Fast Horizontal Contraction without Vertical Strain: Puzzling Interseismic Geodetic Measurements in the Ventura Basin, CA

Ongoing contraction related to the regional-scale left step in the San Andreas fault, (i.e. the Big Bend) has resulted in a complex network of oblique-reverse slip faults that now accommodate shortening across the Ventura basin. Continuous GPS sites from the Plate Boundary Observatory measure horizontal contraction rates across the Ventura basin of approximately 7 mm/yr oriented north-northwest with rates decreasing to the west and east. Inversion of horizontal GPS velocities highlights a zone of localized fast horizontal contraction rates that roughly follow the Ventura basin where seismic velocity models show low modulus sediments. This pattern of localized horizontal contraction can be explained with simple models creeping reverse faults (edge dislocations) at depth; however, such models predict significant uplift gradients that are not observed in the GPS or InSAR data. In fact, the GPS and InSAR show almost no vertical strains in the regions that exhibit fast horizontal strains. Thus, the outstanding unanswered question in the region is: how can interseismic deformation in a contractional setting produce localized horizontal contraction with little to no uplift gradients? To assess whether the simple models are inadequate in their fault geometry, we use a complex interseismic mechanical model incorporating three-dimensional, nonplanar, and geologically constrained fault surfaces from the Southern California Earthquake Center's Community Fault Model (CFM). This model produces very little vertical strains, but cannot match the magnitudes and localization of fast horizontal strains, likely due to the modeled homogeneous rock stiffness. In the end, we suggest that it is possible that a significant portion of the horizontal strains are due to strain localization in the low modulus sediments of the Ventura basin, which may not be released in a future earthquake and potentially mask the interseismic deformation due to faulting. Additionally, the CFM-based model suggests that many faults in the region may have no interseismic signal at all since several faults merge with other faults above their locking depth. Thus, interseismic satellite geodesy may not be able to characterize all potential seismic sources.