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

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 aσ 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.