THE USGS NATIONAL CRUSTAL MODEL FOR SEISMIC HAZARD STUDIES

The U.S. Geological Survey (USGS) National Crustal Model (NCM) is being developed to assist in the modeling of seismic hazards across the conterminous United States, by using geologic information to improve estimates of site response. The NCM is composed of geophysical profiles specifying, among other quantities, depth-dependent seismic velocity and density on a 1-km grid, extending from the Earth’s surface to the mantle transition zone. It is constructed from five primary elements: (1) maps of the depth to bedrock (considered to be the base of Miocene deposits) and basement (the base of Cenozoic and Phanerozoic deposits), which are constructed from local and regional studies and gravity analyses and represent geologic interfaces that may yield strong seismic impedance contrasts; (2) a 3D geologic framework using this information and surface and subsurface geologic maps with ~200 distinct rock types and ~60 rock ages; (3) a petrologic and mineral physics database; (4) a 3D temperature model; and (5) a 3D rock type- and age-dependent porosity model, which uses Biot-Gassmann theory and is calibrated with borehole observations of seismic velocity and density.

Geophysical profiles from the NCM, which are needed for seismic hazard analyses, can be used to extract site response parameters used in existing ground motion models (GMMs), including the time-averaged shear-wave velocity in the upper 30 meters (V_S30) and the depths to 1.0 and 2.5 km/s shear-wave velocity (Z_1.0 and Z_2.5). Other metrics could also be extracted or derived from the NCM such as fundamental frequency, a fully frequency-dependent site response function, or 3D geophysical volumes for wavefield simulations. Application of the NCM may also benefit other aspects of seismic hazard analysis including better accounting for path-dependent attenuation and geometric spreading, more accurate estimation of earthquake source properties such as hypocentral location and stress drop, and improved assessment of non-ergodic ground motion model uncertainty. The NCM documentation and datasets are publicly available (https://doi.org/10.5066/P9T96Q67), and models can be queried using Python, Matlab, or web services (code.usgs.gov/ghsc/nshmp/ncm; earthquake.usgs.gov/nshmp/ncm).