Variations of marine gravity reflect mass changes in Earth’s interior, including magma motions during volcanic eruption, which manifest in submarine mass transports. Absolute gravimeters have been used to detect and study evolutions of magma mass motions, however, only for land volcanoes. Here for the first time, we used radar altimeter-derived time-varying marine gravity fields, corresponding to before, during and after the Nishinoshima volcanic eruption, Izu–Bonin arcs near Japan, to quantify the evolution of undersea volcanic magma mass motions. The magma depths were observed to become shallower and < 2 km, which are almost in exact accordance with seismic results. We find that the magma volume decreased beneath the Nishinoshima volcano, while increased or being fed by deeper reservoirs to the east and northeast of Nishinoshima. We conclude that the Nishinoshima volcano may continue to be active in the future. This study highlights the use of satellite radar altimetry as an innovative and viable tool to study subaqueous volcanism.

海洋重力的变化反映了地球内部的质量变化，包括火山喷发期间的岩浆运动，这在海底质量运输中表现出来。绝对重力仪已被用于检测和研究岩浆质量运动的演化，但是仅适用于陆地火山。在这里，我们首次使用雷达高度计衍生的随时间变化的海洋重力场，对应于日本附近的西之岛火山喷发之前，之中和之后，伊豆-波宁弧，以量化海底火山岩浆质量运动的演化。观察到的岩浆深度变得更浅且小于2 km，这几乎与地震结果完全一致。我们发现，在西之岛火山下方的岩浆量减少了，而在西之岛的东部和东北部则由更深的储层增加或被其供给。我们得出的结论是，西之岛火山将来可能会继续活跃。这项研究强调了将卫星雷达测高技术用作研究水下火山作用的一种创新且可行的工具。