Predicting the evolution of extensional step overs in anisotropic systems with work optimization: Implications for the Hayward-Rodgers creek fault network
By Jessica McBeck, from PGP, University of Oslo.
Recent geophysical imaging of the upper 5 m suggests that the Hayward fault hard-links to the Rodgers Creek fault within the San Pablo Bay, CA, indicating that earthquakes may propagate through the step over. To constrain the geometry of this extensional step over at seismogenic depths, we simulate fault growth and interaction with the modeling tool Growth by Optimization of Work (GROW). GROW predicts fault growth by propagating faults in the orientation that maximizes the change in external work relative to new fault area. Two new implementations to GROW allow exploration of the sensitivity of fault growth to heterogeneities and anisotropy: 1) construction of propagation forecasts of fault growth that indicate a range of highly efficient fault geometries, and 2) fault propagation within anisotropic systems. These implementations applied to fault growth within extensional step overs indicate that early in step over development, a single linking fault propagates across the step over, developing with relatively high propagation power, or rate of efficiency gain. After this first structure develops, faults propagate with reduced propagation power and form increasingly wider basins. The dominant anisotropy direction and strength control fault development. Step over evolution in models with differing initial fault geometries suggest that the Hayward fault links with the Rodgers Creek fault at seismogenic depths if a mapped segment of the Rodgers Creek that potentially extends into the San Pablo Bay is inactive. The model predictions of average slip rate, slip per earthquake and earthquake magnitude closely match paleoseismic estimates.