Confirmation of two types of fracture in granite deformed at temperatures to 300°C

Abstract

Shimada, M., 1992. Confirmation of two types of fracture in granite deformed at temperatures to 300°C. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 259–268. Dry granite samples were triaxially compressed at confining pressures of up to 2 GPa at temperatures to 300°C. The existence of two types of fracture, previously observed in room-temperature experiments ∗ ∗ Shimada et al. (1983); Shimada and Cho (1990). was confirmed. The low-pressure type occurs when the compressive strength is higher than the frictional strength and the high-pressure type occurs when the compressive strength is equal to the frictional strength. The rate of increase of strength with confining pressure in the high-pressure type regime is lowered as temperature increases, as if an ideal Coulomb fracture criterion at a fracture angle of 45° might be applicable with increasing temperature. Near the boundary between the low- and high-pressure type regimes, unusually high values of strength were measured. The specimens after such runs gave the suggestion that both low-and high-pressure types of fracture coexist. There are some discrepancies in the values of strength from room temperature and those performed previously, which are considered to be due to the experimental variability. However, the present experimental results to 300°C confirmed that two types of brittle fracture exist and that the fracture type changes when the compressive strength becomes equal to the frictional strength. This tends to confirm previous speculations that many crustal fractures are of the high-pressure type. If so, the low-pressure type of fracture, in which precursory AE (acoustic emission) activity before final fracturing is observed, would be anticipated only at very shallow depth at ordinary pore-pressure conditions. It would occur at the depth of seismogenic zones only in the case of very high pore pressure.