BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN

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Continuous time variations in relative gravity and tilt were observed by a CG-3 M relative gravimeter at Arimura, Sakurajima Volcano (Southern Japan) during the rapid inflation event on August 15, 2015. The gravity/tilt signals were retrieved from the original data by correcting several disturbances such as instrumental drift and tidal effect. The retrieved gravity change is -5.86±0.27μGal;its amplitude is smaller than the typical uncertainty of relative gravimeters (∼10μGal), but the continuous measurement of relative gravity in a one-minute interval contributed to the detection of the small gravity change in the case of Sakurajima Volcano. The tilt change of 55.9μrad is also retrieved from the CG-3 M’s tilt data. The success in detection of the tilt change shows that the gravimeters can be utilized as portable tiltmeters as long as significant tilt variations are expected at volcanic areas. The observed gravity change is consistent with one of the dike intrusion models provided by Geospatial Information Authority of Japan, if the density value in the dike of 0.98±0.37g/cm³ is assumed.

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