Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−106−Synthesiology - English edition Vol.9 No.2 (2016) feedback mechanism in which the output of the thermocouple that measures the temperature of the top or bottom part of the sample is used as the control signal for the corresponding coil. Similar devices developed overseas use a triple-zone system, but we found that a uniform temperature distribution of the sample can be obtained with the double-zone system.As shown by a schematic diagram of the interior of the pressure vessel (Fig. 7, right side), the sample and the heaters are installed in an extremely tight space. The rock sample at the center of the pressure vessel must reach an extremely high temperature, but the pressure vessel, because it is made of metal, will undergo plastic deformation if its temperature reaches 800 ºC; such deformation would be extremely dangerous because of the high pressure inside the pressure vessel. A technological constraint, therefore, is that only the center part of the very tight interior space can be heated to a high temperature; the interior wall of the vessel cannot overheat. This problem was solved by packing an insulating material between the heater body and the heater case, as well as by the arrangement of the insulating material (shown in Fig. 9). A workable design was achieved through repeated trial-and-error with the help of a private company. The performance test results for the resulting heater (Fig. 10) confirmed that the sample could be maintained at an evenly distributed high temperature (800 ºC) while the temperature of the interior wall of the pressure vessel and the measuring device (the interior load meter) were maintained within a safe range (300 ºC or less).4.5 Integration of the technologies and reproduction of processes by a high-temperature and high-pressure rock deformation experimentThe integration of technologies newly developed at AIST with existing technologies made it possible to conduct rock deformation experiments under high-temperature and high-pressure conditions. By means of such experiments, the mechanisms of fault movement were investigated and the working hypothesis developed on the basis of geological survey observations was tested (Fig. 3). In general, for rocks located deep underground, it is necessary to consider a non-hydrostatic pressure system where various types of pressure are applied from various directions. However, to investigate deformation and fracture, only the differential stress accompanying plate movement needs to be considered as the crustal stress under pressure (hydrostatic pressure), and it can be represented by compression under hydrostatic pressure. Such pressure conditions were reproduced by our experimental device. To accurately measure rock deformation under high-temperature and high-pressure conditions, it was necessary to use gas (high-pressure gas) as the pressure medium. The maximum achievable pressure was 200 MPa. Although the performance of our device is one of the highest of any device of this kind in the world, it can only reproduce conditions at depths of up to about 10 km.5 Proposal of a new conceptWe present as an example the results of an academic study carried out by using the technologies and method developed in this research. In the field of seismology, it is known from geological and geophysical observations that the friction strength of faults in the natural world is weaker than the friction strength of rocks measured in the laboratory. Also, the rock strength changes depending on the rate of deformation, and it also changes with time, as seen by the creep phenomenon. These observations indicate that the properties of rocks, particularly their fracture strength and friction strength, are time dependent. It is important to understand the time dependency of rock properties to clarify earthquake occurrence mechanisms.The occurrence cycles of large earthquakes have long periods of several hundred to a thousand years. Therefore, although direct observation of processes causing long-term changes in fault strength by geophysical monitoring methods is not possible, the friction strength of rocks is also thought to undergo long-term changes. Therefore, we explained the time dependency of friction strength by a model according to which the long-term weakening of fault friction strength is due in essence to the development of micro-fractures and the slow progression of cracks at the tips of asperities where surfaces on either side of the fault plane are in contact, and we conducted an experiment to verify this model. This model assumes that long-term weakening of fault strength is dominantly caused by chemical reactions that progress in the presence of water. Therefore, if such reactions are the effective mechanism, then it should be possible to observe the Temperature (ºC)Time (s)TC1TC2TC3T2T1300002000010000002004006008001000Pc=200 MPaFig. 10 Performance test results of the developed heaterBoth the top (T1) and bottom (T2) of the sample inside the pressure vessel are maintained at a constant temperature. The interior wall of the pressure vessel (TC1 and TC2) and the location of the internal load cell (TC3) are maintained at 300 ºC or less.