THE SHEAR WAVE VELOCITY STRUCTURE OF THE LITHOSPHERE IN CENTRAL AND NORTHERN CALIFORNIA (RAYLEIGH)

The shear wave velocity structure in northern and central California was studied with three component seismographs deployed in arrays to record both local and teleseismic earthquakes. Travel-time analysis of local earthquake shear waves in the Diablo Range and Napa region provided average crustal shear velocity structures for the two regions. The shear velocities are consistent with an upper and mid-crust of Franciscan assemblage rocks and a lower crust of metagabbroic or gabbroic rocks similar to those of oceanic layer 3.;Phase velocity interpretation for shear velocity structure relies upon the assumption of a horizontally layered medium. The Rayleigh waves observed in our experiments have propagated across structures which vary horizontally. To study the effects of lateral velocity variations on dispersion, we simulated Rayleigh wave propagation in laterally varying structures numerically using finite difference techniques. Analysis of several models demonstrated that phase velocities measured over a gradually varying velocity structure are representative of the average plane-layered structure beneath the measuring array. The modeling showed that in such structures directional dependence in Rayleigh wave phase velocities is small. The results of the numerical modeling suggest that the plane-layered interpretations made from the Rayleigh wave observations in California represent the average elastic properties of the lithosphere.;Fundamental mode Rayleigh wave phase velocities in the period band 16-45 seconds were measured along three parallel paths in the Coast Ranges and Great Valley east of the San Andreas fault. The dispersion curves reflect systematic differences in the lithospheric structure from northwest to southeast along the regional structural strike. Inversions of the data gave shear velocities that are compatible with the compressional velocity structure known from seismic refraction experiments in the Coast Ranges. The largest material properties differences beneath the three paths are in the structure of the lithospheric lid. The data do not resolve the lid thickness, but require either an increasing lid thickness from northwest to southeast for a constant lid shear velocity, or an increasing lid shear velocity for constant lid thickness. The data support a recent hypothesis of lithospheric thickening beneath the northern Coast Ranges along the path of migration of the Mendocino triple junction.