Expected seismic shaking in Los Angeles reduced by San Andreas fault zone plasticity

Computer simulations of large (M ≥ 7.8) earthquakes rupturing the southern San Andreas Fault from SE to NW (e.g., ShakeOut, widely used for earthquake drills) have predicted strong long-period ground motions in the densely populated Los Angeles Basin due to channeling of waves through a series of interconnected sedimentary basins. Recently, the importance of this waveguide amplification effect for seismic shaking in the Los Angeles Basin has also been confirmed from observations of the ambient seismic field. By simulating the ShakeOut earthquake scenario (based on a kinematic source description) for a medium governed by Drucker-Prager plasticity, we show that nonlinear material behavior could reduce the earlier predictions of large long-period ground motions in the Los Angeles Basin by up to 70% as compared to viscoelastic solutions. These reductions are primarily due to yielding near the fault, although yielding may also occur in the shallow low-velocity deposits of the Los Angeles Basin if cohesions are close to zero. Fault zone plasticity remains important even for conservative values of cohesions, suggesting that current simulations assuming a linear response of rocks are overpredicting ground motions during future large earthquakes on the southern San Andreas Fault.