A giant underwater landslide that gouged the bottom of Lake Tahoe thousands of years ago sent a tsunami coursing across the lake and left huge ripples of rock that remain today, geologists have discovered.
Three scuba divers, a robot submarine from Santa Clara University and teams from the U.S. Geological Survey and the University of Nevada in Reno combined forces to piece the story together from evidence they gathered during two years spent surveying what lies beneath the lake's storied blue waters just offshore from Tahoe City, Calif..
The scientists say damaging waves from future landslides along the lake's steep underwater cliffs are clearly possible - but just when, they cannot say.
The big slide undermined the ancient Tahoe shore where McKinney Bay on the lake's west side is now and sent cubic miles of boulders, rocks and soil plunging more than 1,500 feet to the lake bottom, and all the way across until they smashed against the eastern shore, 12 miles away.
The waves that hit the lake's far shore must have splashed tremendously high, said James G. Moore of the Geological Survey's regional office in Menlo Park, who led the survey. The results are being published in the November issue of Geology magazine, a journal of the Geological Society of America.
Just what caused the McKinney Bay landslide remains a mystery, Moore said in an interview, but he speculated that the west side of the "ancestral lake" some 2 million years ago must have held a "great pile of sediments uplifted by faulting that sat there for hundreds of thousands of years, just ready to slide."
Any one of the many small-to-moderate earthquakes that are still common throughout the Tahoe region could have triggered the slide and could do so again, Moore said.
The scientists cannot determine the date of the McKinney Bay slide, but cores drilled from the lake bottom by other geologists have revealed an ash layer deposited from the spectacular eruption of long-vanished Mount Mazama that created Crater Lake in what is now Oregon. Radioactive carbon dating of the core set the age at 7,015 years ago - give or take 45 years - so the landslide must have occurred at least that long ago. Moore and his colleagues believe from fossil evidence that the landslide occurred somewhere between then and 15,000 years ago.
Whenever it happened, it sent one towering wall of water across a shallow triangular region of the lake bottom less than 50 feet below the surface, an area known today as the Tahoe City Shelf.
Christopher Kitts, a Santa Clara University engineer and director of the school's Robotic Systems Laboratory, sent his students scuba diving onto the shelf and also led two imaging missions by the robot submarine Triton - designed and built by his students - across the shelf. Dredges, too, dug material from the area as well as from the farthest reaches of the landslide detritus across the lake.
"The tethered Triton is just a one-horsepower workhorse vehicle, a student-designed beast," Kitts said, "but using it successfully on a scientific mission is a great way for the engineering students to take their final exam."
That's how Moore and his colleagues - aided by the diving students and images from the Triton - discovered the ripplelike stony ridges that the tsunami left as litter across the shelf. The ridges, each 6 feet high and more than a mile long, are made of glacial boulders as large as a yard wide and millions of smaller rocks made largely of relatively fresh volcanic material. They all were swept across the Tahoe City shelf by the onrushing force of the tsunami's water.
Other researchers probing the bottom of Tahoe in the past have discovered at least two significant seismic faults on the lake floor and estimate they could trigger earthquakes with a magnitude as high as 7 that would generate tsunamis across the lake, with waves 10 to 30 feet high.