Viscoplastic rheology for characterizing the bulk rheology of fault zone rocks

Hiroki Sone, Mayukh Talukdar, & Zirou Jin

Submitted September 7, 2025, SCEC Contribution #14949, 2025 SCEC Annual Meeting Poster #TBD

Fault zones host abundant fractures, from microscopic to macroscopic scale, created by stress concentrations associated with fault slips. Sliding and closing of these fractures could promote overall viscous behavior of fault zone rocks, which could result in the interseismic evolution of fault strength and stress, as well as accommodate relative plate motions via off-fault deformation. However, there is little understanding about what constitutive description is appropriate to represent such bulk rheology of fault-related rocks at crustal depths. We present data from several different experiments exploring the time-dependent creep behavior of damaged rocks and soft sediments, as well as some attempts to fit the data using various constitutive descriptions. It is clear from room-temperature dry creep experiments on thermally-fractured granite rocks that even in the absence of pore fluid, rocks that typically exhibit brittle characteristics can exhibit time-dependent bulk deformational behavior. It is also common to see in all experiments that creep strain under constant stress decelerates as the sample compacts, which may be described as the evolution of apparent viscosity with porosity reduction. However, strain rates predicted by such simple approach is only able to fit creep strain observed in response to a single step change in stress, but not the entire experiment. We also attempt to fit the experimental data using a viscoplasticity law combined with the Modified Cam Clay (MCC) model. This constitutive description not only treats the viscosity as a mechanical property that evolves with porosity, but also treats stress as an evolving parameter. The viscoplasiticity law has the ability to fit the entire experimental using limited number of prescribed material properties suggesting that validity of the formulation. These findings will help define the appropriate constitutive models to incorporate the community rheology models used in large scale fault mechanics simulations.

Citation
Sone, H., Talukdar, M., & Jin, Z. (2025, 09). Viscoplastic rheology for characterizing the bulk rheology of fault zone rocks. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)