An Iwan-type Plasticity Model for 3D Simulations of San Andreas Scenario Earthquakes

Daniel Roten, Kim B. Olsen, Steven M. Day, & Yifeng Cui

Published August 14, 2019, SCEC Contribution #9617, 2019 SCEC Annual Meeting Poster #005 (PDF)

Understanding ground motions as the coupled response of inelastic off-fault and shallow nonlinear behavior represents a research priority within SCEC5, and the verification and validation of wave propagation codes which accurately model the hysteretic stress-strain relationship of soils and shallow rocks is the focus of a technical activity group on shallow crust nonlinearity.
We have implemented and verified a parallel-series Iwan-type nonlinear model in the CPU version of the AWP finite difference code (named AWP-Iwan). AWP-Iwan was deployed on NCSA Blue Waters and TACC Frontera to simulate a SE-NW rupturing M7.8 earthquake on the southern San Andreas fault with realistic nonlinear rheology in the sedimentary infill of the San Gabriel and Los Angeles basins.

We simulate Masing unloading and reloading behavior by tracking an overlay of 10 concentric von Mises yield surfaces. Lame parameters and failure stresses pertaining to each surface are calibrated to reproduce the stress-strain backbone curve, which is controlled by the reference strain assigned to a given depth level. We analyze the sensitivity of ground motions to the choice of reference strain in sediments, which we define from the Darendeli (2001) empirical relationship for different assumptions on soil composition or the EPRI shear modulus reduction curves for sands. Simulations are carried out using a resolution of 100 m and a minimum shear-wave velocity of 500 m/s.

Inside the Whittier Narrows corridor (strong motion station rus), spectral accelerations at 3 seconds (3s-SAs) are reduced from 1g in the linear case to 0.3-0.6g in the nonlinear case, depending on the choice of reference strain. Plastic simulations obtained with a single von Mises yield surface predict 3s-SAs that are higher than those obtained with the Iwan model, but lower than the linear values (~0.7g at station rus).
Normalized shear modulus reductions reach values of up to 50% near Whittier Narrows and up to 75% in the San Bernardino basin, indicating that the peak strains exceeded the reference strain of ~0.1% (from Darendeli’s relationship) near the surface. These shear modulus reductions result in shorter wavelengths of surface waves propagating through these basins, and effectively limit the maximum frequency which can be resolved in a finite difference simulation. We are currently implementing the Iwan model in the more efficient, GPU-based discontinuous mesh version of the code to address these limitations.

Key Words
Ground motion prediction, nonlinear site response, wave propagation simulation, shallow crust nonlinearity

Citation
Roten, D., Olsen, K. B., Day, S. M., & Cui, Y. (2019, 08). An Iwan-type Plasticity Model for 3D Simulations of San Andreas Scenario Earthquakes. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions