Development of extensional step overs within anisotropic systems: Implications for the Rodgers Creek-Hayward step over

Jessica A. McBeck, & Michele L. Cooke

Published August 15, 2016, SCEC Contribution #6861, 2016 SCEC Annual Meeting Poster #059

The Rodgers Creek-Hayward extensional step over, within the San Pablo bay in northern CA, USA, is estimated to pose one of the highest likelihoods of rupture in northern CA. To better constrain the fault geometry of the step over at seismogenic depths, we simulate the propagation, interaction and linkage of the Hayward and Rodgers Creek faults with the modeling tool Growth by Optimization of Work (GROW). GROW simulates fault development by adding radial elements to growing fault tips that optimize the change in external work on the system. New GROW implementations allow the simulation of fault development within heterogeneous and anisotropic host rock, which facilitates simulating Rodgers Creek-Hayward fault development within the region’s variably deformed and metamorphosed detrital sedimentary rocks dominated by serpentinite. Here, we construct propagation forecast envelopes to reveal the extent of highly efficient propagation paths that deviate from the optimal due to heterogeneous weaknesses. The predicted forecast envelopes and optimal paths produce step over geometries similar to geometries observed in physical and numerical experiments, and inferred from field observations. We also simulate the strength anisotropy with an internal friction of the host rock that varies non-linearly with orientation. Four parameters define this anisotropy: the maximum internal friction of the host rock, the orientation at which the internal friction is lowest, θmin, the value of the minimum internal friction, μmin, and degrees from θmin at which the internal friction reaches the maximum, θsat. To investigate the sensitivity of fault propagation path to anisotropy we vary θmin, μmin and θsat with GROW simulations of a simple extensional step over. In these simple models, the faults are planar and the separation between the faults approximates the separation observed in geophysical imaging of the Rodgers Creek-Hayward step over. Here, θmin and μmin exert a first-order control on fault propagation path, and the tightness of the weak orientations in the host rock, as parameterized with θsat, exerts less influence. θmin orientations that deviate from the existing faults and cross the extensional step over promote fault linkage. Decreasing μmin promotes fault linkage because less frictional work is required to grow the fault along its most efficient path. GROW simulations with initial fault geometries that more closely approximates the observed geometry reveal that the predicted Hayward fault propagation path more closely matches the geophysical data in models with a shorter Rodgers Creek fault, in which the initial fault segments are underlapping.

Key Words
step over, fault propagation, work minimization, anisotropy

Citation
McBeck, J. A., & Cooke, M. L. (2016, 08). Development of extensional step overs within anisotropic systems: Implications for the Rodgers Creek-Hayward step over. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)