Plate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike-slip regime in the Miocene. Strike-slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near-vertical on the surface. However, some previous work suggests these faults have a dipping or listric geometry at depth. We analyze receiver function arrivals that vary harmonically with backazimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault-parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike-slip faults and find consistently NE-dipping lineations in seismicity shallowing in dip from near vertical (80-85) on the Elsinore Fault near the coastal ranges to 60-65 slightly further inland on the San Jacinto Fault to 50-55 on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest
that preexisting fabrics and faults likely act as strain guides in the modern slip regime, with reactivation-like mechanisms operating both above and below the brittle-ductile transition.