Inversions of interseismic geodetic surface velocities often cannot uniquely resolve the three-dimensional slip-rate distribution along closely spaced faults. Microseismic focal mechanisms reveal stress information at depth and may provide additional constraints for inversions that estimate slip rates. Here, we present a new inverse approach that utilizes both surface velocities and subsurface stressing-rate tensors to constrain interseismic slip rates and activity of closely spaced faults. We assess the ability of the inverse approach to recover slip rate distributions from stressing-rate tensors and surface velocities generated by two forward models: 1) a single strike-slip fault model and 2) a complex southern San Andreas fault system (SAFS) model. The single fault model inversions reveal that a sparse array of regularly spaced stressing-rate tensors can recover the forward model slip distribution better than surface velocity inversions alone. Because focal mechanism inversions currently provide normalized deviatoric stress tensors, we perform inversions for slip rate using full, deviatoric or normalized deviatoric forward-model-generated stressing-rate tensors to assess the impact of removing stress magnitude from the constraining data. All the inversions, except for those that use normalized deviatoric stressing-rate tensors, recover the forward model slip-rate distribution well, even for the SAFS model. Jointly inverting stressing rate and velocity data best recovers the forward model slip-rate distribution and may improve estimates of interseismic deep slip rates in regions of complex faulting, such as the southern SAFS; however, successful inversions of crustal data will require methods to estimate stressing-rate magnitudes.

It has been known for decades that the present-day shortening rates across the Western Transverse Ranges (WTR) in southern California are rapid, reaching 10-15 mm/yr near the heavily populated Los Angeles area. However, only recently have geodetic measurements of vertical motion in the WTR been sufficiently dense to resolve a tectonic vertical signal. In this study, we show that much of the geodetically-derived vertical velocity field in the WTR can be attributed to the interseismic signal of strain accumulation on reverse faults. We invert geodetic and geologic data for slip rate and interseismic coupling on faults using a kinematic model consisting of faults embedded in an elastic crust over an inviscid mantle. This method allows us to infer the permanent, long-term component of vertical motions from recoverable, short term motions. We infer that much of the geodetically observed 3-4 mm/yr of differential vertical motion across the WTR, involving subsidence along the Santa Barbara coastline and uplift of the Santa Ynez Range, can be attributed to recoverable elastic deformation associated with interseismic locking on faults dipping under the WTR. The sum of dip-slip rates across the WTR decreases from 10.5-14.6 mm/yr on the east side near Ventura, California to 5-6.2 mm/yr across the western side of the Santa Barbara Channel. The total moment accumulation rate in both the Santa Barbara Channel and the combined San Fernando Valley-LA Basin regions is equivalent to about two M=7 earthquakes every 100 years.