The Effect of Grain Size Reduction During Shearing of Granulated Fault Gouges

Faults that have experienced large slip suggest that shear localization occurs in a narrow zone with a thickness of a few millimeters or even less. This zone is located in the gouge, is composed of highly fractured material and is similar to a granular medium.

Theoretical and numerical analyses show that the thickness of this zone is directly related to weakening, earthquake nucleation and to the overall energy budget during seismic slip. The grain-size distribution and its evolution due to cataclasis, is a central factor that determines the thickness of this zone.

Here, we propose a micromechanical, physics-based (Cosserat) continuum model by considering the grain-size distribution and its evolution during shearing. The associated dissipation due to the grain-size reduction is considered through a thermodynamically consistent approach, which leads to a Cosserat continuum with evolving internal lengths.

This theory allows us to explore the extent of weakening due to grain breakage and to propose a complete framework for studying faults by including full Thermo-Hydro-Chemo-Mechanical couplings. It is shown that the evolving grain-size distribution has a direct impact on the softening branch of the shear stress-strain response curve and influences the transition from aseismic to seismic slip and vice versa.

References

Scholz, C. H. (2002). The mechanics of earthquakes and faulting (Second). Cambridge.

Nguyen, G. D., & Einav, I. (2009). The energetics of cataclasis based on breakage mechanics. Pure and Applied Geophysics, 166, 1693-1724. http://doi.org/10.1007/s00024-009-0518-x

Rattez, H., Stefanou, I., & Sulem, J. (2018). The importance of Thermo-Hydro-Mechanical couplings and microstructure to strain localization in 3D continua with application to seismic faults. Part I: Theory and linear stability analysis. Journal of the Mechanics and Physics of Solids, 115, 54-76. http://doi.org/10.1016/j.jmps.2018.03.004

Rattez, H., Stefanou, I., Sulem, J., Veveakis, M., & Poulet, T. (2018). The importance of Thermo-Hydro-Mechanical couplings and microstructure to strain localization in 3D continua with application to seismic faults. Part II: Numerical implementation and post-bifurcation analysis. Journal of the Mechanics and Physics of Solids, 115, 1-29. http://doi.org/10.1016/j.jmps.2018.03.003