Simulating the recent evolution of the southern big bend of the San Andreas fault, Southern California

The southern big bend of the San Andreas fault has been interpreted to have successively abandoned two strands, the Mission Creek and Mill Creek strands of the San Andreas fault, during the past 1 Ma before taking up activity along its present-day configuration. This evolution is simulated within three-dimensional models to explore the hypothesis that fault systems evolve to increase mechanical efficiency. The three-dimensional boundary element method models are validated by comparison of modeled fault slip rates and uplift rates with geologic data. The model results match well the geologic strike-slip rates and uplift patterns. Overestimation of reverse slip rates along the northern San Bernardino Mountains may reflect model assumptions of fault geometry. The partitioning of slip among active faults changes between the three phases of southern San Andreas fault evolution revealing (1) a trade-off in strike-slip rates between the San Jacinto and San Andreas faults, (2) a trade-off between strike-slip rates on the San Andreas fault and reverse slip along faults associated with uplifting the San Bernardino Mountains, and (3) a trade-off in strike-slip rates along the San Andreas and strike-slip rates along the Eastern California Shear Zone. Mechanical efficiency of the entire model increases from the Mission Creek to Mill Creek fault configuration and decreases from the Mill Creek configuration to the present-day configuration. The decrease in mechanical efficiency with the transition to the present-day fault geometry may reflect gaps in our understanding of tectonic loading and/or fault geometry.