Poroelastic Control of Shear Localization in Fluid-Saturated Fault Zones

Yuhan Wang, & Elias R. Heimisson

Submitted September 7, 2025, SCEC Contribution #14766, 2025 SCEC Annual Meeting Poster #TBD

Shear localization in fault zones, where deformation is confined to millimeter-scale shear zones, exerts a key influence on earthquake nucleation and rupture processes. We develop a hybrid computational framework that couples the spectral boundary integral method (SBIM) for modeling the poroelastic host rock with a staggered-grid finite difference method (FDM) to resolve the shear zone. This integrated SBI–FD scheme enables multi-scale simulations that capture both large-scale fault slip and the evolution of pore pressure within narrow shear zones. We begin with a rate-dependent friction law in which the friction exhibits rate-strengthening behavior, counteracting the effects of thermal pressurization. We investigate two key questions: (1) How do poroelastic effects in the bulk influence slip and pore pressure in the gouge? (2) Given strong pressure gradients across the shear zone, what scalar pore pressure best represents fault strength under the effective stress principle? Simulations show that bulk poroelasticity significantly affects localization, altering the pressure distribution and stability. Under low diffusivity and sealed conditions, the peak pressure approximates the representative value during full localization, but this changes when fluid exchange with the bulk occurs. We propose a new expression for representative pore pressure incorporating shear strain rate and examine its implications for fault strength and gouge evolution. We also capture the migration of localized zones and demonstrate its potential relevance to pseudotachylyte formation. In the following, we extend the model by incorporating rate-and-state friction and rate-wakening friction, enabling us to capture the full spectrum of localization and delocalization within the shear zone over earthquake cycles. Our results show that, after the first seismic event, localization repeatedly occurs at the same position in the shear zone. This suggests that the fault core observed in the field is likely to experience more slip events than previously anticipated.

Citation
Wang, Y., & Heimisson, E. R. (2025, 09). Poroelastic Control of Shear Localization in Fluid-Saturated Fault Zones. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)