The geophysical structure of the Earth’s crust, from the surface to the Moho, plays a major role in seismic hazard by influencing earthquake source properties and wave propagation from the earthquake to the Earth’s surface. We make field measurements and create models to better characterize the crust and resulting earthquake ground motions.
Measurement
Scientists at the USGS maintain a wide variety of instruments for measuring seismic velocities in the crust. We support the engineering community with estimates of the time-averaged shear-wave velocity in the upper 30 meters, termed VS30, via an interactive map and measure shallow velocity profiles that are part of an external database compiled by UCLA. We also measure P-wave and S-wave reflection and refraction profiles to support fault and site response studies (see Data for recent examples).
Modeling
Pacific Northwest
In support of earthquake hazard studies and ground motion simulations in the Pacific Northwest, three-dimensional P- and S-wave velocity (VP and VS, respectively) models incorporating the Cascadia subduction zone were previously developed for the region encompassed from about 40.2°N. to 50°N. latitude, and from about 122°W. to 129°W. longitude. This report describes updates to the Cascadia velocity property volumes of model version 1.3 (V1.3), herein called model version 1.6 (V1.6). As in model V1.3, the updated V1.6 model volume includes depths from 0 kilometers (mean sea level) to 60 kilometers, and it is intended to be a reference for researchers who have used, or are planning to use, this model in their Earth science investigations. To this end, it is intended that the VP and VS property volumes of model V1.6 will be considered a template for a community velocity model of the Cascadia region as additional results become available.
San Francisco Bay Area
A geology-based seismic velocity model is created for the San Francisco Bay Area to support earthquake simulations and ground motion studies.
Central U.S.
Within the upper Mississippi Embayment lies the New Madrid seismic zone, which hosted some of the largest earthquakes in the conterminous United States. The central U.S. seismic velocity model was built to support 3D earthquake simulations and improved estimates of earthquake ground motions.
National Crustal Model
USGS Scientists are also working on a national scale geophysical model to support national seismic hazard assessments. The National Crustal Model is intended to be internally consistent and seamless on a national scale, and care is taken to maximize consistency with existing regional models. An initial version of the underlying 3D geologic framework model is defined for the western U.S. on a 1-km grid.
Other Models
The USGS collaborates with and supports Universities, professionals, and other centers in the development of geophysical models. Examples include models in Southern California, the Wasatch Front in Utah, and Reno, Nevada.
The geophysical structure of the Earth’s crust, from the surface to the Moho, plays a major role in seismic hazard by influencing earthquake source properties and wave propagation from the earthquake to the Earth’s surface. We make field measurements and create models to better characterize the crust and resulting earthquake ground motions.
Measurement
Scientists at the USGS maintain a wide variety of instruments for measuring seismic velocities in the crust. We support the engineering community with estimates of the time-averaged shear-wave velocity in the upper 30 meters, termed VS30, via an interactive map and measure shallow velocity profiles that are part of an external database compiled by UCLA. We also measure P-wave and S-wave reflection and refraction profiles to support fault and site response studies (see Data for recent examples).
Modeling
Pacific Northwest
In support of earthquake hazard studies and ground motion simulations in the Pacific Northwest, three-dimensional P- and S-wave velocity (VP and VS, respectively) models incorporating the Cascadia subduction zone were previously developed for the region encompassed from about 40.2°N. to 50°N. latitude, and from about 122°W. to 129°W. longitude. This report describes updates to the Cascadia velocity property volumes of model version 1.3 (V1.3), herein called model version 1.6 (V1.6). As in model V1.3, the updated V1.6 model volume includes depths from 0 kilometers (mean sea level) to 60 kilometers, and it is intended to be a reference for researchers who have used, or are planning to use, this model in their Earth science investigations. To this end, it is intended that the VP and VS property volumes of model V1.6 will be considered a template for a community velocity model of the Cascadia region as additional results become available.
San Francisco Bay Area
A geology-based seismic velocity model is created for the San Francisco Bay Area to support earthquake simulations and ground motion studies.
Central U.S.
Within the upper Mississippi Embayment lies the New Madrid seismic zone, which hosted some of the largest earthquakes in the conterminous United States. The central U.S. seismic velocity model was built to support 3D earthquake simulations and improved estimates of earthquake ground motions.
National Crustal Model
USGS Scientists are also working on a national scale geophysical model to support national seismic hazard assessments. The National Crustal Model is intended to be internally consistent and seamless on a national scale, and care is taken to maximize consistency with existing regional models. An initial version of the underlying 3D geologic framework model is defined for the western U.S. on a 1-km grid.
Other Models
The USGS collaborates with and supports Universities, professionals, and other centers in the development of geophysical models. Examples include models in Southern California, the Wasatch Front in Utah, and Reno, Nevada.