## 4 Discussion
### Slip Modes in Fluid Injection  
To further study the slip characteristics of faults, changes in the slip rate with time after fault slip initiation were compared and analyzed, as shown in Figure 9. Through the shear tests on cracks, [PERSON] [38] used quasi-static slip to represent fault slip at low speeds(less than 10 \(\upmu\)m/s) and dynamic slip to represent fault slip at high speeds (more than 10 \(\upmu\)m/s). In this article, the same terminology is used to further analyze slip mode.  
As shown in Figure 9a, at a low pressurization rate (0.5 MPa/min), the slip rate remained at about 1.5 \(\upmu\)m/s after slip initiation, indicating that the whole process was a quasi-static slip. At a low pressurization rate, the stress relaxation and the applied shear stress were balanced, resulting in sustained and stable fault creep. At 2 MPa/min, 8 MPa/min, and 32 MPa/min, a dynamic slip stage with a slip rate greater than 10 \(\upmu\)m/s appeared after slip initiation, indicating that the friction coefficient of the fault changed from static friction to dynamic friction during the process of injection-driven shear slip. The difference among them was the subsequent slip behavior. The slip rate rapidly decreased to about 2.4 \(\upmu\)m/s after dynamic slip at 2 MPa/min for quasi-static slip, while dynamic slip occurred again after quasi-static slip at 8 MPa/min, and the slip rate was larger than the first dynamic slip, showing the characteristics of stick-slip. Additionally, the peak slip rate increased with the increase in the pressurization rate. For a confining pressure of 10 MPa, the slip mode changed from dynamic slip to quasi-static slip as the slip rate changed. However, the whole process was characterized by quasi-static slip at higher confining pressures (20 MPa and 30 MPa), and the slip rate was no higher than 5 \(\upmu\)m/s, as shown in Figure 9b. Faults with different initial stress states had no significant effect on the slip  
Figure 8: The distribution of fluid pressure during injection.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Overpressure Ratio \(px\)**} \\ \hline
**Sample No.** & **0.5 MPa/min** & **2 MPa/min** & **8 MPa/min** & **32 MPa/min** \\ \hline Test no.1 & 1.18 & 1.85 & 2.68 & 3.77 \\ Test no.2 & 1.14 & 1.69 & 2.47 & 3.71 \\ Test no.3 & 1.29 & 1.82 & 2.52 & 3.63 \\ Average & 1.19 & 1.78 & 2.56 & 3.7 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The overpressure ratios of three repeated groups under different pressurization rates.  
mode after the onset of slip and had the same transition from dynamic slip with a peak slip rate higher than 10 \(\upmu\)m/s to quasi-static slip with a slip rate lower than 4 \(\upmu\)m/s, as shown in Figure 9c. The slip rate measured above is in the same range as the shear slip rate (10\({}^{-7}\)-10\({}^{-5}\) m/s) observed by [PERSON] during the occurrence of microearthquakes in field tests [38]. Due to the limitations of laboratory experiments, the process of fault reactivation induced by water injection in real geological engineering environments cannot be fully reflected, and there is a certain size effect, which is also the direction of further exploration.