Implementation of Iwan-type Plasticity Model in AWP-ODC

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

Published August 15, 2018, SCEC Contribution #8731, 2018 SCEC Annual Meeting Poster #019 (PDF)

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. These observations motivated one of SCEC5’s research priorities and the creation of a Technical Activity Group, with the goal of understanding ground motions as the coupled response of inelastic off-fault and shallow nonlinear behavior.

We have implemented an Iwan-type nonlinear model in the CPU version of the 3D finite difference (FD) wave propagation code AWP-ODC (AWP-Iwan) which captures the stress-strain relationship of shallow crustal material (weathered rock and sedimentary deposits) more accurately than a single Drucker-Prager yield surface used in past scenario simulations. The code reproduces Masing re-loading and unloading behavior by tracking an overlay of individual von Mises yield surfaces arranged in a parallel-series configuration. Because AWP-Iwan needs to store the stress tensor, the Lamé parameters and the yield stress pertaining to each surface, the implementation requires major changes in the code’s computation and communication routines, and results in significantly increased computational and memory costs compared to simulations using a single yield surface.
To verify the method against established 1D and 2D nonlinear codes we perform 1D simulations for the KiK-net site KSRH10 using plane strain and periodic boundary conditions, as well as 2D P-SV simulations for a generic sediment-filled basin. Synthetic ground motions computed using AWP-Iwan with ~20 yield surfaces are able to reproduce the nonlinear response (i.e., reduced amplification and shift in resonance frequency) predicted by the codes Noah and Noah2D, but the quality of the solution decreases notably if too few (⪅10) yield surfaces are used.

The Iwan model will be further optimized and integrated in the highly efficient and scalable GPU version of AWP which supports simulations on a discontinuous FD mesh.

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

Citation
Roten, D., Olsen, K. B., Day, S. M., & Cui, Y. (2018, 08). Implementation of Iwan-type Plasticity Model in AWP-ODC. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions