USGS Conterminous United States Crustal Model for Seismic Hazard Studies

Abstract:

Lateral and vertical variations of seismic wavespeed in the Earth’s crust can have dramatic impacts on seismic wave propagation. Basins and thin layers of soft sediment, in particular, can substantially amplify ground motions. The amplification typically has a very complex frequency dependence and may consequently impact some man-made structures and not others. Properly characterizing basin parameters required for ground motion modeling, as can be estimated from studies of basin structure, wavespeeds, density and attenuation and impedance contrasts at the basement interface, yields a significant improvement to seismic hazard assessments. Urban Seismic Hazard Assessments, for example in Seattle, Washington, Los Angeles, California, and St. Louis, Missouri, have characterized the near surface in great detail and show more precisely where, by how much, and at what shaking frequency earthquake ground motions will be amplified. Ground motion prediction equations (GMPEs), which are a critical input to the USGS National Seismic Hazard Model, have recently included terms accounting for the effects of basins. To exploit these more sophisticated GMPEs or produce Urban Seismic Hazard like assessments on a broader scale, the USGS has begun a project to compile information needed to produce a crustal model for the conterminous United States, which includes information about shallow seismic wavespeeds. The model will maintain the resolution of component datasets, and an interface to the model will be able to provide values needed for GMPEs, for example, the depths to 1.0 and 2.5 km/s shear wavespeeds, as well as interpolated 3d models for use with various Urban Hazard Mapping strategies. We review: 1) case studies in which the use of basin information has improved seismic hazard assessment; 2) the datasets compiled thus far to produce the USGS Conterminous United States Crustal Model; and 3) how constraints from potential field data might be used to improve knowledge of particular basins.