Frictional Models of Transient Creep on the southern Hayward Fault, San Francisco Bay, California

Since at least the 1920’s, sections of the Hayward fault have exhibited nearly steady surface creep of 5-10 mm/yr. However, theodolite surveys of monuments spanning ~100 m across the fault show a change in creep rates on the Hayward fault following the 1989 Loma Prieta earthquake on the San Andreas Fault. The creep behavior of the fault immediately after the 1989 Loma Prieta earthquake is characterized by a ~6-year period of near-quiescence followed by a rapid transient creep event that accumulated an average of 20-25 mm of displacement. This behavior has been explained as a response to change in the stress state of the fault as a result of 1989 Loma Prieta earthquake. This creep response to a sudden shear stress reduction is consistent with predictions from spring-slider models with slip governed by rate-state friction. Using spring-slider and a boundary element models with the rate- and state-dependent friction laws, we studied this transient behavior and infer friction parameters on the fault. We inverted the data using a Monte Carlo Metropolis algorithm to obtain the posterior probability distributions for rate-state friction parameters of the spring-slider model. The inverted parameters from the spring-slider model were implemented in forward simulations of the spatial and temporal slip evolution on the fault between 1989 and 2007 using the boundary element model. The boundary element simulation produces a transient event similar in space and time to that observed within this time period. We find a critical slip distance, dc, of order 10^-4.3 to 10^-3.5, which is 2-3 orders of magnitude larger than typical laboratory values from small rock samples. The incorporation of the instantaneous change in effective normal stress allowed for an independent estimation of the average effective normal stress within the range of 20-75 MPa over depths of 0-15 km. Estimates of friction parameter a is in the range 0.0025 to 0.035, consistent with laboratory experiments. Friction parameter b is estimated to be of same order as a indicating friction conditions are nearly velocity-strengthening neutral. The inferred slip patch sizes in the spring-slider model implies surface creep on the southernmost Hayward fault extends to a depth of about 10 km with a low effective normal stress.