COMPLEX ANISOTROPY BENEATH PARKFIELD, CALIFORNIA FROM SKS, SKKS, AND PKS SPLITTING

A detailed shear-wave splitting study of the portion of the San Andreas Fault in the vicinity of Parkfield, California is conducted. This study involves 61 broadband stations, from which over 1000 PKS, SKKS and SKS measurements are evaluated and ranked according to their quality. Of these measurements, over 90 well-defined measurements are identified as being of sufficiently high quality to infer the anisotropic characteristics associated with mantle deformation beneath the study area. The results of this study confirm and support findings of some previous studies of the Parkfield area regarding the existence of two-layer anisotropy in the vicinity of the San Andreas Fault. The top layer resides in the lithosphere, with a fast direction roughly parallel to the fault and in the direction of absolute plate motion for the Pacific plate in this region. The lower layer is in the asthenosphere with an E-W fast direction in the direction of absolute plate motion for the North American plate and consistent with the direction of subduction of the Farallon plate. This is the first high-resolution shear-wave splitting study in the Parkfield area.