LATERAL VELOCITY VARIATIONS IN SOUTHERN CALIFORNIA. I. RESULTS FOR THE UPPER CRUST FROM Pg WAVES

The plate boundary and major crustal blocks in southern California are imaged by a tomographic backprojection of the Pg first arrivals recorded by the Southern California Array. The method, formulated specifically for local earthquake arrival times, is a fast, iterative alternative to direct least-squares techniques. With it, we solve the combined problem of determining refractor velocity perturbations and source and station delays. Resolution and variance are found empirically by using synthetic examples. A map showing lateral velocity variations at a depth of approximately 10 km is presented. The results show a strong correlation with surface tectonic features. Clear velocity contrasts exist across the San Andreas, the San Jacinto, and the Garlock faults. The Mojave region has the slowest velocities while the Peninsula Ranges have the highest. The San Jacinto block has velocities intermediate between Mojave and Peninsula Range velocities, and also has early station delays. This may indicate that the San Jacinto block has overridden Mojave material on a shallow detachment surface. No velocity variations are found associated with the Transverse Ranges, which we interpret to mean that the surface batholithic rocks in this area do not extend to Pg depths.