Induced Left-Lateral Creep on the Hayward Fault following the 1906 San Francisco Earthquake

Authors: Schmidt, D. A., Bürgmann, R., and Kenner, S.

Induced creep has been documented on several Bay Area faults following the 1989 Loma Prieta earthquake (M=7.0). Various responses have been observed ranging from accelerated/decelerated creep to a temporary reversal in slip direction. Larger events, such as the 1906 San Francisco earthquake (M=7.7), are expected to include significant postseismic transients thereby imposing a time-dependent creep response on adjacent faults. We explore the possibility of induced creep on the Hayward fault following the 1906 event using a rate-and-state constitutive law to model the temporal slip response given a time-dependent loading curve. This analysis requires advance knowledge of various fault parameters used to specify the rate-and-state formulation. We invert for the A and B parameters which define the weighting of velocity and state dependence, respectively, using the creep response observed along the Hayward fault following Loma Prieta. In addition, a temporal loading curve is calculated using a finite element code to model the coseismic stress change and the postseismic relaxation. Various models representing different structural geometries of the Bay Area are tested. With these inputs we then calculate the temporal creep response along the southern Hayward fault for each model. The rate-and-state formulation predicts induced left lateral creep along the Hayward fault for all structural models. No detailed observations of creep were made immediately following the 1906 rupture that can confirm nor refute our model predictions. Offset cultural features which record the net displacement are used to constrain the average slip rate observed over a range of time periods since 1906. These data suggest that creep rates along the Hayward fault have remained nearly constant since about 1920 until Loma Prieta in 1989. Given the available data, we find that the left-lateral creep predicted by our modelling is acceptable under the condition that right lateral creep resumes prior to 1920. Each model produces varying left-lateral creep signatures which allow us to discriminate between the different shear-zone geometries.