SCEC Award Number 19026 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Numerical modeling of weakening and strain localization on faults experiencing multi-physical mechanisms
Investigator(s)
Name Organization
Hadrien Rattez Duke University Manolis Veveakis Duke University
Other Participants
SCEC Priorities 2c, 3d, 3f SCEC Groups FARM, CS, Seismology
Report Due Date 03/15/2020 Date Report Submitted 03/10/2020
Project Abstract
 The main goal of the SCEC award #118062196 was to investigate the role of different weakening mechanisms on the phenomenon of strain localization inside fault zones and the influence of each of them on the frictional response during seismic slip. It was done by developing a rate-and-state (visco-plastic) model encompassing internal lengths from multi-physical couplings, to provide qualitative assessments on the parameters dominating a fault’s response. The model was implemented in a numerical code, allowing quantifying the role of each of these mechanisms during the seismic slip process. In particular, we aimed at providing scaling laws with respect of the role of each process in seismic slip sequence. The framework was indeed implemented, and in addition to the numerical results, experimental results were obtained to accompany some of the main conclusions of this work. It was shown that the dominant parameters influencing the shearing resistance of laboratory-scale faults are: 1) the sensitivity of the mechanical response (expressed through the viscosity of the visco-plastic law) to the weak phase production, which affects directly the friction coefficient of the material when weak phases are being produced during shearing at high velocities, therefore causing the seismic slip; 2) the thickness of the shear-band (gouge), which controls the amount of mechanical work that is dissipated in heat, verifying that thinner gouges are admitting higher temperatures and being more prone to chemo-mechanical softening.
Intellectual Merit The project aimed at contributing to how the mechanisms of evolving structure, composition and physical properties of fault zones and surrounding rock affect shear resistance to seismic and aseismic slip. Towards this end, the project's major findings were that the dominant properties affecting the frictional resistance of a fault during slip at any velocity are: 1) the sensitivity of the mechanical response of the fault to the weak phase production, which affects directly the friction coefficient of the material when weak phases are being produced during shearing at high velocities, therefore causing the seismic slip; 2) the evolution of the grain size distribution of the fault gouge, which in turn controls its thickness and therefore the amount of mechanical work that is dissipated in heat, verifying that thinner gouges are admitting higher temperatures and being more prone to chemo-mechanical softening.
Broader Impacts The project has aimed at training the next generation of earth scientists. To do so, we have engaged 3 interns in summer activities, performing the triaxial experiments and the digital image correlation. Aspects of the technical tools used in this project (pseudo arc-length continuation, numerical implementation of partial differential equations, higher order continuum mechanics) have enriched the curriculum of the graduate courses of Duke, in particular CEE520: Continuum Mechanics and CEE690:Mathematical Modeling and Nonlinear Bifurcation.

In addition to the above, within the auspices of the project the Multiphysics Geomechanics lab of Duke has enhanced its infrastructure by developing real time image capturing and digital image reconstruction and correlation tools. Through these developments, the project aimed at benefiting the society by advancing our knowledge in the dominant mechanisms controlling seismic slip.
Exemplary Figure Figure 1 of the final report.