Three-dimensional mechanical simulations of the San Andreas fault system within the Coachella Valley in southern California produce deformation that matches geologic observations and demonstrate the first-order impact of fault geometry on uplift patterns. To date, most models that include the Coachella Valley segment of the San Andreas fault have assumed a vertical orientation for this fault, but recent studies of seismicity and geodetically observed strain suggest that this segment of the fault may dip 60°–70° to the northeast. We compare models with varied geometry along this segment of the fault and evaluate how well they reproduce observed uplift patterns in the Mecca Hills and Coachella Valley. Incorporating well-constrained fault geometry in regional models will provide a more accurate understanding of active faulting in southern California, which is critical for rupture and hazard modeling that is used to identify regions most susceptible to earthquake damage.
We have tested three boundary-element method models for the active geometry of the Coachella Valley segment of the San Andreas fault: one contains a vertical Coachella segment, the second contains a northeast ∼65° dipping Coachella segment, and the final alternative contains a vertical Coachella segment plus a subparallel northeast-dipping fault at depth. This final model honors the geometric interpretation of seismicity from the Southern California Earthquake Center Community Fault Model version 4.0. The models containing vertical Coachella Valley segments both produce uplift between the San Andreas and San Jacinto faults that is more uniformly distributed than geologic observations suggest, and these models fail to produce uplift in the Mecca Hills. The dipping model produces tilting of the Coachella Valley consistent with geologic observations of tilting between the San Jacinto and San Andreas faults. The dipping model also produces relative subsidence southwest of the fault and localized uplift in the Mecca Hills that better match the geologic observations. These results suggest that the active Coachella Valley segment of the San Andreas fault dips 60°–70° to the northeast.