Crustal complexity from regional waveform tomography: Aftershocks of the 1992 Landers earthquake, California

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. Th e model is divided into blocks with variable velocity perturbations such th at ray responses are allowed to shift relative to each other to maximize sy nthetic waveform fits to data. An efficient simulated annealing algorithm i s employed in this search. The technique is applied to a collection of 25 a ftershocks (Landers earthquake) as recorded at two stations, GSC and PFO, s eparated by similar to 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 t o +/-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 impl y 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 t he faster velocities on the west.