Arrest and recovery of frictional creep on the southern Hayward fault triggered by the 1989 Loma Prieta, California, earthquake and implications for future earthquakes

摘要

Theodolite measurements across the right-lateral Hayward fault, San Francisco Bay, California, show a dramatic reduction in surface creep rate from 5 to 10 mm/yr before the 1989 Loma Prieta earthquake to nearly zero creep rate after the earthquake. A ～6 year period of nearly zero surface creep was followed by sudden fault creep that accumulated about 20-25 mm of right-lateral displacement followed by an eventual return to a steady creep by year ～2000. This creep behavior can be explained as a result of a sudden shear stress reduction on the fault and is consistent with model predictions for a fault imbedded in an elastic medium with slip governed by laboratory-derived friction laws. We infer friction parameters on the fault using a spring-slider model and a boundary element model with the rate- and state-dependent friction laws. The state (healing) term in the friction law is critical for reproducing the observed evolution of surface creep; a popular simplified rate-dependent friction law is insufficient. Results suggest that the creep event extended to a depth of ～4-7.5 km. The inferred critical slip distance, d_c, is 1-2 orders of magnitude larger than lab values, and inferred au values imply low effective fault-normal stresses of 5-30 MPa. This range of effective normal stress and inversion results for (a – b)σ imply very small values for a – b of 10~(-5) to 10~(-3), suggesting the fault has nearly velocity-neutral frictional properties. Earthquake simulations with such small a – b values show that creeping areas on the Hayward fault may be capable of rupturing during earthquakes.