Variations of absolute gravity accompanying earthquake‐induced changes in subsurface pore water pressure at the Mizunami Underground Research Institute construction site, central Japan

The Tono Research Institute of Earthquake Science has been measuring gravity using an FG5 absolute gravimeter located at the Mizunami Geoscience Academy (MGA) in central Japan since January 2004. Measured gravity decreased immediately following the 2004 earthquake off the Kii peninsula (MJMA 7.4) by about 6 μGal. Here, we investigate the empirical relationship between pore water pressure change in a borehole near the MGA and gravity change measured at the MGA. We reveal that (1) gravity change correlates inversely with pore water pressure change at 81 m below the surface at a particular borehole and (2) several different sets of conversion coefficients from pressure head to gravity can be used to explain 60–70% of gravity variations with less than 2 μGal uncertainty. These newly identified relationships may suggest that an absolute gravimeter alone could be used to observe the change of groundwater quantity.