Model for permeability enhancement from yielding in fault damage zones during the passage of a rupture

Laura Blackstone, & Eric M. Dunham

Published September 8, 2024, SCEC Contribution #13649, 2024 SCEC Annual Meeting Poster #118 (PDF)

When faults rupture, a region of off-fault yielding or plastic deformation, referred to as the damage zone, occurs around the fault surface as secondary fractures form and/or slip. This increases the permeability of the damage zone. The increased permeability, in turn, enhances the transport of fluid and fluid pressure diffusion along the fault, which affects rupture nucleation, propagation, and arrest. Despite the recognized importance of permeability enhancement for induced seismicity, swarm seismicity, and similar phenomena, there are few models to describe permeability evolution with slip and rupture that are appropriate for mature faults with well-developed damage zones.

We address this open problem by modeling the yielding of damage zones in response to the passage of a rupture. We use the Poliakov et al. (2002) solution for a steadily propagating mode II crack with finite peak strength and weakening over a finite-length process (cohesive) zone. This is an elastic solution, but we extend it approximately to the elasto-plastic case by solving for the evolution of plastic strain and stress using a Drucker-Prager model at a set of material points on a mesh extending away from the fault. To facilitate the solution, we neglect stress redistribution by elastic interactions. Then we relate the plastic strain to permeability enhancement. We compare predictions from this model with an ad hoc model, recently introduced by Zhu et al. (2020) to study fault valving and swarm seismicity, which increases permeability toward some specified maximum value over a critical slip distance. This comparison shows that the critical slip distance for permeability enhancement scales with the slip accrued within the process zone at the rupture front. The critical slip distance can be much larger than the slip-weakening or state evolution distance when off-fault yielding provides a significant contribution to total fracture energy. We also find that the critical slip distance increases with rupture velocity. Finally, we note that yielding, and thus permeability enhancement, can occur ahead of the rupture front and prior to the onset of slip, which is not captured in the ad hoc model. We are currently working on different ways to relate the magnitude of permeability increase to plastic strain. Overall this work will inform the choice of permeability enhancement models and their parameters for more realistic modeling of fault zone fluid transport and its coupling to slip.

Key Words
Permeability enhancement, damage zone, fluid transport

Citation
Blackstone, L., & Dunham, E. M. (2024, 09). Model for permeability enhancement from yielding in fault damage zones during the passage of a rupture . Poster Presentation at 2024 SCEC Annual Meeting.

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