Compressional and Shear Wave Velocity Versus Depth in the San Francisco Bay Area, California: Rules for USGS Bay Area Velocity Model 05.0.0 (USGS Open-File Report 2005-1317 by Thomas M.
defining the extent of groundwater aquifers and modeling of subsurface transport of contaminants.įor additional information on 3-D Geologic Models see:.
predicting where destructive liquefaction of the ground may occur.
forecasting strong ground motions that may damage buildings and essential infrastructure or destroy levees in the Sacramento Delta.
Some additional important applications of these new models include: These models have been used in computer simulations of the 1906 San Francisco and 1989 Loma Prieta earthquakes. By assigning velocities to rock types in the 3-D geologic model, geologists can gain an understanding of the extent of areas of low shear velocity that are most likely to experience localized strong shaking and future earthquake damage. Seismic velocities of rocks and sediments at shallow depths determine how strongly an area will shake. At least two properties of the Earth's crust conspired to cause this collapse: it was built on loose soils that shook much more strongly than surrounding regions on stronger ground, and variations in the thickness of the Earth's crust between the epicenter of the Loma Prieta earthquake in the Santa Cruz Mountains and Oakland actually focused energy toward Oakland and downtown San Francisco. The Cypress Structure, the freeway approach to the Bay Bridge from Oakland, collapsed during the Loma Prieta earthquake, killing 42 people. The soft sediments in these basins trap seismic energy and greatly enhance shaking levels relative to surrounding regions. The model also includes the subsurface shape of basins that underlie the Santa Clara Valley, Livermore Valley, and Santa Rosa Plain. The geologic model is a fault block model - that is, the upper 45 km (28 miles) of Earth's crust has been broken up into irregular shaped blocks, bounded by faults. Traditional two-dimensional geologic maps show only the distribution of rock units at Earth's surface. They also include information from boreholes and variations in Earth's gravity and magnetic fields. The 3-D models combine 100 years of surface geologic mapping by the USGS, the California Geological Survey, and many other institutions together with decades of research into the seismic properties of the rocks in the Bay Area. Different colors correspond to different rock types found in the region. Black dots show the epicenters of the 1906 San Francisco earthquake and the 1989 Loma Prieta event. The blue rectangle outlines the region covered by the 3-D geologic and seismic velocity models. Traditional two-dimensional geologic map of the San Francisco Bay region.
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