2022 年 75 巻 p. 83-91
Fluid injection induced earthquakes related to resource mining and geothermal power generation are concerns among many countries. It is difficult to directly observe the spatiotemporal changes in the fracture development and the stress state due to the water injection several kilometers underground, while we can estimate some extent from borehole measurements and seismic activity. We came up with an experiment using the photoelastic effect, which allows us to directly observe the stress on complex structures. Photoelasticity is a phenomenon that enables visualization of the amount of differential stress in a transparent elastic material. This paper introduces a photoelastic experimental method to visually observe the crack propagation and stress change in the crust relating to the water injection. The utilization of a transparent and soft agar gel makes it possible to observe the spatiotemporal stress change during the water injection. A thin acrylic cuboid container filled with agar gel with a 1% concentration is prepared as a simulated crust. The gel sample is placed between the experimental apparatus consisting of polarizers, quarter-wave plates, and a white light source. Water is injected from a syringe into a plastic pipe that resembles a borehole placed in the center of the sample surface. The isochromatic fringes generated by the stress change caused by the water injection are captured on high-speed video. We prepare two models; one is a model with no boundary, and the other has a medium boundary that mimics a weak surface such as a fault. During the water injection, we clearly observe the intense stress concentration on the crack tip and changes in the stress distribution associated with crack propagation. The stress change is also observed in the point far from the crack. The crack growth rate shows quadruple differences between the two types of samples. Visualized stress state in the experiment gives us intuitive insights into the crustal condition during the water injection. The experimental method proposed in this paper will help simulate a variety of crustal conditions and improve the understanding of fluid induced earthquakes.