probably no more than a couple of minutes." Anderson is quick to downplay the risk of tsunamis at Tahoe, however.
"These faults might experience earthquakes of 7.0 or larger once every 1,000 years," he says. Although the likelihood of a Tahoe tsunami happening in our lifetime is slim to none, Anderson advises that if you happen to be at the beach and feel the earth shaking to such a point that it knocks you off your feet, get to higher ground as fast as possible.
If, on the other hand, you are already at least 100 feet above Lake level, he says, "Then get out your video camera. You are about to witness a historic event."