To investigate how the parameters prescribing the slip-dependent constitutive law are affected by seismogenic environments, we conducted a series of triaxial fracture experiments using intact Tsukuba-granite samples, which were deformed at wet, temperature-pressure conditions that correspond to a crustal depth range down to 15 km at a strain rate of 10-5/s. It is conclusively demonstrated that the critical slip displacement Dc is virtually constant at the conditions of depths shallower than 10 km, and that Dc increases with increasing depth at depths greater than 10 km, because of the effect of temperature in the brittle-plastic transition regime. The breakdown stress drop for intact rock is found to be roughly constant (100 MPa) in a depth range down to 15 km. The apparent fracture energy increases with increasing depth at depths greater than 10 km. These results consistently lead to the conclusion that the stability of the breakdown process is enhanced at depths greater than 10 km.