## 4 Discussion and Conclusions
### Implications for Induced Earthquakes  
The injection-induced earthquakes associated with wastewater disposal and shale gas hydraulic fracturing suggest that induced earthquakes usually occur at the shallow sedimentary fault, typically at 3-5 km depths [8; 13; 15; 16; 34]. According to the average overburden pressure gradient in sedimentary faults [33], the lithostatic pressure at this depth is around 50 MPa. Since this research was not aimed at any specific site, a 50 MPa confining pressure was selected and also used in other friction experiments [38; 47; 52]. From the previous studies, fault slip behaviors are mainly affected by the criticality of fault stress [45; 49; 50; 53]. The results of this study can be expanded to other confining pressures and provide a qualitative analysis for different situations.  
Under shallow crustal boundary conditions (i.e., depth \(<\) 3 km, \(T\) \(<\) 100 \({}^{\circ}\)C), laboratory experiments have shown that a great number of rocks observed or inferred to host induced earthquakes (i.e., carbonates, shales, and granites) showed velocity-strengthening behavior, usually considered for aseismic creep [38; 47; 48; 54; 55]. There is a conflict between the observations and the frictional experiments. The experiments performed in this study and previous experiments [27; 28] showed that high-pressure injection can dominate over velocity-dependent effects and induce fault-unstable slip in velocity-strengthening faults that favors aseismic creep. Due to the initial velocity-dependent property, induced earthquakes mainly come from the decrease in fault effective stress; the magnitude of induced earthquakes is relatively small. The induced earthquake associated with the shale gas hydraulic fracturing in the Western Canada Sedimentary Basin [9; 10; 56] and Sichuan Basin, China [14; 15; 16; 57] can be accepted as examples of this mechanism.  
Although injection can outweigh the rate-and-state dependent effects promoting fault dynamic slip, our experimental results show that the velocity-weakening fault showed a faster sliding rate than the velocity-strengthening fault, as it was induced by the pressurized water injection, suggesting that the induced medium-strong earthquakes more likely occur on faults in the underlying crystalline basement. For example, many significant induced earthquakes in Arkansas, Ohio, Oklahoma, and California occurred below the injection zones at a depth of up to 10 km [20; 58; 59]. For the velocity-weakening fault close to its frictional strengths, minor stress perturbations can also induce fault failure, releasing the fault tectonic stress [49; 52; 60]. Therefore, if there is a pre-existing basement fault and the stress level is close to its frictional strength, reservoir impoundment or water injection with several MPa pressures may induce fault sliding and cause strong earthquakes. This mechanism can explain the reservoir earthquakes and strong earthquakes in industrial wastewater reinjection areas, such as several \(M_{\rm W}\) 5-6 earthquakes in the central United States [59; 61; 62].  
Conceptualization, Y.H. and S.M.; methodology, Y.H. and L.Z.; writing--original draft preparation, Y.H.; writing--review and editing, S.M. and X.L.; supervision, S.M.; funding acquisition, S.M. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China, grant number U1839211, and the Spark Program of Earthquake Science and Technology, grant number XH20044.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data available on request from the authors.  
**Acknowledgments:** The authors thank [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] for their help and discussion in the experiments and manuscript writing.  
**Conflicts of Interest:** The authors declare no conflict of interest.