桜島火山における繰り返し相対重力測定(2020年10月および2021年3月)

Abstract

Relative gravity values were repeatedly measured at 19 gravity points in and around Sakurajima Volcano in October 2020 and March 2021, to monitor spatiotemporal mass variations associated with volcanic activity. The gravity values obtained from 1998 to 2021 showed a clear increase of up to +4.3 μGal/yr in the central part of the volcano. The gravity increase was even observed by the LaCoste’s G31 and G375 gravimeters; these gravimeters have been used at Sakurajima Volcano since 1975, and their temporal variations in reading values are negligibly small these days. These facts indicate that the gravity increase observed in the central Sakurajima is the actual one, because the artificial gravity change due to temporal variation of instrumental scale factor is expected to be small for the G31 and G375 gravimeters. The gravity increase can be explained by the point mass increase of 1.9×10¹⁰ kg/yr under the Kita-dake summit at the depth of 3 km below sea level. This result implies that volcanic mass continuously increases under Sakurajima Volcano, even during the quiet period of eruptive activities at the Minami-dake crater.