## 4 Discussion and Conclusions  
### Influence of Rock Permeability  
Recently, it has been suggested that pore pressure increase plays a vital role in injection-induced earthquakes when a fluid pathway to nearby pre-existing faults is available, and the poroelastic stressing plays a dominant role in the case of large distance injection or injection without a hydraulic pathway to the faults [1; 2; 34]. When the pressurized water is injected into the vicinity of the fault, pressurized water is separated from the fault plane, so poroelastic stressing may play a dominant role before the diffusion of pore pressure has begun to take effect. However, the cumulative volume of the pressurized water was tiny due to the small injection hole in the experiments; thereby, the change in the solid matrix stresses caused by injection was slight, suggesting the effect of poroelastic stressing can be ignored. For the mechanism of pore pressure to increase, the pressurized water must diffuse to the fault plane to affect the effective stress of the fault. Thus, the permeability of the rock is the most critical factor affecting injection-induced earthquakes. When pressurized water is injected, the diffusion time of pore water in rock is controlled by compressibility, permeability, and porosity [39]. The permeability of granite and limestone in this experiment were 2.25 \(\times\) 10\({}^{-19}\) m\({}^{2}\) and 3.88 \(\times\) 10\({}^{-22}\) m\({}^{2}\), two orders of magnitude smaller than that of the sandstone, 2.66 \(\times\) 10\({}^{-17}\) m\({}^{2}\) (Table 1). Due to the lower permeability, the pressurized water was sealed in the granite and limestone injection hole. Thus, no changes were observed in fault shear stress and sliding rate (Figure 3a,c), suggesting that injection does not affect the fault slip behaviors. The results of the granite experiment are consistent with those of [PERSON] and others [40], demonstrating that pressurized water injection into the vicinity of the fault has little effect on fault stress.  
On the contrary, pressurized water can quickly penetrate the sandstone to the fault plane due to the higher permeability, so the injection caused a decrease in the shear stress and increased the sliding rate (Figure 3b). The experimental results showed that whether the pressurized water acting near the fault zone can induce fault sliding is mainly controlled by the permeability. Therefore, pressurized water injection can induce earthquakes on  
Figure 8: **(a)** Shear stress, injection pressure, and slip rate of the limestone fault at 100 \({}^{\circ}\)C. **(a1,a2)** are enlarged views of the time window denoting the 1 MPa and 15 MPa pressurized water injection. **(b)** The apparent friction coefficient changes with time. The black dotted lines denote the locations of enlarged views. The blue dotted lines denote the processes of pressurized water injection. The red dotted lines denote the steady-state friction strength (\(\mu_{\text{ss}}\)) in different loading velocity.  
nearby faults in an area with high-permeability rocks, such as sandstone [41; 42]. However, pressurized water injection cannot affect the fault stress in a short time in areas with low-permeability rocks, such as granite and limestone. Nevertheless, if the injection lasts long, it may also affect the fault stress and induce earthquakes.