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Rate and Pressure Dependence of Dilatancy and Fault Strength in Partially‐Drained Laboratory Fault Zones

R. Affinito,D. Elsworth,T. Mittal,M. Scuderi,C. Marone

Published 2025 in Journal of Geophysical Research: Solid Earth

ABSTRACT

The dynamics of fluid flow within faults plays a critical role in the evolution of fault strength through the seismic cycle. The key processes that control how fluids affect fault slip behavior are the evolution of fault porosity and fluid recharge and drainage during slip that, in turn, determine dilational strengthening or compaction weakening. Despite the significance of these processes, high‐fidelity lab measurements that include the evolution of porosity, fluid pressure and frictional properties are sparse. Here, we report such data for drained and undrained velocity‐stepping experiments from 3 to 300 µ/s on natural fault gouges from the seismogenic zone of injection well 16A (2,050–2,070 m) of the Utah FORGE enhanced geothermal site site. We conducted a suite of experiments under constant normal stresses (44 MPa) and pore fluid pressures (13, 20, 27 MPa) corresponding to pore fluid factors between 0.3 and 0.65. We carefully monitor the volumetric strain and show that the dilatancy coefficient of the material ranged from 5 to 12×10−4 $12\times 1{0}^{-4}$ , and showed minor sensitivity to fluid boundary conditions. In some cases, we see that larger slip velocities cause a transition from dilatancy strengthening to compaction weakening via fluid pressurization. Two effective fault permeability terms were required to model that fault‐fluid response with slip. We posit that the spatial‐temporal pattern of pore‐throat size and connectivity creates a spectrum of fault drainage conditions, ultimately controlling the mode of fault slip.

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