Two new studies suggest the Lake Tahoe region in California and Nevada has gone longer than usual without a large earthquake and may be due for a magnitude-7 temblor capable of spawning a tsunami that could flood shoreline communities.
The research targeting three major faults was conducted by scientists at the Scripps Institution of Oceanography at the University of California-San Diego.
They determined that earthquakes as large as magnitude-7 historically have occurred every 2,000 to 3,000 years in the Tahoe basin.
But they found that the largest fault in the basin — the West Tahoe Fault running along the west shore of the lake and out the south end just west of South Lake Tahoe — appears to last have ruptured between 4,100 and 4,500 years ago.
The new data suggests that the most recent ruptures along the West Tahoe and Incline Village faults each produced nearly offsets of about 13 feet. The most recent event along the Incline Village Fault occurred about 575 years ago, they said.
“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 (1,600 feet) of overlying water, which is capable of spawning a large tsunami wave,” said Graham Kent, a research geophysicist at Scripps.
Jeff Dingler, lead author on a paper in the April online issue of Geological Society of America Bulletin and 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.
Scripps’ Neal 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 another paper published in the April issue of the Bulletin of the Seismological Society of America, Scripps graduate student Danny Brothers 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 over 31 miles.
When combined with the rupture offset size observed across the fault from CHIRP imagery, the analysis suggests an upper limit of a magnitude-7.3 earthquake for the basin’s most dangerous fault.
The researchers said the new analysis — along with a slip-rate approaching 0.8 millimeter per year and the rupture timeline taking place between 4,100 and 4,500 years ago — places the West Tahoe Fault near the end of its characteristic earthquake cycle.
The researchers cautioned that some degree of variability is to be expected. Earlier research at the University of Nevada, Reno, suggests such an earthquake could produce tsunami waves some 10 to 33 feet high.
At 1,645 feet deep, Lake Tahoe is one of the one of the world’s deepest freshwater lakes. It covers 191 square miles in a basin prone to earthquakes and catastrophic landslides.
A quake with a magnitude of 6 can cause severe damage, while one with a magnitude of 7 can cause widespread, heavy damage. An increase in one full number — from 6.5 to 7.5, for example — means the quake’s magnitude is 10 times as great.
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.
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