3D DENSITY AND VELOCITY MODEL OF THE LOS ANGELES BASIN

We present a 3D velocity model and a new 3D density model of the LA basin. Both the density structure and velocity structure must be defined in a 3D model for its use in performing simulations of earthquake wave propagation with a spectral element method, to predict the distribution of hazardous ground shaking during large events. The LA basin velocity model was constructed using sonic log and stacking velocity information, provided by oil industry sources and not previously incorporated into southern California velocity models. The density model is based upon a new database of approximately 300 oil industry density logs from across the Los Angeles basin. These logs use gamma ray emissions to determine formation density at samples of about one meter. We have developed an empirical relation between velocity (Vp) and density by comparing data from approximately 30 wells in which we have both sonic and density logs. For the remaining wells with only density logs, we have derived relationships between depth and density, and between lithology and density, by defining the empirical depth vs. density functions for the three main stratigraphic sub-divisions of the SCEC Phase 2 model (Quaternary to base Pico Fm., top Repetto Fm. to top Mohnian, and top Mohnian to basement). The density-depth and density-velocity relations will provide independent rules that can be employed to define density and velocity structure in areas where data does not exist, or in other areas with similar lithology to the Los Angeles basin.