Three-dimensional observations of faulting process in Westerly granite under uniaxial and triaxial conditions by X-ray CT scan
Introduction
Detecting fracture nucleation and understanding the process of faulting are important keys to earthquake prediction. Precursory changes in some physical properties before faulting have been reported in laboratory experiments (e.g., Yanagidani et al., 1985; Reches and Lockner, 1994). In order to appropriately interpret these precursory signals, it is essential to understand the faulting phenomenon itself. Investigations of faulting in rock samples have been carried out since the 1960s and fall into three types. The first is observing crack systems by means of microscopy. Hallbauer et al. (1973)used a stereo-microscope to observe cracks on the surface of rock samples. Fractured samples have been observed using an optical microscope (e.g., Koide and Hoshino, 1967; Moore and Lockner, 1995) or a scanning electron microscope (e.g., Tapponnier and Brace, 1976; Batzle et al., 1980; Kranz, 1983; Shimada and Cho, 1990). Observation with an optical or scanning electron microscope is one of the most useful methods for studying the development of microscopic crack systems in a sample, although the number of observable cross-sections is small. This type of observation cannot be made non-destructively and is essentially two dimensional. The second type of investigation is observing physical properties, such as P-wave velocity (e.g., Yukutake, 1989, Yukutake, 1992) and acoustic emission events (e.g., Lockner et al., 1992). This approach allows one to discern changes in physical properties as they occur, and makes it possible to perceive the sequential development of cracking three-dimensionally, although it requires us to relate the changes in physical properties to cracking activity. The third type of investigation is observing sample interiors with an X-ray CT scanning system, which allows one to non-destructively obtain an unlimited number of images of sample cross-sections (e.g., Wellington and Vinegar, 1987; Raynaud et al., 1989; Johns et al., 1993; Nishizawa et al., 1995; Kawakata et al., 1997). Kawakata et al. (1997)observed the faulting process under triaxial condition by X-ray CT scanning. The X-ray CT scanning method is effective because it does not disrupt the use of other sampling methods. X-ray CT scanning also provides three-dimensional images of fault systems, and they reveal the orientation of fault planes not visible externally.
In the present study, to capture images of fault systems developed within the interior of samples undergoing faulting processes, an X-ray CT scanning system was used. The three-dimensional reconstruction of X-ray CT images was also done. Westerly granite samples were compressed under a confining pressure of 100 MPa (triaxial conditions) and 5 MPa (∼ uniaxial conditions), and recovered after unloading at various points following peak stress, in order to record the sequential development of faulting. We will describe three-dimensional faulting processes in Westerly granite and show that the predominant mode of cracking changes from uniaxial to triaxial.
Section snippets
Experimental procedures
We used cylindrical samples from a single block of Westerly granite cored perpendicular to its rift plane. The samples were 50.0 mm in diameter by 100.0 mm in length, and the ends were ground parallel, to a tolerance of ±0.01 mm. Samples were jacketed from the inside outward with three heat shrinkable tubes of PFA (Perfluoroaloxy), silicone rubber, and FEP (Fluorinated Ethylene Propylene), then loaded under confining pressures of either 100 or 5 MPa in a triaxial cell. We adopted a confining
Observation procedures of sample interiors with an X-ray CT scanning system
We used a conventional medical X-ray CT scanning system. The X-ray beam was generated at 120 kV by a 175 mA current for 4.0 s. The CT number, which is a function of X-ray attenuation coefficient (Johns et al., 1993), is obtained over the scanned volume of a 160-mm-diameter by 1.0-mm-thick cylinder that was divided into 512×512×1 voxels. Then, the resolution of the resulting images is about 0.3 mm in a direction parallel to scanned cross-sections and 1.0 mm in a direction perpendicular to
Samples after triaxial loading
X-ray CT images of WG-T2, WG-T3, and WG-T4, which experienced a certain amount of stress drop under triaxial compression, show only a single fault plane that is inclined at about 30° to the maximum compressive axis down to certain depths (Fig. 3, Fig. 4). Kawakata et al. (1997)gave surface images of the same samples observed with a stereo-microscope (Fig. 5). The fault traces of them correspond to the fault planes in the X-ray CT images. Images near the surface, as seen in Fig. 3a1,b1,c1, show
The faulting process and fracture modes under triaxial conditions
The slip vector for shear fracture in laboratory experiments is subparallel to the vertical plane, hence, the fault growth parallel to the surface trace is in-plane growth and that perpendicular to the surface trace is anti-plane growth. Under triaxial conditions, fault growth both along the dip direction (in-plane fault growth) and perpendicular to the dip (anti-plane fault growth) was suggested in images from surface observation (Fig. 5), as well as in images produced by an X-ray CT scanning
Conclusions
Compression tests under triaxial and uniaxial conditions were done with cylindrical samples of intact Westerly granite under dry conditions at ambient temperature to investigate the faulting process of crustal rocks in detail. The faulting process was controlled by using circumferential displacement as a feedback signal, and samples were unloaded rapidly at various points following peak stress. After observing their surfaces with a stereo-microscope, the samples were analyzed with an X-ray CT
Acknowledgements
The authors are greatly indebted to Ken Ikehara, Koji Masuda, and Yoshito Nakashima for their kind assistance in using the X-ray CT scanning system, and also thank Shoichi Yamazaki for his support in carrying out the experiments. A number of helpful comments in review from Terry Engelder, Joanne T. Fredrich, Naoto Yoshioka, and Ernest H. Rutter are greatly appreciated.
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