Dynamic Rupture Models for the Southern San Andreas Fault

Geoffrey P. Ely, Steven M. Day, & Jean B. Minster

Published 2010, SCEC Contribution #1147

Dynamic rupture, and resultant ground motions up to 0.25\,Hz, are simulated for a $M_w7.6$ earthquake on the southern San Andreas Fault. Spontaneous rupture is modeled with slip-weakening friction, and viscoelastic wave solutions are computed with an explicit support-operator method. Piecewise planar geometry is used for the fault surface. Initial traction conditions are derived from inversions of the $M_w7.3$ 1992 Landers strong ground motion records. The fault geometry and traction distribution borrow heavily from the TeraShake2 simulations by \citet{Olsen2007}. Heterogeneity in the traction model leads to focusing of the rupture front, in some cases producing super-shear rupture velocity in areas of high initial traction (asperities). Rupture focusing sometimes occurs between the asperities, with the notable result that the highest peak slip rates occur in areas of low initial traction. Low frequency ground motion agrees with TeraShake2, though amplitudes are smaller due to the lower overall event size (TeraShake2 simulated a $M_w7.7$ event). Separate solutions are computed for version 3.0 and 4.0, respectively, of the Southern California Earthquake Center Community Velocity Model (SCEC-CVM). We also compare the case of a flat ground surface (a common simplification made for finite difference calculations such as TeraShake) to the case of the ground surface conformed to regional topography. We find that the differences in the velocity models and the ground surface representations have minimal effect on the early stages of rupture (before the event has reached its full size) but the effects become substantial in the later stages of rupture. As first seen in the TeraShake1 simulations \citep{Olsen2006}, stronger than expected ground motions occur at the site of Montebello, due to a basin wave guide, though the effect is not as strong in version 4.0 of the SCEC-CVM relative to version 3.0. The overall distribution of simulated peak ground velocities is consistent with those derived from the empirical model of \citet{Campbell2007} for $M_w7.6$, in the sense that the bulk of simulated PGVs are within the 16--84\% probability of exceedance (POE) range. Those simulated PGVs that would correspond to lower POE in the Campbell/Bozorgnia empirical model are principally associated with basin wave-guide and directivity effects.

Ely, G. P., Day, S. M., & Minster, J. B. (2010). Dynamic Rupture Models for the Southern San Andreas Fault. Bulletin of the Seismological Society of America, 100(1), 131-150. doi: 10.1785/0120090187.