We have developed a Borehole tiltmeter (Ishii et al., 2002) and installed it at the bottom of two boreholes site (STG100 and STG200) in the Mizunami underground Research Laboratory (MIU) in the Tono region, central Japan, as a part of the research. The Tono region is a relatively stable block of Cretaceous Toki granite uncomformable overlain by Tertiary layers (Mizunami sedimentary formations) with a thickness of approximately 100-200m (King et al, 1999, JGR). The STG100 site is located in the Tertiary layers, and the STG200 site in the Toki granite.
As research advances, the following results were obtained: (1) Amount of tilt-down variation at the STG100 site is approximately 1.7×10－5 radian/yr, and variation for the STG200 site is approximately 1.0×10－5 radian/yr. (2) Direction of observed tilt-down variation at both site are almost south-southeast.
In addition to above observation result, groundwater with volume of 700m3 was generally pumping a day in MIU (Kimata et al, 2015, Engineering).We consider that observed tilt-down variations may be caused by the “source” located in neighboring south-southeastern area of MIU.
