The influence of complex fault geometry on uplift patterns in the Coachella Valley and Mecca Hills of Southern California

Strike-slip faults are often interpreted to have vertical dip in the subsurface following Anderson's theory of faulting. Because we often lack direct data to confirm or deny vertical dip, we may be able to use geomorphic expression to infer subsurface active fault geometry. Vertical or dipping strike-slip faults may have the same strike-slip rates but different dip-slip rates so that observed uplift patterns can be used to discern between alternative fault geometries. We investigate the Coachella Valley segment of the San Andreas Fault as an example. Detailed 3D surfaces for active faults in southern California have been compiled within the Southern California Earthquake Center Community Fault Model (CFM) based on data from geologic maps, seismic reflection data, and microseismicity. 3D mechanical models that use the CFM version 3.5 and apply plate motions on the boundaries produce fault slip rates and uplift patterns that match geologic observations, validating that the CFM nicely represents active fault geometry in many portions of southern California. One region of lingering mismatch is the Coachella Valley segment of the San Andreas fault (SAF). Model uplift patterns produced by a vertical Coachella segment do not match the pattern of sedimentation in the Coachella Valley and rapid ongoing uplift in the Mecca Hills northeast of the SAF. While the Coachella Valley segment of the SAF has previously been modeled as vertical, seismic and structural studies suggest a steep (60-70°) northeast dip on the main segment that shallows to the northwest, where the fault merges at depth with the north-dipping San Gorgonio Pass fault zone. A northeast dip to the Coachella Valley segment of the SAF is also consistent with the inferred position of locking depth of this fault from InSAR data. The most recent version of the CFM (v. 4.0) has seen the addition of several secondary faults in Indio Hills and Mecca Hills as well as the addition of a dipping fault at depth striking sub-parallel to the Coachella Valley segment. We compare uplift patterns and slip rates produced by mechanical models with a variety of alternative fault configurations— a northeast dipping Coachella segment and a vertical Coachella segment with a sub-parallel northeast dipping fault (CFM 4.0). Both of these Coachella Valley alternatives are have been run with and without the five secondary faults on the northeast side of the Coachella segment. Strike-slip rates do not change significantly with vertical or northeast dip to the Coachella segment. However, the modeled northeast dipping Coachella Valley segment tilts the Coachella Valley to the east, better matching observed sedimentation patterns. The addition of secondary faults east of the Coachella segment into the model produces localized uplift at the Mecca Hills, a site of substantial documented uplift. A second set of models investigates the effect of fault geometry on interseismic deformation and compares model results to permanent GPS station velocities. These variations in configuration of the Coachella Valley segment and associated secondary faults demonstrate the impact of fault geometry on uplift/subsidence patterns and slip rates in the Coachella Valley and Mecca Hills.