2022 年 75 巻 p. 93-105
The Tono Research Institute of Earthquake Science, located in Mizunami-City, Gifu Prefecture, has developed a comprehensive set of borehole crustal activity observation devices and is conducting deep underground observations on earthquakes and crustal movements. A geological fault, which is called the NNW fault, runs in the vicinity of the observation points. This paper reports on pumping experiments that were carried out in February 2001 and April 2002, in the DH2 borehole (depth: 500 m) near the NNW fault. The purpose was to investigate strain behavior near the fault. Changes caused by the pumping experiments were observed by water-level gauges and strainmeters at borehole observation points TGR350 (depth: 350 m) and TGR165 (depth: 165 m), and by a three-component extensometer with a length of 30 m installed in a horizontal vault observation point, TGRvault. The horizontal distance from the DH2 borehole to the TGR350 and TGR165 boreholes is about 271 m. The distance from DH2 to the extensometer installed in the TGRvault is about 193 m. When pumping was performed in DH2, the water level in TGR350 dropped; when pumping ceased, the water level rose. As the water level went up and down, changes were observed in each strainmeter. The maximum shear strain observed in the strainmeters of TGR350 was in the NNW direction from the viewpoint of geophysics. It showed a right lateral shift variation when the water level was falling and a left lateral shift variation when the water level was rising. The NNW fault is a right lateral fault. The strainmeters of TGR165 observed a different strain variation from that of the TGR350 strainmeters. Unlike the TGR165 and TGR350 strainmeters, the TGRvault extensometer in the horizontal vault showed similar fluctuations to those of the water level in all three of its components. The NNW fault is considered to extend to a depth of about 1 km, but The TGR350 strainmeters are located in the lower sparsely fractured domain (LSFD) at the uniform base of the Toki granite at a depth of 350 m, where fractures are few. The TGR165 strainmeter is installed in the Toki granite, which is the base of the upper higher fracture domain (UHFD), at a depth of 165 m, where fractures are more significant. The TGRvault extensometers in the horizontal vault are installed in the sedimentary layer close to the surface. The results of the pumping experiments can be attributed in part to differences in the response of the medium to groundwater fluctuations, due to differences in the number and distribution characteristics of the fractures in the medium where the observation instruments are installed, resulting in the differences in strain behavior.