3D Geologic Framework for use with the U.S. Geological Survey National Crustal Model, Phase 1: Western United States

A 3D geologic framework is presented here as part of the U.S. Geological Survey National Crustal Model for the western United States, which will be used to improve seismic hazard assessment. The framework is based on 1:250,000 to 1:1,000,000-scale state geologic maps and depths of multiple subsurface unit boundaries. The geology at or near the Earth’s surface is based on published maps with modifications to remove discontinuities across state borders. Extrapolation of rock type and age in the subsurface is achieved by iterative stripping of units of a given age, nearest neighbor interpolation of the remaining units, and constraints on basement geology. The subsurface depth of the interfaces between units is determined by a range of models with varying quantity and quality of constraints. Bedrock depth is derived primarily from a proxy model with added geophysical constraint in some areas. The depths to the base of Cenozoic and Phanerozoic sedimentary and extrusive volcanic rocks are constrained by geophysical methods in many areas. Elsewhere, a simple method is used to estimate their subsurface depth based on the distance to the edge of the geologic units. The remaining continental units are evenly distributed above, below, and between depending on age. The oceanic crust is treated as a simpler four-layer model with the added complexity of subduction beneath the North American plate along the Cascadia subduction zone. Refinements to this technique can be accomplished in future versions of the model with more specific information including the location of faults to produce discontinuities in geologic structure and additional information obtained from boreholes and geophysical studies. Further improvements to the geologic framework can be made by incorporating information from more local studies, for example, hydrogeologic studies.