Induced creep was documented on several Bay Area faults following the 1989 Loma Prieta earthquake. Various responses were observed ranging from accelerated/decelerated creep to a temporary reversal in slip direction ( Galehouse, 1997). Larger events, such as the 1906 San Francisco earthquake, 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 (Linker et al., 1992). 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 (Kenner et al., 1998). 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 (Lienkaemper et al., 1997). These data suggest that averaged 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 modeling is acceptable under the condition that right lateral creep resumes by the 1920's.
Each model produces varying creep signatures which allow us to discriminate between the different shear-zone geometries. Three different rheological models are used to compute the stressing history imposed on the Hayward fault following the 1906 event (Figure 18.1). Both a purely elastic model and an elastic model that includes vertical shear zones beneath the San Andreas, Hayward, and Calaveras faults produce a transient left-lateral response followed by a return to the secular right lateral creep rate in 20 to 40 years. A model that includes a horizontal shear zone at a depth of 15 km predicts a prolonged initial left-lateral response with a slow return to the secular slip rate over a century. Our modeling suggests that in future seismic events, surface creep records could provide an additional source of information to discriminate between differing models of upper crustal rheology.
Galehouse, J. S., Effect of the Loma Prieta earthquake on fault creep rates in the San Francisco Bay region, U. S. Geological Survey Professional Paper 1550D, 193-207, 1997
Kenner, S., and P. Segall, Time-dependence of the stress shadowing effect and its relation to the structure of the lower crust, Geology, 27, 2, p. 119-122, 1999
Lienkaemper, J. J., and J. S. Galehouse, Revised long-term creep rates on the Hayward fault, Alameda and Contra Costa counties, California, USGS open-file report 97-690, 1997
Linker, M.I., and J. H. Dieterich, Effects of variable normal stress on rock friction: observations and constitutive equations, J. Geophys. Res., 97, pp. 4923-4940, 1992