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

Several recent studies in the Lake Tahoe basin have called attention to the potential for serious earthquakes and earthquake-related tsunamis. Tahoe City is the location of the outlet for Lake Tahoe, and therefore the history of lake level fluctuations should be recorded in sediments beneath the outlet. Based on borehole data (Tahoe City Public Utilities District), the sediments consist primarily of a thick sequence of lacustrine silts and clays with interbedded sands. Beneath this unit is an older Quaternary-Tertiary (?) sand and gravel sequence of unknown origin. The lacustrine deposits locally rest upon 2.0 Ma latites, which in turn rest upon the older sand and gravel sequence. In and around Tahoe City, several fault scarps displace units less than 2.0 Ma old. Additional important questions include the stability of the Lake Tahoe dam site at Tahoe City and the sequence and extent of lake level high stands.

We are integrating various types of geophysical and stratigraphic data to further define and describe the Tahoe City sub-basin. New gravity data provides an estimate of basin depths and helps define subsurface faults. Preliminary analysis of this data suggests the maximum basin depth is 180 m near the outlet. Refraction microtremor (REMI), refraction, and reflection surveys yield information about stratigraphy and shear wave velocities of the Quaternary deposits. The average shear wave velocity at 30 meters depth (V₃₀) obtained for this area is 334 m/s. These values correspond to a NEHRP soil rating of D which is similar to that found in the San Francisco Bay area. Soils with these NEHRP ratings tend to show a significant amplification of shaking posing a danger to structures. Work in progress includes detailed mapping, stratigraphic analyses of cores taken in and around Tahoe City and additional geophysical surveys. We are combining these data sets with earlier gravity and seismic data to produce geologic cross sections with information on basin depths and Quaternary faults. A goal of this ongoing study is to integrate all available data to identify evidence for past lake level fluctuations.