2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 318-5
Presentation Time: 9:00 AM-6:30 PM


MCBECK, Jessica A., Department of Geosciences, University of Massachusetts Amherst, 611 North Pleasant St, Amherst, MA 01003, COOKE, Michele, Department of Geosciences, University of Massachusetts Amherst, Amherst, MA 01003 and MADDEN, Elizabeth H., Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003, jmcbeck@gmail.com

Strike slip fault systems often develop through the linkage of fault segments within extensional stepovers. Constraining the fault geometry, and resulting mechanics, of the extensional stepover in the San Pablo Bay, between the Hayward and Rodgers Creek faults, can shed insight on seismic hazard in the San Francisco region. To investigate the mechanical efficiency and evolution of this enigmatic fault network, we simulate the propagation of faults within the stepover with the numerical modeling tool Growth by Optimization of Work (GROW). GROW predicts the evolution of a fault system by analyzing the gain in efficiency, or change in external work, produced by propagation and interaction. We load the San Pablo Bay stepover models with right-lateral velocity and normal compression that reflects a range of seismogenic depths. We perform distinct GROW analyses with different initial fault configurations that represent potential fault geometries at the onset of interaction between the faults. Throughout the development of each fault network we analyze the evolution of external work, and change in external work (ΔWext) due to fault growth, interaction and linkage. The GROW analysis with initially overlapping fault segments separated by 5 km predicts that the Hayward and Rodgers Creek faults propagate toward one another in gently curved paths that form a symmetrical pattern. The curved path of the fault segment representing the Hayward fault disagrees with the observed planar fault trace, which suggests that this fault may precede the southern propagation of the Rogers Creek fault. The GROW analysis of the fault network including both segments of the southern end of the Rodgers Creek fault shows that propagation from the tip of the northeastern segment initially produces a significantly smaller change in work than propagation from the southwestern segment. Additionally, under a range of loading conditions, the southwestern segment continues to propagate for a longer period of model time than the northeastern segment. These results are consistent with geologic data suggesting that the southwestern segment is active and the northeastern segment presently does not have a significant slip rate.