The Influence of Grain Crushing and Pore Collapse on the Formation of Faults

N. A. Collins-Craft; I. Stefanou; J. Sulem; I. Einav

doi:10.1029/2025JB033062

During an earthquake, slip occurs in a localized shear zone that features a heavily granulated fault core that can be characterized as a shear band. We study the formation of this fault core in a granular rock such as sandstone by developing a model of crushable granular media within the framework of Breakage Mechanics. This model accounts for the evolution of the grain size distribution, while also accounting for the co‐evolution of the solid fraction. An enrichment with the Cosserat continuum allows for the model to predict finite‐width shear bands. The model is then calibrated against experimental data taken from tests on Bentheim sandstone, and a parametric study of the mechanical parameters is conducted using linear stability analysis. We find that for deeply‐buried rocks the shear bands have a compactive component, and the initial value of the solid fraction does not play a strong role in the initial band thickness, but can influence the rate of delocalization of the band. Post‐localization behavior is studied with the finite element method, which shows the formation of zones of dilation outside the band in addition to the compaction within the band. Using a modified Kozeny–Carman permeability law, it is shown that within the band the permeability reduces by several orders of magnitude, but can increase outside the band. Our results highlight the importance of modeling grain size and solid fraction evolution as they exert a controlling influence on hydromechanical properties that play an important role in fault formation and seismic slip. Plain Language Summary During an earthquake, all of the sliding motion is accommodated in a very narrow zone. Within this zone, the material has much finer grains than in the surrounding rock. To understand how this arises we develop a new model that tracks how the grains become finer, as well as how the porosity of the material reduces. The model can predict how wide the narrow zone of fine material is. We show that during the sliding process, strain localizes in a narrow compacting shear band where significant grain crushing is accompanied by porosity reduction whereas porosity increase occurs outside the band. These changes in porosity and grain size strongly impact the permeability of the medium, reducing it by orders of magnitude in the compacting slip zone. This process is very important as materials through which it is difficult for fluids to flow are known to be more prone to earthquakes. Key Points A new model accounts for the coevolution of the grain size distribution and porosity in crushable granular media subject to shearing Linear stability analysis and the finite element method are used to study the localization width of the model at depth Orders‐of‐magnitude decrease in permeability is predicted to occur inside the fault core, but permeability can increase outside it

The Influence of Grain Crushing and Pore Collapse on the Formation of Faults

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