Current research
3D geologic map of Silicon Valley
About
the map What info. goes into the map?
The "real" 3-D geologic map
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3D geologic map - a new way of looking at Silicon Valley
What information goes into the map?
Defining critical surfaces
In order to define the shapes and locations of critical
surfaces that make up the 3D geologic map, accurate quantitative knowledge of the positions (latitude, longitude,
depth) of points that lie on the surfaces is needed. Such positional information comes from a variety of different
sources, some based on indirect inference or geophysical interpretation. Several of the more common sources of
positional data are described below. |
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Surface geology provides information on the units to be included, their inter-relationships in both space and time, the major structures present in the map volume, and point data defining the 3-D locations of critical surfaces (faults, contacts, etc.) where they intersect the topographic surface. These point data are combined with other point data from the subsurface to define the full 3-D representation of the critical surfaces.
The magnetic field is used to determine the subsurface shape and location of the buried Logan Gabbro (Jgb) southwest of the San Andreas fault, and various serpentinite bodies northeast of the fault. Small '+' symbols represent earthquakes from 1984 to present.
Lithologic, geophysical, geochemical, and isotopic
logs from water wells, coupled with the geology of adjacent outcrops and fundamental concepts of depositional
environments and sedimentary architecture are used to produce a 3-dimensional model of Cenozoic sedimentary sequences
in the Santa Clara Valley. These modeling efforts focus on determining 3-dimensional sedimentary characteristics
that can act as surrogates for physical properties such as permeability, density, and seismic wave velocity. The
modeling is iterative and subject to repeated testing by comparison with the results of an active drilling program
being conducted cooperatively by the U.S. Geological Survey and the Santa Clara Valley Water District. The program
is designed to drill seven 300 m wells within the valley.
The gravity field, in conjunction with surface geology,
well data, seismic reflection and refraction profiling, and other subsurface data, is used to define the 3-D location
and shape of the contact between the Cenozoic deposits and the pre-Cenozoic bedrock. This contact, which typically
is a strong density contrast in the San Francisco Bay area, displays substantial relief, especially in the Cupertino
and Evergreen basins.
The 3-D distribution of recent earthquakes beneath
the Santa Clara Valley and vicinity is used to define the location and shape of active fault surfaces within the
map volume. Active faults include the Calaveras fault, the Loma Prieta fault, the Hayward fault, a concealed basinward
strand of the Monte Vista fault system, and a possible steep fault within the Diablo Range defined by a band of
earthquakes known informally as the Mount Lewis Trend. |
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