https://phys.org/news/2018-07-deep-subterranean-japan-volcanoes.html

The research team from the University of Miami's (UM) Rosenstiel School of Marine and Atmospheric Science and Florida International University analyzed deformation data from 32 permanent GPS stations in the region to identify the existence of a common magma reservoir that connected the two volcanoes.

Leading up to the eruption of Kirishima, which is located in the densely-populated Kagoshima region, the Aira caldera stopped inflating, which experts took as a sign that the volcano was at rest. The results from this new study, however, indicated that the opposite was happening—the magma chamber inside Aira began to deflate temporarily while Kirishima was erupting and resumed shortly after the activity at Kirishima stopped.