## 3 Results
### Pressurized Water Injection into the Fault Plane  
Typical results for the pressurized water being injected into the fault planes are presented in Figure 4. For each experiment, we injected 1 MPa and 15 MPa pressurized water into the borehole of the samples and performed the velocity stepping test in the dry and wet samples with 1 MPa and 15 MPa pressurized water. Overall, all samples showed stable slip at room temperature, and the fault sliding rate decreased with the decreasing loading rate. When 1 MPa pressurized water was injected into the faults (\(\sim\)2% of the confining  
Figure 3: Shear stress, injection pressure, and fault sliding rate for (**a**) granite, (**b**) sandstone, and (**c**) limestone faults as the 15 MPa pressurized water is injected into the vicinity of the fault. (**a1–c1**) are enlarged views of the time window denoting the pressurized water injection. The black dotted lines denote the locations of enlarged views. The blue dotted lines denote the processes of pressurized water injection. pressure), three faults showed slight changes both in the shear stress and fault sliding rate (Figure 4(a1-c1)), indicating that low-pressure injection had negligible effects on fault stress and fault slip. However, the 15 MPa pressurized water injection (\(\sim\)30% of the confining pressure) made the shear stresses decline significantly, and the fault sliding rate increased considerably (Figure 4(a2-c2)), suggesting that the injection induced the fault reactivation. As the fault reactivation induced by water injection, three faults showed significant differences in the shape of shear stress decreasing, stress drop, slip duration, and fault displacement (Figure 4). The shear stress of granite and limestone faults decreased like an exponential curve. However, the shear stress of the sandstone fault decreased like a quasi-linear curve as a whole and showed synchronous fluctuation as the pore pressure changed during the injection. When the faults reached a new steady state, a \(\sim\)6.9 MPa stress drop, \(\sim\)290 s slip duration, and \(\sim\)0.28 mm fault displacement were observed in the granite fault (Figure 4(a,a2), Table 3). For the sandstone fault, these parameters were \(\sim\)10.4 MPa, \(\sim\)145 s, and 0.19 mm, respectively (Figure 4(b,b2), Table 3). They were \(\sim\)18 MPa, \(\sim\)490 s, and \(\sim\)0.57 mm, respectively, in the limestone fault (Figure 4(c,c2), Table 3). Note that the permeability of sandstone is much higher than granite and limestone (Table 1), so the injected fluid could more easily dissipate in the matrix, which means that the sandstone fault required more injection time to form the 15 MPa pore pressure. However, the time for the fault to reach a new steady state in the sandstone experiment was less than that in  
Figure 4: Shear stress, pore pressure, and fault sliding rate change with time for (**a**) granite, (**b**) sandstone, and (**c**) limestone faults as the water is injected into the fault plane. (**a1–c1**) are enlarged views of the time window denoting the 1 MPa pressurized water injection. (**a2–c2**) are enlarged views of the time window denoting the 15 MPa pressurized water injection. The black dotted lines denote the locations of enlarged views. The blue dotted lines denote the processes of pressurized water injection. the other two samples, suggesting that the shear stress of the sandstone fault decreased faster than that of the granite and limestone faults. In addition to the mechanics data, the fault sliding rate showed apparent differences in the three faults. The sliding rate of the granite and limestone faults increased monotonously, but the sliding rate of the sandstone faults varied with the injection pressure fluctuation. The maximum sliding rate of the sandstone fault was 2.19 \(\upmu\)m/s, significantly exceeding that of the two other faults, which were 1.57 \(\upmu\)m/s and 1.7 \(\upmu\)m/s, respectively (Figure 4). When the injection stopped, the fault sliding rate slowed again, showing different behaviors in the three faults, too.