2022 年 68 巻 p. 49-68
Relative gravity measurements are one of the most useful methods for investigating spatiotemporal mass variations in volcanoes. In order to quantify gravity variations accurately, scale factor （SF） and resultant conversion function should be calibrated for each relative gravimeter in advance. Conventionally, the SF value has been treated as a constant for each relative gravimeter. However, a recent study showed that the SF of Scintrex CG5-300500033 gravimeter indicates a dependence on its reading value. The reading value dependence of SFs has not been fully investigated for other relative gravimeters including LaCoste gravimeters, which have long been used for relative gravity measurements in volcanoes. In this study, we made calibration of the SFs for three LaCoste gravimeters （G534, G605 and G680） and one Scintrex gravimeter （CG5-150241330）, by measuring relative and absolute gravity values at four primary gravity points in Japan with the maximal gravity difference of 660 mGal. We found that the SFs vary with reading values almost linearly, by up to 4×10－4 for the 500 mGal gravity difference. We also estimated the absolute gravity values at five secondary gravity points located near active volcanoes, by measuring relative gravity differences between the primary and secondary gravity points using the four relative gravimeters described above. The SF-derived systematic error was then corrected by considering the reading value dependence of the SFs. As a result, the absolute gravity values at all of the secondary gravity points were precisely determined with < 20 µGal standard deviation by using the reading-value-dependent SFs; the average of the SF’s error became smaller by 72 and 25% than in the cases where the SFs were assumed to be 1.0 and a constant value for each gravimeter, respectively. By comparing the estimated absolute gravity values with those measured by absolute gravimeters, an absolute gravity change of ＋240 µGal was identified at the AVL gravity point in Aso Volcano from May 2010 to April 2021. We conclude that this large change has been caused mainly by the coseismic ground subsidence due to the 2016 Kumamoto earthquake.