3D Simulation of Large San Andreas Scenario Earthquakes Using a Multi-Surface Plasticity Model

Date: 4/25/2019

Time: 08:45 AM

Room: Elliott Bay

Strong ground motions recorded on vertical arrays indicate that site response formalism (decoupled from source and path effects) fails to reproduce empirical surface-to-borehole transfer functions in the majority of cases due to the presence of lateral heterogeneities. Understanding ground motions as the coupled response of inelastic off-fault and shallow nonlinear behavior requires 3D wave propagation codes which reproduce laboratory observations of the hysteretic stress-strain relationship in sediments and weathered rocks more accurately than bilinear (e.g., Drucker-Prager) yield criteria used in past scenario simulations.

Towards this goal we have implemented an Iwan-type plasticity model in the 3D finite difference (FD) code AWP. The code, named AWP-Iwan, reproduces Masing re-loading and unloading behavior in three dimensions by tracking an overlay of individual von Mises yield surfaces arranged in a parallel-series configuration. The implementation was verified against the 1D FD code Noah by simulating the response of the KiK-net site KSRH10 using plane strain and periodic boundary conditions. We also performed verification runs against Noah2D by carrying out 2D P-SV simulations for an asymmetric sediment-filled basin. Synthetic ground motions computed by AWP-Iwan using 20 yield surfaces were found to be consistent with the reference solutions in the time and frequency domain.

AWP-Iwan was deployed on NCSA Blue Waters to simulate a M 7.8 earthquake on the southern San Andreas fault with realistic near-surface nonlinear behavior in the sedimentary basins. These simulations confirm the importance of nonlinear effects on long-period surface waves during a ShakeOut-type earthquake scenario, with spectral accelerations at 3s reduced by 50% in Whittier Narrow and downtown Los Angeles with respect to a linear solution. Normalized shear modulus reductions reach values of up to 75% in the San Bernardino basin and up to 50% in the main waveguide along the Whittier Narrows corridor, indicating that peak strains exceeded the reference strain of 0.1% at the surface.

Presenting Author: Daniel Roten

Authors