Team Led by Scripps Develops New Profile for Lake Tahoe Earthquake Risk

Scripps Institution of Oceanography / University of California, San Diego

The deep, cobalt-blue waters of Lake Tahoe can mean different things to different people. For residents and tourists of the popular resort destination in the western United States, the lake's waters are a primary component of the area's serenity and beauty. For scientists, the lake's depth and rich color are an impediment to studying several important geological characteristics beneath the lake's basin.

Now, a team led by researchers at Scripps Institution of Oceanography at the University of California, San Diego, has used a novel combination of scientific instruments to produce the first estimates for earthquake activity of several faults in the region.

The team's methods and results are described in the May issue of Geology.

The scientists' new 60,000-year record of fault movement, or "slip rate," melds several emergent technologies and data sources. Scripps Institution's Graham Kent and his colleagues calculated the potential for a large, magnitude-seven earthquake occurring approximately every 3,000 years in the area.

Such an earthquake could produce tsunami waves some three to 10 meters high, research by Kent's colleagues at the University of Nevada, Reno, has shown.

"Such an event would carry the potential for significant damage in the Lake Tahoe region, particularly through tsunami waves-up to 10 meters in height-that would emerge with little or no warning and slosh back and forth across the lake for an extended period of time," said Kent, a geophysicist at Scripps's Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "There are thousands of people on the beaches here in the summertime, so it's important to find out more about the history of such events in the area."

Ongoing and future research by the scientists will involve cataloging individual fault ruptures over the past 10,000 to 20,000 years to assess where each fault lies within its earthquake cycle, a series scientists call "strain accumulation and rupture."

"We are attempting to quantify the recurrence intervals for Lake Tahoe to see how likely such an event might be in the future, especially in light of our new results suggesting a large magnitude earthquake could occur approximately every 3,000 years," said Kent.

Lake Tahoe, which straddles the California and Nevada border in the Sierra Nevada region, is one of the world's deepest freshwater lakes. At more than 1,600 feet deep, the lake covers 193 square miles over a fault basin proven to be prone to earthquakes and landslides.

A native of Lake Tahoe, Kent has been studying the geophysical components of the region for years and says the character of the fault architecture and its history have been difficult to probe because of the lake's depth.

To get a clearer picture, he and his colleagues used a device known as a CHIRP developed by Scripps Institution's Neal Driscoll, a coauthor on the new study. The digital CHIRP profiler shoots acoustic signals at the lake floor to penetrate sediment layers and derive information about its seismic history. The researchers also used airborne laser technology and an additional acoustic mapping system to uncover several different aspects of lake characteristics. Finally, they extracted deep- and shallow-water sediment cores to analyze and date the lake's geologic history first-hand.

In addition to Kent and Driscoll, Scripps Institution coauthors on the paper include Jeff Babcock, Alistair Harding and Jeff Dingler; other coauthors include G. G. Seitz of San Diego State University; J. V. Gardner and L. A. Mayer of the University of New Hampshire; C. R. Goldman, A. C. Heyvaert and R. C. Richards of the University of California, Davis; R. Karlin of the University of Nevada, Reno; C. W. Morgan of AVALEX Inc.; P. T. Gayes of Coastal Carolina University; and L. A. Owen of the University of Cincinnati.

The research was funded through grants from the National Science Foundation, Lawrence Livermore National Laboratory and the National Earthquake Hazard Reduction Program (United States Geological Survey).