Strain localization and crustal thickness variation across major strike slip faults in southern California revealed by receiver function studies

The degree to which faults are localized or distributed within the continental lithosphere has been a controversial subject for decades. Reflection and refraction studies indicate that the San Andreas fault (SAF) extends into the lower crust as a narrow fault zone in northern California and along the Mojave segment of the fault. However, the discreteness of lithospheric deformation along the southernmost segments of the SAF system (e.g., the Elsinore, the San Jacinto and the Coachella segment of the San Andreas) remains unclear. We have begun a detailed study of how the distribution of strain at depth is in southern California using P receiver functions to image crustal and lithospheric thickness variations. We specifically focus on the depths of the Moho and lithosphere-asthenosphere boundary (LAB) underneath the major faults. Our results for the geometry of crustal and lithospheric interfaces provide insight into the strain localization in this region. Our preliminary results using events recorded by Southern California Seismic Network and USarray reveal complex crustal and lithospheric structure in this region. A SW-NE profile across the Elsinore, San Jacinto, and and San Andreas faults indicates that the Moho dips to the southwest. Moreover, receiver gathers at certain stations near the faults show a strong back-azimuthal variation. Using simple velocity models to project the Ps conversions to depth, we find that some of the back-azimuth variation can be related to different sides of the faults. These back-azimuthal variations of the Moho signal indicate three-dimensional complexity beneath the central San Jacinto fault that may suggest strain localization at these depths. We assess the robustness of these features with respect to the choice of velocity model and method resolution using synthetic waveform modeling.