SCEC Award Number 15116 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title The Effects of Plasticity and the Evolution of Damage Zones in Earthquake Cycle Simulations
Investigator(s)
Name Organization
Brittany Erickson Portland State University
Other Participants Two individuals listed in the proposal work plan have agreed to be unfunded collaborators.
SCEC Priorities 3f, 3c, 2e SCEC Groups Seismology, CS, FARM
Report Due Date 03/15/2016 Date Report Submitted 03/14/2016
Project Abstract
 We are developing an efficient, computational framework for simulating multiple earthquake cycles with heterogeneous material properties and off-fault plastic response. The method is developed for heterogeneous elastic materials in both anti-plane and plane-strain settings. To incorporate plasticity, we are considering the classical, anti-plane problem where a vertical strike slip fault is governed by rate-and-state friction. Rate-independent plasticity is assumed and stresses are constrained by a Drucker-Prager yield condition. The off-fault volume is discretized using finite differences satisfying a summation-by-parts rule. Slow tectonic loading is accounted for at the remote boundaries which are displaced at a slow plate rate through weak enforcement of boundary conditions. Time-stepping is done through an incremental solution process which makes use of an elastoplastic tangent stiffness tensor. Quasi-dynamic events nucleate on the fault at depth and propagate up towards the free surface. These events generate perturbations in off-fault stresses and plastic flow ensues when stresses reach the yield surface, generating off-fault plastic strain. Assuming a depth-constant yield stress, the amount of cohesion affects the magnitude and off-fault extent of plastic strain. Isotropic hardening is included in the constitutive theory to ensure a unique solution, thus the yield surface expands with plastic flow. Consequently, the first rupture in the cycle generates the most damage, and each subsequent earthquake generates a decreasing amount of additional plastic strain until the response is effectively elastic.
Intellectual Merit We are developing a robust, physics-based earthquake cycle model accounting for off-fault yielding over multiple event sequences in order to address several scientific questions put forth by SCEC related to the influence of plasticity on earthquake cycle. How does plastic response during the earthquake cycle affect nucleation and propagation during individual events and the recurrence intervals between events? How do damage zones evolve with increasing cu-mulative slip and how do they affect subsequent rupture? Important and unsolved problems include the relationship between the degree of off-fault yielding and mechanical properties of fault zone material, how damage zones evolve with increasing cumulative slip and how these damage zones affect subsequent rupture.
Our model is currently developed for heterogeneous elastic problems in both the antiplane and plane-strain (Erickson and Day, 2016) settings where interseismic loading is imposed at the remote boundary. Spontaneous, quasi-dynamic events nucleate at the fault governed by rate-and-state friction with fault-variable frictional properties. For the plane-strain setting, our initial investigation was to explore the influence that material contrast has on the earthquake cycle over the course of many hundreds of years. We found that the presence of bimaterial properties influences the earthquake nucleation site, such that rupture in the preferred direction (that is, in the direction of particle motion of the side of the fault with lower shear wave velocity) is favorable. For large values of the critical slip distance Dc, events propagating in the preferred rupture direction occur for a wide range of material contrasts.
Broader Impacts Results from our bimaterial study may shed light on our understanding of rupture directivity on large strike-slip faults (like the San Andreas Fault in California) which occasionally host events rupturing in the non-preferred direction. Because increased ground motion damage and triggered seismicity are often observed in the forward direction of rupture, an under-standing of rupture direction would be helpful in improving seismic hazard estimates. Furthermore, studies of off-fault yielding and how the evolution of damage zones affects future rupture will give an improved understanding of seismic hazards, which is one of SCEC’s long term goals.
Exemplary Figure none submitted