The in situ injection experiment and its concept are detailed in (6). In short, the experiment was conducted at a depth of 282 m within the LSBB (Laboratoire Souterrain à Bas Bruit) underground laboratory in France (41). The protocol consists of injecting water at a pressure of 0 to ~3.5 MPa into a segment of an inactive ~500-m-long regional fault zone. At the location of the experiment (fig. S5), the fault has a dip angle of 70° and cuts through limestone rocks, with a strike-slip to normal cumulated slip of a few meters (42). We drilled the fault with a 20-m-long vertical borehole (diameter, 16 cm) and selected a 1.5-m-long section isolated between two inflatable packers to inject 950 liters of water in the fault. In the sealed section, we placed, on either side of the fault, a special instrument based on fiber optic Bragg sensors, the SIMFIP (Step-Rate Injection Method for Fracture In-Situ Properties) probe (43), to monitor synchronously at high frequency (500 kHz) the fluid pressure (sensitivity of 1 kPa), the flow rate (sensitivity of 0.1 liter per minute), the fault-parallel displacement (i.e., “fault slip”), and the fault-normal displacement (i.e., “fault opening”) (sensitivity of 1 μm). Three seismometers in nearby boreholes (3 to 5 m from the injection well) monitored the seismic activity. During the experiment, the temperature was 12.5°C and remained constant when water was injected. Hydraulic (i.e., porosity and permeability) and elastic properties of rock and fault were estimated from laboratory and in situ tests (6, 42). A normal stress (σn) of ~4.25 ± 0.5 MPa and a shear stress (τ) of ~1.65 ± 0.5 MPa acting on the fault were estimated (6). We used the hydraulic testing of pre-existing fractures method (44) and the nonreversible displacements measured with the SIMFIP probe to extract both the reopening pressure and the slip vector on the reactivated fault planes and inverse the stresses from these measurements using a forward fully coupled numerical analysis of 10 tests (6, 11).

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