SCEC Award Number 16143 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A Continuum Model for Fault Zone Viscoplasticity: Compaction, Dilation, and Pore Fluids
Investigator(s)
Name Organization
Ahmed Elbanna University of Illinois at Urbana-Champaign
Other Participants Xiao Ma (PhD Student, UIUC)
SCEC Priorities 3a, 3c, 3e SCEC Groups FARM, CS
Report Due Date 03/15/2017 Date Report Submitted 03/27/2017
Project Abstract
 The scientific objective of this proposal is to predict the strength evolution and strain localization patterns in fluid infiltrated fault zones under different loading conditions in the presence and absence of vibrations. Methodology: We use the Shear Transformation Zone theory to describe viscoplasticity in sheared granular layers. We developed an implementation of the theory in the finite element software MOOSE (from Idaho National lab). We have furthermore implemented pore fluid pressure evolution model driven by both diffusion and inelastic dilatancy. For the effect of vibrations, we have completed the study in an idealized 1D fault zone and we are currently testing the 2D implementation. Main results: (1) A working validated implementation of the plasticity model, (2) Identification of brittle to ductile transition in sheared granular materials as a function of initial porosity, pressure, and dilatancy, (3) Reproducing complex strain localization patterns (including R, X, Boundary and Y-bands) while tracking their evolution history, and (4) Development of a new hybrid (finite-difference/ spectral boundary element) numerical scheme that enables coupling small scale fault zone physics with large scale elastodynamics. Significance: The proposal is an important step towards developing a theoretically sound framework for inelasticity and shear banding in granular materials. It opens new opportunities for multiscale modeling of earthquake ruptures that couple large scale elastodynamics with small scale inelastic processes in fault gouge. Coupling with pore fluids (as in the current implementation) will enable investigating competition between inelastic dilatancy and thermal pressurization that has not been addressed before
Intellectual Merit The project addresses short-term objectives in Fault and Rock Mechanics (3a, 3c, and 3e) by developing models that quantifies the influence of small scale processes on large scale rupture re-sponse. A better quantification of this issue will aid long-term objectives in Earthquake Source Physics and Ground Motion, informing models of fault system evolution and dynamics, and physics-based hazard analysis. Understanding the complex behavior of fault zone and its influence on rupture dynamics is also essential for the interpretation of seismic observations and for the problem of seismic inversion. It is also essential for evaluating impacts of future seismic events on Southern California. The proposal develops a unique and novel methodology for modeling gouge visco-plasticity considering compaction, dilation, pore fluids, shear band complexity and gouge spatial heterogeneities.
Broader Impacts The activity contributed to the training of 1 graduate student Xiao Ma who is currently conducting his PhD at UIUC on inelasticity and fracture in amorphous solids. The activity supported the PI’s travel to attend the annual meeting and continue his interactions/ explore new collaboration opportunities with other SCEC scientists. The computational methods developed as part of this proposal have applications beyond fault mechanics as they are relevant to analyzing deformation and failure in a broad range of amorphous materials including metallic glasses and lithium ion batteries. The activity contributes to SCEC efforts in developing fundamental models for multiscale deformation in fault zones which will enhance our physics based earthquake rupture simulations and improve our ground motion prediction tools on the long run. This will contribute to combating the heavy toll that earthquakes take on our society through making better informed decisions in the context of seismic hazard and risks.
Exemplary Figure Figure 1: Figure 1: (a) Predictions of localization patterns in a computational model with STZ theory (top) consistent with the schematic complex patterns observed in the field and lab (bottom). The contours in the top plot represent the distribution of effective temperature (darker is higher disorder and more localization). The angle between the bands will depend on the pressure sensitivity and dilation. (b) Brittle to ductile transition in sheared gouge layer (loading rate =0.4m/s) as a function of initial preparation. The loading protocol is shown in the insert. Initially dense gouge is brittle (blue curve) and exhibits strain localization (top contours) while initially loose gouge is ductile (red curve and bottom contours). [Wiggles in the pre-peak regime are well-resolved. They occur due to load ramping and inertia effects. Slower ramping eliminate them.] (adapted from Ma and Elbanna (2017))