For the safe long-term functioning of a geological waste disposal system, it is important that groundwater flow and its chemical composition are maintained in favorable conditions. One reason for the significant changes in groundwater flow and chemistry conditions is the mixing of upwelling mantle-derived fluids through the crust. However, it is impossible to investigate the underground conditions at such depths of deeper than several thousands of meters through drilling surveys.
 Herein, we hypothesized that crustal fracture zones act as fluid pathways and focused on seismic anisotropy as a clue to identifying their distribution. Seismic anisotropy is the directional dependence of the seismic velocity. The anisotropic intensity in the crust changes depending on factors such as fracture density and could be revealed through seismic wave analysis.
 The results we obtained in Kyushu, southwestern Japan, showed that strong anisotropy appeared around Kirishimayama volcano. This strong anisotropy may reflect the existence of volcanic fluid pathways.
 In recent years, the relationship between upwelling fluids and earthquake occurrence has attracted attention. Fluids destabilize faults and are one of the causes of earthquake generation. Although there are still challenges to overcome, such as more detailed studies, this research effort contributes not only to geological waste disposal but also to earthquake disaster prevention.

This study was funded by METI (Grant number JPJ007597), as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-Term Geosphere Stability on Geological Disposal Project of Radioactive Waste (FY 2021–2022).”