Living with faults: Lake Tahoe could be quaking
Mountain-building is a violent business. The Sierra Nevada may not be growing as dramatically as in eons past, but the tectonic forces and fault mechanisms that uplifted the range are still present.
The Sierra Nevada are part of the North American Cordillera, an extensive mountain belt that runs from Wyoming to California. To the casual hiker, climber or skier, the range may seem stable, but it is really in a constant state of uplift that helps offset the weathering process that would otherwise slowly wear the mountains down. Mountains rise along faults and earthquakes result from this building process. Lake Tahoe lies on the western rim of the Basin and Range province, one of the most seismically active regions in the United States.
In years past, scientists studying the bottom of Lake Tahoe have found evidence of huge landslides that they figured were triggered by ancient earthquakes of magnitude 7.0 or larger. Chunks of rock and dirt the size of city blocks broke free and smashed into the lake during powerful temblors thousands of years ago. Geologists have been warning the three major faults that lie directly beneath Lake Tahoe are capable of generating a 7.1-magnitude quake, enough movement to produce tsunami waves 30 feet high.
Two new studies released by scientists at the Scripps Institution of Oceanography confirm the potential magnitude threat and also suggest the West Tahoe Fault is geologically overdue for an adjustment. The largest of the three Tahoe faults, this fracture line runs for 31 miles along the west shore of the lake, under Baldwin Beach, through Fallen Leaf Lake and then under Echo Summit.
According to a paper published in the April issue of Geological Society of America Bulletin, the researchers used a high-resolution seismic technique to investigate active normal faulting across the Tahoe Basin. They also used the global positioning system (GPS) to estimate the off-set movement that puts increasing stress on a fault. A second study by a Scripps’s graduate student involved core sampling of sediment layers at Fallen Leaf Lake, where the West Tahoe Fault crosses the southern end of the lake.
The combined studies provide an unprecedented view of lake bed deformation and time scale of seismic activity. The new data indicate historically, a major earthquake as large as magnitude-7 will shake the region every 2,000 to 3,000 years. The scientists found the West Tahoe Fault last ruptured more than 4,000 years ago, positioning the quake-maker near the end of its characteristic earthquake cycle. In contrast, the Incline Village Fault ruptured relatively recently, about 575 years ago, but both faults have generated offset movements of about 13 feet.
But don’t panic and sell your lakefront home ” scientists estimate the risk of a magnitude 7 quake under Lake Tahoe in the next 50 years to be between 3 and 4 percent, far less than the perennial dangers of forest fires and floods in the region. Experts do suggest, however, that if you are near the lake’s shoreline and feel a severe tremor that lasts for more than 10 seconds, “first duck and cover and then sprint 30 feet in elevation.”
” Weather historian Mark McLaughlin is writing a book about the history of Tahoe winter sports. He can be reached at firstname.lastname@example.org
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