Work Optimization Predicts the Evolution of Extensional Step Overs Within Anisotropic Host Rock: Implications for the San Pablo Bay, CA

Abstract

AbstractRecent geophysical imaging indicates that the Hayward Fault hard links to the Rodgers Creek Fault at 5 m depth within the San Pablo Bay, CA, suggesting that earthquakes may be able to rupture continuously through the fault network. To investigate fault propagation, interaction, and linkage in segmented fault networks, including those within the San Pablo Bay, we simulate the development of two idealized, underlapping faults within an extensional step over at seismogenic depths using work optimization. We test the sensitivity of fault growth to strength anisotropy, material heterogeneities, and initial fault geometry. The optimal faults propagate toward each other until linking with the other fault at its tip and form a single hard‐linked transverse fault. These faults propagate with relatively high propagation power or rate of efficiency gain. Less efficient faults form wider basins and develop with reduced propagation power. Models with initial fault geometries that more closely match the shallowly imaged Hayward and Rodgers Creek faults suggest that the faults link at seismogenic depths if a mapped segment of the Rodgers Creek that extends into the San Pablo Bay is currently inactive. Predictions of average slip rate, slip per earthquake, and earthquake magnitude from these models closely match paleoseismic estimates. The hard linkage of the Hayward and Rodgers Creek faults imaged in the near‐surface, and predicted by these models, increases local seismic hazard by increasing the upper limit of throughgoing earthquakes to M 7.6.