APPLICATIONS OF THE USGS NATIONAL CRUSTAL MODEL FOR SEISMIC HAZARD STUDIES

The U.S. Geological Survey (USGS) National Crustal Model (NCM) provides geophysical parameterization to assist in the modeling of seismic hazards across the conterminous United States. The NCM is composed of geophysical profiles on a 1-km grid, extending from Earth’s surface into the upper mantle. It is constructed from a three-dimensional (3D) geologic framework and geophysical rules defined by (1) a petrologic and mineral physics database; (2) a 3D temperature model; and (3) a calibrated rock type- and age-dependent porosity model. The NCM uses this information to provide estimates of site response parameters for existing ground motion models (GMMs), including the time-averaged 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 useful in ground-motion modeling could also be extracted or derived from the NCM such as sediment thickness, travel times, fundamental frequency, a frequency-dependent amplification function, or 3D geophysical volumes for wavefield simulations. Application of the NCM may also benefit other aspects of seismic hazard analysis including path-dependent attenuation and geometric spreading; more accurate estimation of earthquake source properties, such as hypocentral location and stress drop; and calculation of crustal strength profiles that inform estimates of seismogenic zone depth.

We present the results of several applications, two of which are being considered in the 2023 update of the USGS National Seismic Hazard Model. In the first application, we combine maps of Z_1.0 and Z_2.5 from the USGS San Francisco Bay Area 3D seismic velocity model and the NCM to estimate basin amplification in the Great Valley of California. In the second, we use temperature profiles derived from the NCM thermal model and Southern Methodist University’s heat flow map to estimate crustal strength and the depth of the brittle–ductile transition, which supplements seismicity-based estimates of seismogenic depth for earthquake ruptures. Lastly, we compute travel times through Phanerozoic sediments in the central and eastern United States, which correlate well with earthquake ground motions and may be used to reduce uncertainty associated with site effects in earthquake hazard analyses.