The experiments were performed in a rotary-shear low- to high-velocity frictional testing machine equipped with a pressure vessel
25 (link),33 (link) (Supplementary Fig.
S1a, b). Main parts of the vessel are shown in Supplementary Fig.
S1b. The rock used is dolerite named “Fengzhen black” from Inner Mongolia, north China. The dolerite cylinders for experiments were made by using a few tool machines including coring machine, cylindrical grinder, diamond saw and surface grinder. The samples ready for the experiments are 39.98 ± 0.01 mm in diameter and ~20 mm in length, with the edge near one of the end surfaces cut to form two parallel surfaces for the purpose of torque transmission (Supplementary Fig.
S1c). The slip surfaces were roughened with 80# silicon carbide after being leveled carefully with a surface grinder. Some of such roughened level surfaces were further pitted with small holes of 2 mm in diameter and ~1.5 mm in depth. Both of these two kinds of slip surfaces (referred to as flat and pitted slip surfaces, respectively; Supplementary Fig.
S1d) were used in the experiments to evaluate the effects of the amount of water within the simulated faults on TP weakening. The existence of the pits only results in a difference in the area of slip surface by 2.25 percent, so it only exerts negligible effects on the
σn_eff for a given normal load. To suppress the sample failure due to thermal fracturing, for each rock cylinder, an aluminum ring tightly fitting around the cylinder was set about 2 mm away from the slip surface (Supplementary Fig.
S1c).
The experimental conditions are summarized in Supplementary Table
S1. In the experiments, the samples were immersed in deionized water of about 200 ml. We used a gas booster to pressurize pure nitrogen and a precision pressure-reducing regulator to control the nitrogen pressure at the upper part of the pressure vessel, through which the desired pore water pressure inside the vessel was obtained (Supplementary Fig.
S1b). For a given set point of the pressure-reducing regulator, if the downstream pressure increases due to temperature rise or volume change, the poppet valve inside the regulator would be closed, making a closed system of the vessel during the experiments. It is worth noting that any changes in local pressure could be significantly buffered due to the high compressibility of nitrogen and the relatively large volume of water in the vessel. For the sample configuration we used, the pore pressure could impose a downward force (
Fpp) that reduces the net axial loading on the slip surface. In the data processing, we used the recorded data of
Pp and axial force from the air actuator (
Fa), and the calibrated relation
25 (link) between
Fpp and
Pp to determine
σn_eff (= (
Fa –
Fpp)/
A = [
Fa – (177.9*
Pp [MPa] + 90.5)]/
A, where
A is the area of the slip surface). The changes in bulk
Pp only brought about small deviations (<0.3 MPa) from the initial
σn_eff during the experiments.
The sheathed thermocouple of 0.5 mm in diameter was used to monitor the temperature evolution in selected experiments. The thermocouple hole of 2 mm in diameter was drilled through the stationary rock cylinder, and the thermocouple was fixed in the hole by using the high-temperature waterproof adhesive, with its tip end exposing on the slip surface (Supplementary Fig.
S1b, c).
In each experiment, before shearing the dolerite samples at the equivalent slip rate
24 (link) of 2.0 m/s, we preslid the samples for two revolutions (~166 mm in equivalent displacement) under the slip rate of 5 mm/s and the normal stress same as that in the main test. This adjusts the alignment of rock cylinders and helps to get better quality experimental data.