Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−107−Synthesiology - English edition Vol.9 No.2 (2016) effect of water on the long-term weakening of fault strength in the laboratory. We studied the effect of water on the friction strength of a fault under temperature conditions higher than those actually found deep underground where the fault movement was assumed to occur by conducting compression fracture experiments with cylindrical rock samples (Fig. 11). The rock samples were myloniteTerm 1 sampled from the Hatagawa fracture zone, Fukushima Prefecture. This rock had been deformed and fractured while it was deep underground, and the cylindrical sample was cut such that its long axis was oriented at an angle of 30 degrees to the planar structureTerm 2 of the rock. When compression stress was applied axially to the sample, a fault plane formed parallel to the planar structure. With continued compression, the fault plane slipped and the frictional strength could be measured. Two types of experiment were conducted under constant pressure: one without water (dry condition) and the other with water (wet condition). Each experiment was performed at four temperatures: room temperature, 200 ºC, 400 ºC, and 600 ºC.The results showed that frictional strength hardly changed even under high-temperature conditions in a waterless environment, but the frictional strength decreased as the temperature increased under the wet condition (Fig. 12). Because the pressure conditions were the same, the data suggest that the strength decrease was due to chemical reactions and not to the physical mechanism known as the effective pressure law, which attributes the strength Differential stress (MPa)Axial strain (%)Axial strain (%)Differential stress (MPa)WETDRY64200200400600100012008006420020040060010001200800600400200RT600400200RTFracture strengthFrictional strengthFig. 12 Measurement resultsUnder the dry condition (DRY), frictional strength was almost constant (area inside the red frame) and independent of temperature, but under the wet condition (WET), the strength level decreased (shown by arrow) as the temperature rose. Measurement results in (left) the absence of water (DRY) and in (right) the presence of water (WET). Axial strain (%)Differential stress (MPa)Frictional strength of fault planeFracture strength642002004006008001000Fig. 11 Photograph of a sample after the experiment and the resulting stress-strain curveThe fracture strength of the rock and the frictional strength of the fault were measured. When the differential stress reached the fracture strength, a fracture plane formed in the rock sample. The differential stress needed to cause the fracture plane to slip is used to calculate the frictional strength.