Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−108−Synthesiology - English edition Vol.9 No.2 (2016) TerminologiesTerm 1.Mylonite: Rocks that have been ductilely deformed and formed in the high-temperature zone of deep faults (ductile shear zone).Term 2.Planar structure: A two-dimensional rock element. The term also applies to structures formed by deformation processes.ReferencesK. Yamaoka: Nankai Trough Jishin (Nankai Trough Earthquake), Iwanami Shoten (2016) (in Japanese).Seismology in Japan: Present state and seeds for the 21st century, Jishin, 2 (61), Special Edition (2009) (in Japanese).C. Scholz: The Mechanics of Earthquakes and Faulting (Second Edition), Cambridge Univ. Press (2002).T. Yoshioka: Evaluation of earthquake occurrence from active faults—Evaluation of rupture probabilities of active faults using the Cascade Earthquake Model based on behavioral segmentation, Synthesiology, 2 (3), 194–200 (2009) (in Japanese) [Synthesiology English edition, 2 (3), 177–183 (2009)].Y. Okamura: Reconstruction of the 869 Jogan tsunami and lessons of the 2011 Tohoku earthquake—Significance of Long-term processFrictional strength of faultTemperature (ºC)80060040020000.50.60.70.80.91.0DRYWETFig. 13 Temperature dependency of the frictional strength of the fault plane in rock samplesBy subjecting the sample to higher temperature conditions than those experienced underground, the processes occurring in the area of contact along the fault plane were accelerated, allowing processes that normally occur over a long period of time to be observed. The measurement results show that the frictional strength decreased in the wet condition (WET) compared with that in the dry condition (DRY).decrease to an increase in the pore water pressure. Under the assumption that the processes of this phenomenon, which in nature progress slowly, are dependent on chemical reactions, we obtained results that support the inference that the speed of progression can be increased by raising the temperature to increase the rate of the chemical reactions. In fact, the results showed that higher temperatures allowed long-term processes to be observed (Fig. 13).This study clarified the mechanism whereby fault strength is decreased over a long time period. The fault strength decreases and eventually becomes lower than the present crustal stress; when this point is reached and a fracture is triggered, an earthquake occurs. Through the modeling of this process, simulations of earthquake occurrences can be refined. In earthquake forecast studies, to be able to conduct simulations with a numerical model, it is first necessary to construct a physical model based on an accurate understanding of the mechanism of earthquake occurrence. Our results demonstrated an important mechanism leading to the occurrence of an earthquake, a phenomenon that cannot be directly observed on a human timescale. By quantitatively evaluating the mechanism and incorporating the results into a numerical model, we can construct a refined earthquake occurrence model. In the future, it will be necessary to develop a constitutive equation for earthquake occurrence in a mathematical form that can be incorporated into a numerical model and to determine the necessary parameter values and their dependency on environmental conditions such as temperature and pressure. By increasing the precision of earthquake forecasts, we expect to be able to deliver more accurate information to society.6 Future issues and prospectTo verify earthquake forecast models, we developed technologies and methods for accelerating and investigating geological phenomena that ordinarily occur on a thousand-year timescale in a laboratory rock experiment. Here, we combined existing methods and technologies with newly developed technologies and incorporated them into our research scheme (Fig. 3). We have provided the core technology to some universities in Japan.Although our research is still in the data collection stage at this point, we were able to investigate and publish a new concept about time-dependent fault strength. Therefore, we have taken one step toward understanding earthquake phenomena.The next step is to obtain data and develop a model that can be used in computer simulations to provide physical and geological evidence for future earthquake forecasts. We hope to work toward constructing a model to make accurate forecasts possible.AcknowledgementsThis research was conducted with the help of many colleagues. I am particularly grateful to Koichiro Fujimoto, Takashi Arai, Miki Takahashi, Keigo Kitamura, Kazuo Mizoguchi, and Norio Shigematsu. The internal heater was developed with the help of Kenichi Iryo and Akira Ogura (currently, Pre-Tech Co., Ltd.).