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

We have simulated several scenarios of dynamic rupture propagation for the 1994 Northridge, California, earthquake, using a three-dimensional finite-d ifference method. The simulations use a rate- and slip-weakening friction l aw, starting from a range of initial conditions of stress and frictional pa rameters. A critical balance between initial conditions and friction parame ters must be met in order to obtain a moment as well as a final slip distri bution in agreement with kinematic slip inversion results. We find that the rupture process is strongly controlled by the average stress and connectiv ity of high-stress patches on the fault. In particular, a strong connectivi ty of the high-stress patches is required in order to promote the rupture p ropagation 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 obtai ned in the kinematic inversion results. However, when the amount of rate-we akening is increased, the overall moment drops dramatically unless the aver age prestress is raised to unrealistic levels. A velocity-weakening paramet er 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 negligibl e 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 d ynamic simulation provides a good fit to strong motion data at sites NWH an d 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 simul ations.