This study aims to understand the effect of injection rate on injection-induced fracture activation in granite. We performed water injection-induced slip tests on samples containing either a smooth or a rough fracture at four different injection rates under undrained conditions and monitored the acoustic emission (AE) signals during the tests. Experimental results reveal that the critical activation fluid pressure is related to the injection rate, pressure diffusion rate, stress state, and fracture roughness. For the smooth fracture, as the injection rate increases, the critical activation fluid pressure increases significantly, while the injection rate has little effect on the critical activation fluid pressure of the rough fracture. The quasi-static slip distance of fractures decreases as the injection rate increases, with rough fractures exhibiting a greater overall slip distance compared to smooth fractures. The number of AE events per unit sliding distance increases with the injection rate, while the global b value decreases. These results indicate that higher injection rates produce more large-magnitude AE events and more severe slip instability and asperity damage. We established a linkage between fluid injection volume, injection rate, and AE events using the seismogenic index (S). The smooth fracture exhibits a steadily increasing S with the elapse of injection time, and the rate of increase is higher at higher injection rates; while the rough fracture is featured by a fluctuating S, signifying the intermittent occurrence of largemagnitude AE events associated with the damage of larger fracture asperities. Our results highlight the importance of fracture surface heterogeneity on injection-induced fracture activation and slip. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The thermal effect has a significant impact on the activation and slip characteristics of fractures. In this study, four pairs of granite fractures were treated by temperatures T ranging from 25 degrees C to 900 degrees C. The fractures were then employed to carry out triaxial unloading-induced shear slip experiments. The step unloading of confining pressure s3 was used as a disturbed stress to activate fractures that were in a near-critical stress state. The slip characteristics, frictional behaviors, as well as damage modes of fractures with different T, were systematically investigated. The results show that at T = 25 degrees C and 300 degrees C, no stick-slip events were observed, and the slipping process of the fractures was characterized by aseismic slip and creep, respectively. For T = 600 degrees C and 900 degrees C, the fractures slipped stably, with occasional interruptions by episodic stick-slip events. Ultimately, they entered the dynamic slip stage after a series of consecutive stick-slip episodes. With increasing T, the number of sheared-off asperities increases due to thermal damage, which in turn leads to an increase in the occurrence of stick-slip events. The slip modes of the fractures transited from friction strengthening to friction weakening. As T increased from 300 degrees C to 900 degrees C, a considerable quantity of generated gouge layer acted as a lubricant for the slipping of fractures. This resulted in a notable increase in the proportion of aseismic slip, which rose from 24% to 54%. As the temperature increased from 25 degrees C to 900 degrees C, the crack length increased exponentially from 2.975 mm to 45.349 mm. For T = 600 degrees C and 900 degrees C, the duration between stick-slip events decreased as stick-slip events occurred more frequently. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).