A nonlinear least squares modeling procedure has been developed to estimate simultaneously hypocenter parameters, station corrections, and velocity model parameters by using P wave (or S wave) arrivals from local earthquakes at a regional array. This procedure is applied to P wave data obtained from the 14-station telemetered seismograph network in the Puget Sound region of western Washington. Forty selected earthquakes with a depth distribution from near the surface to over 50 km were used, providing a stable inverted model having a large step increase in velocity in the upper 10 km of the crust and exhibiting a comparatively low velocity gradient between 10- and 40-km depths. Transition to an upper mantle velocity of 7.8 km/s is indicated at approximately the 41-km depth, earlier measurements of an anomalously low upper mantle city west of the Cascade Range thus being confirmed. The inverted indicates the presence of a low-velocity zone at the base of the crust, although the exact configuration of this zone is difficult to determine because of a lack of resolving power at critical depths. Relocation of a number of earthquakes using the derived model indicates a substantial reduction in average residuals and a corresponding increase in the general confidence level of hypocenter locations for the region.