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We used a gPhone (serial number 90), the newest spring-type gravimeter manufactured by Micro-g LaCoste Inc., to acquire high-quality, continuous gravity records, both below-and aboveground. At a depth of 100 m, when the gPhone was situated under an uncon ned aquifer, the standard deviations of the residual gravity based on rst-and second-order curve ttings are 4.2 and 2.7 μGal, respectively. Some gravity decreases caused by rainfall were clearly observed, and unknown gravity variations may also have occurred. Alternatively, when the gPhone was placed aboveground on the ank of a high mountain, the standard deviation of the residual gravity was 1.7 μGal for both the rst-and second-order curve ttings. The rainfall amount and snow depth can explain most of the residual gravity. On the basis of these results, we propose to detect and correct hydrological gravity responses using multiple gravimeters to study gravity signals from deep within the earth.

Water‐level data recorded during 1989–1999 at 16 closely clustered wells near the Tono mine in central Japan are searched for possible premonitory changes before earthquakes. The results suggest that some water‐level drops at a sensitive well before an M5.8 local earthquake on 1997 March 16 and some large distant earthquakes during 1994–1995 in Japan may have been premonitory in nature. The high sensitivity of this well to earthquake occurrence is attributed to the near‐critical hydrological conditions and the relatively high permeability of the tapped aquifer.

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.