3-DIMENSIONAL GEOLOGIC MAP OF THE SANTA CLARA ("SILICON") VALLEY, CALIFORNIA

The USGS recently began a long-range project to construct a 3D geologic map of the Santa Clara ("Silicon") Valley, southern San Francisco Bay area, California. This is part of a larger project that also involves developing techniques for constructing 3D models, defining uncertainties associated with geologic elements and properties, and designing procedures for visualizing, providing user access to, and releasing 3D geologic information. This multipurpose map is intended to provide a quantitative basis for modeling processes including groundwater flow, contaminant dispersion from naturally occurring mercury and asbestos, ground shaking, seismic wave propagation, and tectonic strain accumulation. The fundamental map architecture is defined by critical surfaces (faults, intrusive contacts, unconformities, other depositional contacts) interacting to form volumes, which ultimately are assigned properties based on geologic identity, geometric position, or both. Quantitative definition of critical surfaces is based mainly on surface geology, drillhole data, cone penetrometer testing, gravity and magnetic modeling, seismic reflection and refraction profiling, and seismicity. Critical surfaces are assembled into a 3D map using EarthVision modeling software.

The map volume is 45X45 km by 14 km deep, and spans the valley floor and surrounding hillsides between the San Andreas and Calaveras faults. It is divided by 12 major faults into blocks, within which the Cenozoic section is represented by up to three layers, and the Mesozoic section by more than six units. The map exists in the computer as 1) a set of numerical grids that quantitatively define the positions of the critical surfaces, 2) a set of instructions that specify how these surfaces interact when they encounter each other, and 3) the software to assemble the surfaces according to the specified instructions and to assign properties. The present 3D map includes the fundamental geometry, architecture, and interaction instructions, although quantitatively, many of the surfaces exist only as approximately defined surrogates. This framework allows us to progressively refine the individual surfaces without altering the fundamental model architecture.