The USGS 3-D Geologic and Seismic Velocity Models of the San Francisco Bay region provide a three-dimensional view of the geologic structure and physical properties of the region down to a depth of 45 km (28 miles). Construction of this 3D Bay Area model has been a joint effort of the USGS Earthquake Hazards Program and the USGS National Cooperative Geologic Mapping Program.
The purpose of the Three-Dimensional Community Velocity Model for Southern California is to provide a unified reference model for the several areas of research that depend of the subsurface velocity structure in their analysis. These include strong motion modeling, seismicity location, and tomographic velocity modeling. It is also hoped that the geologic community will find the basin models useful because they are based on structures and interfaces that are largely derived from geologic structure models. The deeper sediment velocities themselves are obtained from empirical relationships that take into account age of the formation and depth of burial. The coefficients of these relationships are calibrated to sonic logs taken from boreholes in the region. Shallow sediment velocities are taken from geotechnical borehole measurements. Hardrock velocities are based on tomographic studies.