Akio Cho

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.

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.

Compressive failure of mudstone samples containing quartz veins using rapid AE monitoring: the role of asperities

This paper presents the results of an ongoing experimental investigation of compressive failure of homogeneous and heterogeneous rocks. We used a rapid data acquisition system to monitor the spatio-temporal distribution of acoustic emission (AE) during fault nucleation under conditions of either constant-rate loading or static loading. In order to examine the effect of asperities on faulting, we conducted a series of experiments on mudstone with quartz veins. The bedding planes of the mudstone were oriented at an angle of 30° with the maximum compressive stress and were expected as the fracture plane. The quartz veins are much stronger than the bedding plane of mudstone and thus the samples model faults having strong asperities. Experimental results show that: (1) AE activity initiated close to the peak stress, and almost all AE hypocenters appeared at the intersections of the veins and the fault planes, suggesting that the vein asperities control faulting; (2) the b-value changed with time and shows multiple large and short-term fluctuations; (3) the change of the b-value correlated closely with the spatio-temporal hierarchy of the fracture process; and (4) fault segments along the bedding plane show behavior of slip having large compressive deformation before the peak stress, while the vein asperities show large precursor dilatancy prior to dynamic rupture. These experimental results are helpful for the understanding of seismic precursor phenomena associated with strain localization, dilatancy, change in level of ground water, as well as the b-value.

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.