We conduct experimental and numerical studies to recover fluid pressure heterogeneity in rock fractures and simulate localized shear slip of highly pressurized fracture segments, which are difficult to be directly observed in field and laboratory investigations. The shear-flow experiments coupled with the COMSOL simulation reproduce the fluid pressure non-uniform distribution along the sawcut fractures in both granite and shale samples. Our data show that the high pressure appears around the injection borehole, while the pressure remains zero in the other areas. The friction experiments based on the rate-and-state friction law examine the slip characteristics of the sawcut fracture with various fluid pressures in the shale sample. The results show that the slip behavior evolves from velocity-weakening to velocity-strengthening as the fluid pressure increases, and infer that the localized shear slip occurs in the velocity-weakening segment near the injection borehole. The unstable slip can propagate along the fracture, and finally induce the instability of the entire fracture. This study provides a novel option to uncover the mechanism of injection-induced seismicity, which could help us mitigate the risks of the anthropogenic earthquakes.

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