Viscoplastic deformation of naturally damaged rocks from the San Andreas Fault from creep and pure shear experiments

Nairong Du, & Hiroki Sone

Submitted August 30, 2026, SCEC Contribution #15361, 2026 SCEC Annual Meeting Poster #052

Shallow fault zone rocks are often damaged by fault activities, making them more susceptible to bulk plastic deformation upon mechanical loading. Because viscoplastic deformation can relax differential stress over time and accommodate distributed off-fault deformation, it is important to constrain the bulk constitutive behavior of fault zone rocks to better quantify fault stress accumulation and strain budgets.

We obtained naturally damaged fault rock core samples from boreholes drilled through the Mohave segment of the San Andreas Fault. In previous experiments, we conducted triaxial creep experiments to investigate the time-dependent deformation of the fault rocks under constant deviatoric stress. The strain data could be characterized through regression of a Perzyna viscoplastic constitutive law. However, we have thus far only described the volumetric component of the deformation due to the lack of understanding in how shear deformation is affected by volumetric compaction. Pure shear experiments were thus conducted where the total volume of the sample remained constant during loading. Since there is no volumetric strain, the shear strain could be resolved from the observed deformation.

We were able to conduct pure shear experiments on two samples. The samples were axially loaded at displacement rates from 0.05 µm/s to 1 µm/s. Each pure shear stage lasted several minutes to several hours, depending on the loading rate. The samples deformed plastically with each stage, irrespective of the loading rate. The initial effective stress paths evolved along an elliptical Modified Cam Clay yield surface. The mean effective stress, p, decreased as differential stress, q, was increased. However, later, effective stress paths of both samples departed from the yield surface. The mean effective stress gradually transitioned from decreasing to increasing, and the effective stress paths merged onto a straight line for later stages. By tracking the ratio between differential stress and shear strain rate, we provide an estimate of the bulk shear viscosity of the fault rock samples used in the experiments, which could be used in future numerical models with viscous fault damage zones.

Citation
Du, N., & Sone, H. (2026, 08). Viscoplastic deformation of naturally damaged rocks from the San Andreas Fault from creep and pure shear experiments. Poster Presentation at 2026 SCEC Annual Meeting.

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