Mitsuhiko Shimada

Three-dimensional observations of faulting process in Westerly granite under uniaxial and triaxial conditions by X-ray CT scan

Abstract Observations of spatial fault development in granite undergoing compression provide new insights into the process of faulting. Dry intact Westerly granite samples were loaded under a confining pressure of 100 MPa (triaxial conditions) and 5 MPa (∼ uniaxial conditions), and the progress of faulting was controlled by maintaining the increment of circumferential displacement at a constant rate, which apparently stiffened the machine. The samples were unloaded after they experienced some degree of stress drop and were successfully recovered before faulting progressed further. A conventional medical X-ray CT scanning system was used to image the sample interiors. Three-dimensional fault systems were detected with sequential X-ray CT images. It was found that three-dimensional reconstruction by X-ray CT images yields not only three-dimensional images of the fault system, but also provides fault cross-section images with much less artificial noise (artifacts) than does direct X-ray CT imaging. Three-dimensional images show that a fault system that developed under uniaxial conditions is much more complicated than a fault system produced by triaxial conditions. In addition, the fault plane produced under uniaxial conditions is inclined at a lower angle to the maximum compressive axis than under triaxial conditions. Comparing X-ray CT images, we show that a fault nucleates locally on the sample surface just after peak stress, then develops into the final fault plane in the residual stress stage of the complete stress–strain relationship under triaxial conditions.

Lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures

Abstract A model is proposed for the strength of the crust or lithosphere, in which the peak strength is in the brittle regime, but not on the brittle-ductile transition. The model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high-pressure type when the compressive strength equals the frictional strength. The model assumes that a high-pressure type of fracture is dominant in the earths lower crust, except where very high pore pressures exist. It takes account of the scale effect on the strength of rocks. The strength of the high-pressure regime is estimated as a function of depth, or pressure and temperature, using experimental results derived at high confining pressures and temperatures. In the upper part of the brittle zone, where the frictional strength is lower than the compressive strength, Byerlees friction law is used. In the ductile regime, the steady-state creep law is used. Two cases of lithospheric strength, dry and wet (hydrostatic water pressure), respectively, are evaluated for thrust, strike-slip and normal faulting mechanisms. With the exception of a “wet” lithosphere and a normal faulting mechanism, the strength between the frictional and creep regimes is approximately constant with depth, although the peak strength is in this part.

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.

Two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth's crust

Abstract Four silicate rocks (granite, gabbro, dunite and eclogite) were triaxially compressed up to a confining pressure of 3 GPa at room temperature and were found to exhibit brittle fracture behaviour. From the measurements of the compressive and frictional strengths and the activity of acoustic emission (AE), and the observations of the microstructure of fractured specimens, the pattern of fracture was found to change when the compressive strength became equal to the frictional strength. Two types of fracture mechanism, “low-pressure” and “high-pressure” types, were inferred. The low-pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers. The features of high-pressure type fracture are similar to those observed between brittle fracture and ductile creep in high-temperature triaxial experiments: microcracks are not concentrated close to the main fault, and the main fault is sharp and oriented at approximately 45° to the stress direction. This suggests that the high-pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature. For granite and gabbro samples of a few centimeters size, high-pressure type fracturing begins at ∼ 0.8 GPa confining pressure, while for dunite and eclogite it begins at ∼ 1.0 and ∼ 2.0 GPa respectively. The size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens > 1 m. This estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 MPa. The significance of this is that it may be necessary to model earthquakes using the high-pressure type fracturing, because in the absence of pore pressure 60 MPa confining pressure corresponds to a depth of 3 km. Faulting processes in the crust, including both the initial faulting and the movement of existing faults, were inferred using the experimental results and the estimated size effect on strength.

The observations of faulting in westerly granite under triaxial compression by X-ray CT scan

Abstract Observations of the spatial development of cracks in granite undergoing triaxial compression provide new insights into the proces of fault formation. Dry intact Westerly granite samples were loaded under confining pressure of 100 MPa, and the progress of fault formation was controlled by maintaining the increment of circumferential displacement at a constant rate. The samples were unloaded when they experienced stress drops and were successfully recovered without severe crushing. A stereo microscope was used to observe the spatial pattern of cracks on the sample surfaces, and an X-ray CT scanning system was used to non-destructively image the sample interiors. Microscope observations show that several fault-like linear cracks developed on the surfaces of all the samples and have an angle of about 30 degrees to the maximum compression axis. This suggests that fault nucleation began just after peak stress. The X-ray CT images show that a fault-like linear crack grew inward toward the sample interior as axial stress decreased. It is concluded that the fault nucleated locally on the sample surface just after peak stress, and developed into an apparent fault by the plateau stage of the complete stress-strain relationship.

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 ∗ 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.

Ductile behavior of a fine-grained porous basalt at room temperature and pressures to 3 GPa

Abstract A dry fine-grained porous rock (Yakuno basalt with grain size of 0.05–0.4 mm and 7% porosity) was triaxially compressed up to 3 GPa confining pressure at room temperature. The axial differential stress was increased stepwise at fixed intervals. At each step, in the ductile regime of the rock (above 300 MPa confining pressure), the observed strain exhibited time-dependent creep behavior. The linear time-dependent strain was well represented by a power law constitutive equation. The stress exponent had a value of ∼ 3 at 1.5 GPa and lower confining pressures, and decreased as confining pressure increased above 1.5 GPa, asymptotically approaching ∼ 1. This suggests there is a pressure-induced transition of the cataclastic flow mechanism, even at room temperature. Microstructures showed highly granulated grains in former pore spaces. In the host areas, as well as in the granulated areas, extensive microcrack formation was observed at lower confining pressures but there was less microcrack formation at higher confining pressures. This contrast probably corresponds to the change in the flow mechanism. The equivalent viscosity was estimated as a function of differential stress and strain rate.

Gross structure of a fault during its formation process in Westerly granite

Abstract We investigated the gross structure of a fault in three dimensions during its formation process. The observed sample was Westerly granite recovered from compression after peak stress but before dynamic rupture. The sample was infused with a fluorescent resin, mechanically stripped, then observed with a stereo-microscope under UV light. The main fault plane was found to be macroscopically elliptical, and the fault trace on each polished surface was identical to that detected by X-ray CT scanning. Evidence suggests that the fault plane widened its cracked zone as it grew. The gross structures of the fault system, such as a step-over structure and sub-faults parallel to the main fault, were also detected. We conclude that the step-over structure was formed during the fault formation process.

Melting of albite at high pressures in the presence of water

Abstract The melting of albite has been investigated in the presence of water in the pressure range up to 30 kb, from the point of view that the effect of water on the melting of silicates is important for the investigation of the state of the earths interior and the earths thermal history. The melting relations have been determined under the following conditions: (a) that the water pressure is equal to the total pressure or the albite melt is saturated with water, and (b) that albite melt contains a fixed amount of water (10 wt% and 14 wt%) and is saturated with water up to a certain value of pressure but undersaturated in the higher pressures. As oressure increases, the melting point of albite, in the presence of a fixed amount of water, decreases from 1118°C along the water-saturated melting curve up to a certain value of pressure. In the higher pressure region albite begins to melt incongruently, and the temperatures of the beginning of melting decrease with increase of pressure along the water-saturated melting curve and the temperatures of the end of melting increase approaching asymptotically the curve for the dry condition. In the region between the solidus and the liquids, crystal and liquid co-exist. The value of pressure, where the incongruent melting begins, is in good agreement with the results of the solubility of water in albite melt.