INTEGRATED GEOPHYSICAL STUDIES OF THE TAHOE CITY SUB-BASIN, LAKE TAHOE, CALIFORNIA

The Lake Tahoe basin has the potential for large magnitude earthquakes even though it has been commonly considered a seismically “quiet” region. The recent 5.0 magnitude earthquake on June 26, 2005, located a few miles north of Lake Tahoe, is the largest and shallowest in an unusual series of earthquakes that may have begun with a deep magma injection event. From August to December of 2003 the Nevada Seismological Laboratory observed a swarm of ~1600 earthquakes with a maximum magnitude of 2.2 located 21-33 km beneath the California-Nevada state border in north Lake Tahoe. Because of this newly revealed seismic potential, it is important to understand the subsurface geology around Lake Tahoe to accurately assess the true seismic hazard of the region.

Tahoe City provides an opportune locality to study subsurface geology because of the accessibility for geophysical surveys, the previous work done here, and the Lake Tahoe Dam and outlet is located here. Our project is integrating geophysical and stratigraphic data to further define and describe the Tahoe City sub-basin and its shaking potential. Borehole data show the sediments consist primarily of a thick sequence of lacustrine silts and clays with interbedded sands, which overlies an older Q-T (?) sand and gravel sequence of unknown origin. The youngest lacustrine deposits locally rest upon 2.0 Ma latites, which in turn rest upon the older sand and gravel sequence. Near the outlet, several fault scarps displace units less than 2.0 m.y. old. We collected new gravity data to provide an estimate of basin depths across the outlet and help define subsurface faults. Refraction microtremor surveys have yielded information about stratigraphy and shear velocities of the Quaternary deposits. The shear wave velocity to 30-m depth obtained for this area is only 334 m/s. These values correspond to a NEHRP soil hazard class of D, similar to that found in other lacustrine basins of the region. Soils in this NEHRP class tend to show significant amplification of shaking, posing increased hazard to structures. We executed a seismic reflection survey across what we believe to be one of the younger scarps to characterize the faulting within this area. We are combining stratigraphic with gravity and seismic data to produce geologic cross sections having information on basin depths and Quaternary faults.