Viscoplastic fault rock properties from creep experiments on naturally damaged rocks from the San Andreas Fault

Nairong Du, & Hiroki Sone

Submitted September 7, 2025, SCEC Contribution #14807, 2025 SCEC Annual Meeting Poster #TBD

Shallow fault zone rocks are often damaged by fault activities, making them more prone to bulk plastic deformation upon mechanical loading. A significant fraction of such plastic deformation can also be time-dependent. Because viscoplastic deformation can relax differential stress over time and accommodate 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. The boreholes were drilled as part of a seismic hazards assessment surrounding the Elizabeth Tunnel by the Los Angeles Department of Water and Power. The boreholes plunge 55-65° to the northeast and have a maximum depth of 140m across the 150 m wide fault zone. Faulted and deformed granodiorite to granitoid gneiss bedrocks were recovered around the primary cataclasite/gouge zones. Samples included pulverized fault rocks, which still retained the protolith textures but were crushable by hand. The samples were held under constant deviatoric stress with confining pressure under dry conditions. Preliminary results show that the naturally damaged rocks can creep under this condition without pore fluid at room temperature. The amount of creep strain increases with time and increasing axial stress. The strain data is further characterized through regression of a Perzyna-Modified Cam Clay viscoplastic constitutive law. The results provide information on the realistic constitutive formulation and parameters to be used to represent fault damage zone rocks.

Citation
Du, N., & Sone, H. (2025, 09). Viscoplastic fault rock properties from creep experiments on naturally damaged rocks from the San Andreas Fault. Poster Presentation at 2025 SCEC Annual Meeting.

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