We performed laboratory experiments using a biaxial apparatus (BRAVA) configured in a double direct shear (DDS) configuration within a pressure vessel to allow a true triaxial stress field (16, 18) (fig. S1). Normal and shear stress were applied via fast-acting hydraulic servo-controlled rams. Applied loads were measured internally to the pressure vessel via strain-gauged hollowed load cells (LEANE International model CCDG-0.1-100-SPEC) positioned at the ram nose, with an accuracy of ± 0.03 kN over a maximum force of 1.5 MN, that were calibrated regularly. Displacements were measured via linear variable displacement transformers (LVDTs), referenced at the load frame and the moving piston, with an accuracy of ± 0.1 μm (fig. S1A). Load point displacement measurements were corrected for the stiffness of the testing apparatus, with nominal values of 386.12 kN/mm for the vertical frame and 329.5 kN/mm for the horizontal frame. Confining pressure (Pc) and upstream and downstream fluid pressure (Ppu and Ppd) were applied using three independent hydraulic fast-acting servo-controlled intensifiers (fig. S1A). Fluid and confining pressure are measured via diaphragm pressure transducers accurate to ±7 kPa and displacements via LVDTs. Pore fluid pressure was applied using water in equilibrium with CaCO3, similar to the fluid that circulates within the carbonate-bearing fault zone, and confining pressure was applied using a hydrogenated paraffinic white oil (Vaseline oil viscosity, ISO 15). All the output signals were recorded using a simultaneous multichannel analog-to-digital converter with a 24-bit channel resolution at a sampling rate of 10 kHz and then averaged for storage at rates between 1 Hz and 10 kHz. All the experiments were recorded at a minimum recording rate of 10 to 1000 Hz.

The DDS configuration consists of a three-steel block assembly that sandwiches two identical layers of powdered (grain size, <125 μm) simulated fault gouge. The forcing blocks are equipped with conduits for fluid flow that directly connect the fault gouge with the upstream and downstream fluid pressure intensifiers (fig. S1B). Sintered porous frits (permeability, 1 × 10−14 m2) are press fit within cavities in the forcing blocks to allow a homogeneous distribution of fluids on the sample surface. The porous frits are equipped with grooves (height, 1 mm; spacing, 0.8 mm) to localize shear deformation within the gouge layers and avoid shear at the boundary. For this configuration, the nominal frictional contact area is 5.54 cm by 5.55 cm, and we refer all the measurements of stress, displacement, and pressure changes to one layer.

Samples were prepared using leveling jigs to achieve a uniform layer thickness of 5 mm. After the layers were constructed, each side of the DDS shear was weighted to ensure a uniform starting sample density (i.e., porosity). At this stage, the central block was secured to the side blocks, and the sample was jacketed to separate the gouge layers and pore fluid from the confining oil (fig. S1C) [see details in (18)]. Pore fluid pressure lines were then connected to the sample assembly and the sample positioned within the pressure vessel.

Each experiment followed a common loading up procedure. For our sample geometry and dimension, the effective normal stress (σn′) acting on the gouge layers is given by σn=n+ Pc) − Pf. We first applied the confining pressure stepwise until we reached the target value of 4 MPa, which was maintained constant throughout the experiment. Next, we advanced the horizontal ram to apply a constant normal stress of 1 MPa that was maintained throughout the experiment by controlling the piston in a load-mode feedback control. With this configuration, we are capable to resolve fine details of the evolution of gouge layer thickness (h), after correcting for the geometrical layer thinning due to our DDS geometry [e.g., (35)]. At this point, fluid saturation begun by increasing the fluid pressure (0.5 MPa) at the upstream intensifier, while the downstream line was opened to the atmosphere. We waited for flow through, and once it was established, meaning that all the fault gouge was fully saturated, we closed the downstream intensifier to the atmosphere and waited for fluid pressure equilibration.

Injection experiments consisted of a three-stage experimental protocol (fig. S2). First, we advanced the vertical ram at a constant displacement rate of 10 μm/s until we reached a steady-state strength (fig. S2A). This stage is necessary to localize shear within the simulated fault gouge. Then, we stopped the vertical ram and let the sample relax until a residual shear strength was achieved, which implies that the sample reached a best packing configuration and is representative of ancient fault zone. The third stage consists of creep deformation, where we set a constant shear stress on the fault, which, in this case, was ~50% relative to the steady-state strength as it was retrieved from in situ experiments, and measured the resulting fault slip. The creep test started at a constant fluid pressure value of 0.5 MPa, and we let the fault creep for 1 hour to achieve a steady secondary creep. Subsequently, the fluid pressure was increased stepwise, at steps of 0.5 MPa every 2.5 min (table S2), from the upstream intensifier, and we recorded the fluid pressure at equilibrium, after the fluid pressure front diffused within the gouge layers, at the downstream intensifiers (fig. S2C).

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