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.
Compared to what we know about earthquake hazards in California, less is known in the Central US. The main reason for this difference is the relatively few numbers of large earthquakes in the Central US compared to California. The more frequent occurrence of small and larger earthquakes in California gives scientists data that can be used to infer the effects of historical earthquakes in the region and estimate the effects of future large earthquakes. A lack of large earthquakes in the Central US means a lack of data on stronger ground motions from big earthquakes. When there is little observational data, scientists use models to generate estimated data. A model for estimating the effects of an earthquake includes an earthquake source and the earth through which the seismic waves travel. A research paper on this modeling effort (4.7MB PDF) was published in the August 2015 issue of the Bulletin of the Seismological Society of America.
Model overview. The regional structure is constructed by extending the physical properties of most of the well-defined units to less well-constrained areas in the Mississippi Embayment and the CUS: Paleozoic, Crystalline Upper Crust, Lower Crust, Modified Lower Crust and Upper Mantle. The Mesozoic to Cenozoic materials are represented by several units within the Mississippi Embayment and a Vs30 based layer outside the embayment.