The morphology of strike‐slip faults: Examples from the San Andreas Fault, California

Fault zones are seldom straight nor are fractures within them continuous. Permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip. These strain fields may be manifest as vertical deformation near the fault. Strain fields that increase area result in subsidence, and strain fields that reduce area result in uplift. Although these effects are modified by erosion and deposition, the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike. Graphical views of strain fields associated with simple vertical strike‐slip faults are derived using boundary element methods. If the fault is straight and parallel to the applied shear strain, the hills and valleys resulting from finite fault rupture are separated by the fault trace. This is rarely observed in nature. Instead, strike‐slip faults cut through hills and valleys apparently of their own making. We show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain. Coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments. We test our theory with examples from parts of the San Andreas fault system in California. The morphologies of the Parkfield region and the southern San Andreas fault in the Coachella Valley are shown to be consistent with plausible distributions of slip on mapped surface faults. The location of the Salton Sea appears to be a consequence of a reduction of slip on the San Andreas fault toward the Brawley seismic zone.