Poromechanical modeling of strain localization during earthquake rupture

Strain localization – manifesting as narrow shear bands in brittle rock masses under compressive stresses – is a critical yet contentious phenomenon in earthquake dynamics. Understanding the mechanisms driving this localization is essential, as it influences fault weakening and energy dissipation during seismic events. In this study, we investigate the spontaneous formation of highly localized shear zones, with thicknesses less than 1 cm, within fluid-saturated granular fault gouge using one-dimensional poromechanical numerical simulations. We utilize H-MEC (Hydro-Mechanical Earthquake Cycles), a novel two-phase flow numerical framework that couples solid deformation with pervasive fluid flow (Dal Zilio et al., 2022). This continuum-based model employs a staggered finite difference–marker-in-cell method, accounting for inertial wave-mediated dynamics and fluid flow in a poro-visco-elasto-plastic compressible medium. Global Picard iterations and adaptive time stepping enable accurate resolution of both long- and short-term processes, spanning timescales from years to milliseconds. Our simulations incorporate two frictional laws: the conventional rate-and-state-dependent friction and a newly developed rate-dependent friction. The rate-and-state model, while effective in various contexts, proves ill-posed in localization scenarios due to the absence of a diffusive term in the state variable, causing localization to collapse into a single grid cell regardless of resolution. Conversely, the rate-strengthening friction model with pore pressure diffusion governs localization through fluid pressure diffusion within the poroelastic medium. This approach eliminates the need for classical phenomenological parameters such as the evolutionary effect (b) and the characteristic slip distance (L), resulting in shear zones with finite thicknesses less than 1 cm for slip velocities on the order of meters per second. Additionally, under lower effective normal stress, the model predicts slow-slip events that localize over broader shear zones ranging from 4 to 6 meters. We further perform a linear stability analysis to delineate the poromechanical conditions that drive fluid-induced earthquakes. Our findings suggest that strain localization serves as a dynamic fault-weakening mechanism during seismic events, where the formation of shear bands reduces sliding stress and decreases frictional energy dissipation along the fault. This study provides a physically robust representation of strain localization, enhancing our understanding of the precursory processes leading to earthquakes and potentially informing early warning systems.

• Dal Zilio, L., Hegyi, B., Behr, W., & Gerya, T. (2022). Hydro-mechanical earthquake cycles in a poro-visco-elasto-plastic fluid-bearing fault structure. Tectonophysics, 838, 229516 (https://doi.org/10.1016/j.tecto.2022.229516).