Abstract: The finegrained massive granite outcrop samples with a weak or broken surface were selected, the force critical angle which led to the new rupture surface of the rock was studied under high temperature and high pressure simulated in the lab. Under the temperature 100 ℃, confining pressure 100 MPa and the temperature 200 ℃, confining pressure 200 MPa, the experiments were made separately through changing the angle with principal stress. The results showed that when the angle α was 0°~30°, the rock steadily slipped along the original rupture surface; when the angle α was 30°~60°, the fault was dominated by stickslip; when the angle α was 60°~90°, mainly led to the generation of the new rupture surface; the critical angle likely to generate a new rupture surface ranges between 50°~65°; the new rupture occurred when the angle α was greater than 65°. The phenomenon, which occurred on rocks, such as stable sliding, stickslip, and a new rupture surface generation, was explained by analyzing the relationship between the stress from different slices (σ) and strain rate (ε) . As is showed in Mohr′s circle theory, under the complex fracture background, the rock stress angle has dominant effect on the generation of the new fracture, and for a given action condition, the critical angle 60° led to the new fracture was possibly the dangerous value which could cause the strong earthquake.

Key words: high temperature and high pressure, new fracture, strong earthquake, critical angle