Depth to basement and thickness of unconsolidated sediments for the western United States—Initial estimates for layers of the U.S. Geological Survey National Crustal Model

Abstract

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Science plan for improving three-dimensional seismic velocity models in the San Francisco Bay region, 2019–24

“…The USGS National Crustal Model (NCM) is being developed (Boyd, 2019a(Boyd, , 2019bShah and Boyd, 2018) to assist in the modeling of seismic hazards across the conterminous United States, initially by improving estimates of site response. This model is based in part on existing geologic and geophysical models and is composed of geophysical profiles, devoid of discontinuities across constituent model boundaries and county, state, and country borders.…”

“…1. Depth to bedrock and basement (Shah and Boyd, 2018; These elements make use of a host of geology, borehole, gravity, thermal, and seismic datasets. The model is defined on a 1−km 2 grid and, when combined with Biot-Gassmann and mineral physics theory, describes how density and seismic velocities change as a function of porosity, saturation, composition, temperature, and pressure.…”

“…Thickness of unconsolidated sediments in the western United States fromShah and Boyd (2018). Numbered polygons show the bounds of regional models superimposed on top of the background model of Pelletier and others (2016).…”

3D geologic framework for use with the U.S. Geological Survey National Crustal Model, Phase 1—Western United States

“…Following Shah and Boyd (2018), who compiled data from multiple sources to produce maps of the depth to bedrock and basement in the WUS, bedrock is defined as the depth to the base of unconsolidated sediment, that in many cases, is assumed to be the depth to pre-Miocene strata (see for example, Pelletier and others, 2016). In the study of Shah and Boyd (2018), basement, however, has two definitions.…”

“…Each geophysical profile within the NCM is based on subsurface geology, pressure, and temperature coupled with mineral and rock physics theory and porosity and attenuation models. The primary elements used to construct the NCM are: (1) depth to bedrock and basement (Shah and Boyd, 2018); (2) 3D geologic framework (this report); (3) petrologic and mineral physics database (Sowers and Boyd, 2019); (4) 3D temperature model; (5) calibration of a porosity and attenuation model; and (6) validation. These elements make use of a host of geology, borehole, gravity, and seismic datasets to connect subsurface geological variability with geophysical attributes and seismic response.…”