Velocity models are developed for the region in and around the southern Diablo Range, lying just east of seismically active portions of the San Andreas and Calaveras faults in central California. An iterative damped least-squares inversion procedure is used to simultaneously estimate hypocentral parameters, station delays, and velocities in a horizontally layered crustal model. Arrival times from both earthquakes and explosions of known location and origin time are used as data. The procedure used has the potential to detect low-velocity layers if they exist. The models obtained show a rapid increase in velocity to about 5.5 km/sec at 3 km depth, another rapid increase to about 6.0 km/sec at 6 km depth, and a roughly linear increase below 6 km with the velocity reaching approximately 6.7 km/sec at a depth of 15 km.