Hideo Yukutake

Fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission

Abstract Three dry silicate rocks, gabbro, dunite and eclogite, were triaxially compressed up to a confining pressure of 3 GPa at room temperature. These rocks exhibited brittle fracture behavior up to the highest confining pressure. The change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission. The existence of the “high-pressure brittle-fracture” phase is proposed. The fracture strength increased with increase of confining pressure. The increasing rate of strength was lowered at a value of confining pressure: at about 0.8 GPa on gabbro; at about 1.0 GPa on dunite; and at about 1.5 GPa on eclogite. At lower confining pressures than the above value, the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased. At the higher confining pressures, however, the acoustic emission rate did not increase rapidly before final fracture, and stayed constant to the fracture. The similar behavior was shown on the granite studied previously. It is interesting that the frictional strength forms the boundary between “low- and high-pressure brittle-fracture” phases.

Thermal conductivity of NaCl, MgO, coesite and stishovite up to 40 kbar

Abstract The thermal conductivity of NaCl, MgO, coesite and stishovite have been measured as a function of pressures up to 40 kbar (4 Gpa) at room temperature. Polycrystalline coesite and stishovite were synthesized under high pressures and temperatures at our laboratory. An improved version of the comparative method suitable for the thermal conductivity measurement of small samples under high pressures was designed. The zero-pressure values are 0.0189 and 0.0412 cal. cm −1 s −1 °C −1 for coesite and stishovite, respectively. The thermal conductivities were found to increase linearly with pressure, and the increase rates relative to their zero-pressure values were 3.1, 0.68, 0.39 and 0.90% per kbar for NaCl, MgO, coesite and stishovite, respectively. On the thermal conductivity of coesite, the pressure dependence is small and the zero-pressure value is almost the same as that of polycrystalline quartz. On the other hand, the zero-pressure value of stishovite is 2.2 times as large as that of coesite.

In situ measurements of elastic wave velocity in a mine, and the effects of water and stress on their variation

Abstract In situ wave velocity measurements were carried out in the Ikuno mine of Japan to detect the travel time variation of less than 0.5 μs over distances of about 11 m, using ultrasonic waves. The measurements were conducted continuously every 90 min over an 8-day period. Ultrasonic waves, emitted by a generator assembled from lead zirconite titanite discs, were received by accelerometers with frequency responses up to 16 kHz. We found both long-term systematic and short-term periodic variations in the observed travel time, and examined the correlation between the short-term periodic variations and earth tides, and also the relationship between the long-term variations reaching 7.5 μs (corresponding to a velocity change of 0.3%), and atmospheric pressure and rainfall. The stress sensitivity of the in situ rock caused by opening and closure of dry cracks was considered insufficient to solely explain the observed semidiurnal variations reaching a range of 2.5 μs (0.1%). The semidiurnal variations were then found to be explainable by changes in water fraction of less than 0.5% in cracks and joints. To study the physical properties of the crust, it will be useful to perform such a continuous high precision observation in mines as well as between boreholes.

Fracture nucleation process in intact rocks

Abstract Yukutake, H., 1992. Fracture nucleation process in intact rocks. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 247–257. We produced tomographic images for cross-sections of granite samples, by measuring the elastic wave velocity along 20 ray paths at high sampling rates during triaxial compression tests. The dynamic fracturing process just before ultimate fracture was investigated from the rapid changes in the associated tomographic images. It was found that microcracks concentrated locally in several regions as axial load was increased, and the velocity then recovered in some of the intensively microcracked regions 6–60 s before fracture. Subsequently, the amplitude of the elastic waves through the most intensively microcracked region also recovered. In a few cases, the frequency of the elastic wave through the most intensively microcracked region became extremely high. We consider that fracture nucleated in the intensively microcracked regions, and infer that several macrocracks occur due to an abrupt coalescence of microcracks in the fracture nucleated region. A crack length in the process zone for brittle fracture was evaluated from the stress drop observed just before fracture and the published value of the shear fracture energy for Westerly granite. We expect, from the results of both tomography and stress drops, that subsequent instability occurred when one of larger macrocracks formed by the coalescence of the macrocracks reached a critical length in the most intensively nucleated region.