Tsunamis May Have Rocked Lake Tahoe

The Lake Tahoe Basin is not considered one of the more seismically active parts of California.

But new research by a University of Nevada geologist suggests that there have been 7.0-magnitude earthquakes and lake tsunamis in the last 10,000 years. Such activity, he stressed, is extremely rare but will happen again in the future.

In a talk at the Squaw Valley Institute, Richard Schweickert said he and a University of Nevada professor emeritus, Mary Lahren, have found deposits on the west side of the lake that indicate a tsunami with 30-foot-high waves occurred about 7,000 years ago.

Lake Tahoe has a depth of 1,640 feet and sufficient water to allow for tsunamis, he said. The generator of that phenomenon could be a large landslide or the rupture of one of the three active faults existing under the lake.

But a minor earthquake is not enough to start a tsunami. Scientists believe a quake must be at least a magnitude 6.5 to do so.

Eddie Bernard, director of the Pacific Marine Environmental Laboratory in Seattle and perhaps the nation’s foremost tsunami expert, said a Lake Tahoe tsunami was “perfectly conceivable,” although he too emphasized it would not be a common event.

“There can be a lot of shaking and a lot of displacement in a body of water this size,” he observed.

Schweickert noted that a Japanese scientist, Kenji Satake, has done computer models of a Tahoe tsunami and found that waves as high as 33 feet could strike the shore.

Satake said his modeling indicated a tsunami’s largest waves might hit Sugar Pine Point, Rubicon Point and the casinos in South Lake Tahoe.

Schweickert said the Tahoe faults included one on the northeast side of the lake along the north shore, a second stretching from north to west named the Tahoe-Sierra frontal fault zone, and a third running north to south near the northwest side of the lake near Tahoe City.

A magnitude 4.9 quake near Incline Village in 1998 was the most recent moderate temblor in the Tahoe area. A magnitude 5.6 quake near Truckee in 1966 was probably not related to the faults in or near the lake, Schweickert said.

The tsunami deposit that he and Lahren found extends about 10 miles along the McKinney Bay shore of the lake from Sunnyside through Tahoma, Schweickert said.

But, the scientist said, the waves could have been generated not by an earthquake but by the collapse of a 200-foot-high glacial moraine -- an accumulation of earth and stones deposited by a glacier -- into the lake. That would mean a landslide caused the tsunami.

The Tahoe Basin is believed to have been formed 3.6 million years ago by fractures in the Earth’s crust, with the Carson Range of mountains being uplifted to the east and the Sierra Nevada to the west. Lake Tahoe filled the cavity left in between.