Seismic images of the deep structure of the San Andreas fault system, Central Coast Ranges, California.

Three-dimensional inversion of teleseismic P wave travel time residuals recorded at the US Geological Survey central California array has resolved small-scale (?? tens of kilometers) crustal and upper mantle heterogeneity down to depths of 90 km beneath the California coast ranges. Upper crustal lateral velocity variations of +-8% correlate closely with surface geology. Lower-than-average velocities are associated with thick Tertiary sedimentary fill and higher-than-average velocities with basement exposures. Lower crustal velocity heterogeneity of +-4% appear to reflect crustal thickness variations. A thinner crust is indicated southwest of the San Andreas fault and northwest of San Pablo Bay. A linear zone of low-velocities (0 to 4%) subparallel to the San Andreas fault was resolved in the upper mantle. The preferred interpretation is that the low-velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths (??45 km) beneath the coast ranges. Such an unusual mantle structure may have been produced by the northwestward migration along the California coast of a transiently unstable Mendocino triple junction. The inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle. -Author