Fault slip rates and interseismic deformation in the western Transverse Ranges, California

To better constrain fault slip rates and patterns of interseismic deformation in the western Transverse Ranges of southern California, we present results from analysis of GPS and interferometric synthetic aperture radar (InSAR) data and three-dimensional mechanical and kinematic models of active faulting. Anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous GPS site locations. GPS measures contraction rates across the Ventura Basin of similar to 7 mm/yr oriented west-northwest with rates decreasing to the west and east. The Santa Barbara channel is accommodating similar to 6.5 mm/yr in the east and similar to 2.5 mm/yr in the western portions of N/S contraction. Inversion of horizontal GPS velocities highlights a zone of localized fast contraction rates following the Ventura Basin. Using a mechanical model driven by geodetically calculated strain rates, we show that there are no significant discrepancies between short-term slip rates captured by geodesy and longer-term slip rates measured by geology. Mechanical models reproduce the first-order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the Ventura Basin due to the inadequate homogeneous elastic properties of the model. Existing two-dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the GPS data. Mechanical models predict zones of fast contraction in the Santa Barbara channel and offshore near Malibu, suggesting that offshore faults represent a significant seismic hazard to the region. Furthermore, many active faults throughout the region may produce little to no interseismic deformation, making accurate seismic hazard assessment challenging.