Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−103−Synthesiology - English edition Vol.9 No.2 (2016) a timespan of about a thousand years, and the processes causing those changes must take place over this extremely long period of time. These processes slowly change the fault strength and deform the rocks. When water is present, chemical interactions between water and rocks must occur in the high-temperature and high-pressure environment deep underground. Although fault movement is imaged as a “slip,” isn’t friction really a micro-fracture occurring at the point where the surfaces on either side of the fault plane are in contact? Don't chemical reactions occur between rocks and water that affect the fracture process in this microscopic domain? We proposed a working hypothesis that such processes are important, namely, that friction really consists of micro-fractures caused by chemical reactions between rocks and water in the area of contact, specifically at the tip of asperities of the fault plane (Fig. 5). A rock fractures when a stress that surpasses its strength is applied, and the fracture progresses at the tips of cracks within the rock. According to this reasoning, a fracture or a change in state will not occur if the applied stress does not reach the fracture strength, nor will the crack continue to grow. However, cracks and fractures are known to slowly progress even in environments where the stress is less than the fracture strength. This phenomenon is called stress corrosion, and it is explained by a mechanism in which the rock materials or other substances react with water or other components in the surrounding environment and their strength decreases.4.2 Materials testing technologySlow deformation and slip are thought to occur deep underground when force (crustal stress) is applied to rocks and faults. Therefore, to reproduce processes occurring deep underground, we measure the deformation and slip that occur when we apply force to rock materials. The method and technique used here are the same as those used in materials testing. Figure 6 shows an apparatus used in this research that was originally designed for materials testing. When this testing apparatus was acquired by AIST (Geological Survey of Japan) in the 1980s, it was used mainly to test the fracture toughness of rocks. For our earthquake research, we brought this apparatus, which had been lying unused since the project for which it had been acquired had been completed, back into service. The basic function of this apparatus is to apply upward and downward forces on materials (in this case, rocks) to deform and fracture them. A control unit (on the left in Fig. 6) controls the upward and downward movement of the piston that applies force to the sample in the materials testing device (on the right in the figure). A servo-controller causes the position of the piston, and thus the load applied to the sample material, to change at a constant velocity or according to some pre-set function (such as a sine function), and the changes in the position and load are measured. We replaced the original analog control unit with the latest digital control technology. This materials testing apparatus was used as the basic framework of our device for measuring the deformation and fracturing of rock samples and the force applied to the samples.4.3 High-pressure technologyBecause the aim of this endeavor was not merely to conduct materials testing of rocks but to investigate how processes differ between the high-temperature and high-pressure conditions that prevail deep underground and those in the laboratory environment, we developed technologies to reproduce high-temperature and high-pressure conditions in which we could perform deformation experiments.To produce a high-temperature and high-pressure environment in the laboratory, a solid or a fluid is used as the pressure medium, which is sealed inside a pressure vessel (sealed container). The pressure inside the vessel (confining pressure) is increased as the interior volume is decreased by inserting a Fig. 6 Materials testing apparatus used for materials science investigationsRocks were used as the test samples. Changes were made to this device in this study.NanometerScale: MeterFracture at the crack tipPartial contact of planeFriction, slipFig. 5 Schematic diagram of friction and fracturesOn a spatial scale of centimeters to meters, friction can be understood as a force resisting slip along a flat plane. However, at a small spatial scale of millimeters or less, the two surfaces on either side of the plane can be seen to not be in continuous contact, but only in partial contact. Micro-fractures occur where asperities on each surface contact the other surface. Such fracture processes in the contact area are the essence of the friction phenomenon. At a nanoscale, micro-fractures at the tips of the asperities progress slowly when water is present by means of chemical reactions.