Why are oceanographers working at Tahoe?
In 1998, most of us were fascinated to see the contours of the bottom of Lake Tahoe, courtesy of the U.S. Geological Survey. We learned a new word, bathymetry, which means mapping underwater surfaces, and hung reproductions of lake bottom contours on the wall for family and friends to admire.To seismologists, however, this underwater map was much more. It was a treasure trove of information. They could identify three major faults, plus some minor ones, along the bottom of the lake. And an immense landslide was evident, a slump that had formed McKinney Bay on the west shore of the lake.
Seeing faults and landslides is one thing. Learning when they were formed and what they mean is a much more difficult thing. And who could do it better than an organization that specializes in studying things under hundreds of feet of water? Thats where the Scripps Institution of Oceanography, associated with the University of California at San Diego, comes in. The Scripps staff has sophisticated sonic probes that can extract information from sediments at the bottom of Lake Tahoe, and an impressive way of integrating and visualizing all the information through a computer program.At Tahoe, the sonic probe is towed behind the UC Davis research vessel, the John LeConte. It emits a series of sonic bursts that penetrate sediments down to some 35 meters. Reflections from layers of sediments are received on the boat and reconstructed to provide a picture of a slice through the sediment layer. Because of the noise made by the sonic probe, the instrument is called a Chirp.From those pictures of sediment slices, researchers can see the size of the displacements across the faults and determine the severity of the quakes that have occurred along the bottom of the lake over the past 60,000 years. They can measure the thickness of sediment that has covered the displacement and estimate when the displacement last occurred.
Scripps and University of Nevada at Reno researchers have collected corings of sediments above and around the faults to determine the dates of the sediment layers that are displaced. The cores penetrate sediments up to eight meters. The sediment layers are age-dated using both Carbon-14 and optically stimulated luminescence techniques.According to a paper published last year in the Journal Geology, the McKinney Bay landslide was estimated to have occurred some 60,000 years ago and could have caused a tsunami wave some 100 meters high.Compare that with estimates of potential tsunami waves of 10 meters that could be caused by a magnitude 7 earthquake in the depths of the lake.The paper describes work done by a team of scientists, led by G. M. Kent of Scripps and that included five scientists from Scripps, and scientists from San Diego State University, University of Nevada at Reno, University of California at Davis, Coastal Carolina University, University of Cincinnati, and AVALEX, Inc.According to Kent and his associates, Lake Tahoe sits astride the boundary between the relatively stable Sierra Nevada Block and the more active Basin and Range Block. Three major fault lines run north-south along the bottom of the lake the West Tahoe Fault which becomes the Dollar Point Fault at its northern end, the Stateline Fault, and the Incline Village Fault. And just over the hill, at the eastern base of the Carson Range, the Genoa Fault runs parallel to the faults along the bottom of the lake.The team, using the Chirp instrument, airborne laser bathymetry, and corings, found that the West Tahoe Fault had vertically displaced submerged shoreline terraces by 10-15 meters over a 20,000 year time span. A portion of the McKinney Bay slump that lies across the Stateline Fault was displaced by 21 to 25 meters. They estimate minimum slip rates ranging from at 0.2 to 0.6 millimeters per year for the faults.
That might not seem like a lot, but it amounts to some 20 inches per thousand years, and seismologists deal in events that could be many thousands of years in the making. They explain, however, that the displacements dont move at a steady rate. They are formed by several magnitude 7 earthquakes.Using all the information that they had gathered, the Scripps scientists and their collaborators made an estimate of how often a major earthquake, one that could generate a 10-meter high tsunami wave, might occur. Their estimate was that such an earthquake could occur every three thousand years.Kent and his colleagues will be continuing their investigations of Tahoes earthquake faults this summer. Their knowledge of these faults is integrated into a computer generated visualization of the lakes bottom features, including details about the faults and landslides. Projected on a wall-to-wall screen, it takes the viewer on a fascinating trip along the nether regions of the lake.Its planned to show these visualizations at the new Thomas J. Long Science Education Center, being developed on the Sierra Nevada College campus. Itll be ready in a few months. Dont miss it.Questions or comments? Send them to email@example.com
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