Constraints on stress and friction from dynamic rupture models of the 1994 Northridge, California, earthquake

We have simulated several scenarios of dynamic rupture propagation for the 1994
Northridge, California, earthquake, using a three-dimensional finite-difference method. The simulations
use a rate- and slip-weakening friction law, starting from a range of initial conditions of stress and
frictional parameters. A critical balance between initial conditions and friction parameters must be met
in order to obtain a moment as well as a final slip distribution in agreement with kinematic slip inversion
results. We find that the rupture process is strongly controlled by the average stress and connectivity of
high-stress patches on the fault. In particular, a strong connectivity of the high-stress patches is required
in order to promote the rupture propagation from the initial nucleation to the remaining part of the
fault. Moreover, we find that a small amount of rate-weakening is needed in order to obtain a level of
inhomogeneity in the final slip, similar to that obtained in the kinematic inversion results. However,
when the amount of rate-weakening is increased, the overall moment drops dramatically unless the
average prestress is raised to unrealistic levels. A velocity-weakening parameter on the order of 10 cm
per second is found to be adequate for an average prestress of about a hundred bars. The presence of
the free surface and of the uppermost low-impedance layers in the model are found to have negligible
influence on the rupture dynamics itself, because the top of the fault is at a depth of several kilometers.
The 0.1–0.5 Hz radiated waves from the dynamic simulation provides a good fit to strong motion data
at sites NWH and SSA. Underprediction of the recorded peak amplitude at JFP is likely due to omission
of near-surface low velocity and 3-D basin effects in the simulations.