Uncovering Lake Tahoe's Faults

Two new papers provide a deeper look at earthquake vulnerability for the Lake Tahoe region

For more than a decade, scientists at Scripps Institution of Oceanography at UC San Diego have been unraveling the history of fault ruptures below the cobalt blue waters of Lake Tahoe one earthquake at a time. Two new studies by the Scripps research team offer a more comprehensive analysis of earthquake activity in the Lake Tahoe region, which suggest a magnitude-7 earthquake occurs every 2,000 to 3,000 years in the basin, and that the largest fault in the basin, West Tahoe, appears to have last ruptured between 4,100 and 4,500 years ago.

"These studies taken together show that the West Tahoe Fault is capable of a magnitude-7 earthquake - similar to large earthquakes that have occurred on the nearby Genoa Fault - but with the added danger of nearly 500 meters (1,640 feet) of overlying water, which is capable of spawning a large tsunami wave," said Graham Kent, a research geophysicist at Scripps.

These studies, led by Kent and Scripps colleagues Neal Driscoll, Jeff Babcock, Alistair Harding, Jeff Dingler, and Danny Brothers, collected new data on earthquake history along three active faults in the region.

Lake Tahoe, which straddles the California and Nevada border amid the mountains of the Sierra Nevada, is one of the world's deepest freshwater lakes. At more than 501 meters (1,645 feet) deep, the lake covers 191 square miles in a basin prone to earthquakes and catastrophic landslides. The West Tahoe Fault runs along the west shore of the lake and comes onshore at Baldwin Beach, then passes through the southern third of Fallen Leaf Lake, where it descends into Christmas Valley near Echo Summit.

Dingler, lead author on a paper in the April online issue of Geological Society of America Bulletin  and a former Scripps Oceanography graduate student, used a high-resolution seismic imaging technique, known as CHIRP, to supply a comprehensive view of faulting beneath the lake.  Driscoll developed the new digital CHIRP profiler for this study, which provided an unprecedented picture of deformation within the sedimentary layers that blanket the floor of Lake Tahoe, laying the groundwork for more detailed fault studies that continue today.

In a complementary paper, published in the April issue of the Bulletin of the Seismological Society of America (BSSA), Brothers, a Scripps graduate student, investigated the rupture history of the West Tahoe Fault in greater detail. Using comprehensive CHIRP and coring surveys of Fallen Leaf Lake, where the West Tahoe Fault crosses the southern end of the lake, the study confirmed the suspected fault length of more than 50 kilometers (31 miles). When combined with a nearly 4-meter (13-foot) high rupture offset size observed across the fault from CHIRP imagery, the analysis suggests an quake as large as magnitude-7.3 earthquake is possible along the basin's most dangerous fault.

By studying past earthquakes, researchers can estimate future earthquake probability, information crucial to assessing the seismic hazard in a region such as Lake Tahoe, where numerous faults appear to have produced magnitude-7 ruptures during the last few thousand years.

"Once we have this information, we can start informing people of potential risks," said Brothers.

—Annie Reisewitz

July/August 2009