A THREE-DIMENSIONAL GEOLOGIC MAP OF THE CENTRAL CALIFORNIA COAST RANGES

We have constructed a 3D geologic map of west-central California for the region from Monterey Bay to the Santa Barbara Channel and from the San Andreas Fault westward to the continental shelf, including the whole crust and uppermost mantle. The map volume is divided by 18 faults into 19 blocks, each having a distinct lithostratigraphic makeup. Each block-bounding fault satisfies some of the following: 1) recognized as important in San Andreas system; 2) >100 km long (including aligned faults with consistent structural style); 3) accommodates large total offset (>1 km vertical, >5 km horizontal); 4) is Quaternary active; 5) represents a profound physical property boundary. Ambiguity in the shapes and hierarchy of the Rinconada and Oceanic Faults resulted in 4 different 3D maps. The main purpose for constructing the 3D map was to support a new probabilistic seismic hazard assessment of the central California Coast Ranges via better understanding of the subsurface geometry of the faults, but the map is designed for other applications as well. For example: The present map can support computer-based predictive ground-shaking estimates from scenario earthquakes once physical properties are assigned based on geology, and calibrated with the recent San Simeon and Parkfield earthquakes. Simulations of ground shaking based on a similar 3D map have been successful for the 1906 San Francisco earthquake and for a suite of scenario earthquakes on the Hayward Fault. The central California Coast Ranges have a complex tectonic history since inception of the San Andreas Fault system involving 100s of km of lateral slip on bounding faults, 10s of km of lateral slip on internal faults, kms of shortening across thrust/reverse faults, rotation of the western Transverse Ranges, and development of large deep sedimentary basins. The 3D geologic map and its associated geologic and geophysical data can test models of tectonic evolution of the Coast Ranges by providing 3D geometric, volumetric, and offset constraints. Quantitative ground-water flow modeling is routinely used for evaluating and managing ground-water resources and requires detailed 3D geologic aquifer models. The present 3D geologic map lacks the detail needed to directly support flow modeling, but provides the foundation upon which to add detailed geologic aquifer models.