Plasticity Throughout the Earthquake Cycle

Brittany A. Erickson, Eric M. Dunham, & Jeremy E. Kozdon

Published August 15, 2016, SCEC Contribution #6960, 2016 SCEC Annual Meeting Poster #045

We are developing an earthquake cycle model to simulate multiple ruptures in complex geometries, with material heterogeneity and off-fault plastic response. This initial study focuses on the 2D antiplane shear case of ruptures on strike-slip faults with rate-and-state friction and off-fault plasticity. Both rate-independent plasticity with hardening and viscoplasticity are considered, where stresses are constrained by a Drucker-Prager yield condition. The volume is discretized using a finite difference method with interseismic loading applied by imposed motion of the remote boundaries rather than through backslip. Quasi-dynamic events nucleate at depth and propagate toward the free surface, carrying stress perturbations that result in off-fault plastic strain. With a depth-dependent yield stress, the magnitude and extent of plastic strain is affected by cohesion, viscosity, and isotropic hardening. For rate-independent plasticity with hardening, the yield surface expands with plastic flow. Consequently, the first rupture in the cycle causes plastic strain, but for all subsequent events the off-fault material responds elastically. For perfect viscoplasticity, all ruptures generate plastic strain. If hardening is included in the viscoplastic model, the first event causes the most plastic strain (in a ~100 m region away from the fault at the free surface for most scenarios), with a decreasing amount of additional plastic strain for each subsequent rupture. In all cases we found that a shallow slip deficit occurs with the first event, but that the evolution of the deficit with each subsequent event is dictated by the plasticity model. In particular, there is almost no change in the deficit when rate-independent plasticity is used and a continuous increase with the viscoplastic models. Integration of the off-fault plastic strain from the viscoplastic model reveals that a significant amount of offset is accommodated by inelastic deformation (~1 m per 10 ruptures). Motivated in part by these results, we are developing a more complete cycle model that can account for interseismic loading and dynamic rupture. To do this, we are developing a unified code capable of simulating both quasi-static and dynamic rupture problems. The dynamic rupture code is based on a discontinuous Galerkin method, and will be capable of handling complex geometries, heterogeneous material properties, and utilizing dynamically adapted grids.

Key Words
earthquake, cycle, plasticity, shallow slip deficit

Citation
Erickson, B. A., Dunham, E. M., & Kozdon, J. E. (2016, 08). Plasticity Throughout the Earthquake Cycle. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)