We construct a three-dimensional P wave velocity model of the southern California crust by combining existing one-dimensional models, each describing a region defined by surface geology, and calibrate the model with travel times from three explosions. The model is expressed as blocks, each of a given slowness. The variance of the P wave travel time residuals of ≈1000 earthquakes relocated in and near the Los Angeles basin, where the model is most detailed, is half that of the catalog locations in the standard one-dimensional model for southern California. Starting from the forward model, we invert ≈21,000 P wave arrivals from earthquakes for hypocenters and block slownesses using the technique of Roecker (1981). The variance of these P wave travel time residuals decreases 47% during the inversion. Many of the blocks representing the upper crust and midcrust are well sampled and well resolved. The resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions. For example, the velocity structure of the Los Angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the Peninsular Ranges. Moho is between 26 and 32 km depth. In contrast, the Ventura basin has mostly slower sediments above a deeper, higher-velocity basement. Compared to catalog locations, relocations in the final three-dimensional model of 98 ML≥4 earthquakes throughout southern California tend to deepen below sediment filled valleys and basins, shallow in regions without sedimentary cover, and have a 44% lower P wave travel time residual variance. ¿ American Geophysical Union 1992