We construct a two-dimensional velocity section sampling the Mojave crustal block in southern California by modeling shear wave (SH) seismograms. Our approach uses individual generalized rays computed from a layered model. The model is divided into blocks with variable velocity perturbations such that ray responses are allowed to shift relative to each other to maximize synthetic waveform fits to data. An efficient simulated annealing algorithm is employed in this search. The technique is applied to a collection of 25 aftershocks (Landers earthquake) as recorded at two stations, GSC and PFO, separated by ~200 km, which bracket the event population along the Landers fault system. The events are assumed to have known mechanisms and epicenters, but both their depths and origin times are allowed to vary. The results indicate considerable variation, especially in the top layer (up to ¿13%), which mirrors surface geology. Best fitting models contain a low-velocity zone in the lower crust if we constrain the crustal thickness (29 km) from receiver function analysis. Reduced lower crustal velocities imply crustal weakening, which appears compatible with the shallow seismogenic zone found in the northern end of this section. There is also evidence for a lateral jump in velocity of several percent across the San Andreas with the faster velocities on the west. ¿ 2001 American Geophysical Union