Critical Factors That Control Hydrothermal Circulation Within Active Volcanoes: Constraints From Numerical Simulation Based on a Resistivity Structure Model

Numerical modeling is a valuable approach for quantitative elucidation of the highly complex hydrothermal processes within active volcanoes. Various schemes have been developed for the numerical simulation of volcanic hydrothermal systems, generally employed in combination with other geophysical and geochemical monitoring techniques. However, a scheme for constructing realistic permeability structures (crucial for simulations) remains unexplored. As the first step toward establishing such a scheme, we conducted numerical simulations to explore the effect of lithology variations on hydrothermal circulation within active volcanoes. These simulation models were constructed based on the electrical resistivity structure model of the Kusatsu-Shirane Volcano (KSV) in Japan. Key factors include the permeability of the host rock and underlying basement, permeability reduction in the ductile region, and the geometry of a silica sealing layer. Of these, the silica sealing layer was a significant factor in reproducing the actual observations. The simulations indicated that the permeability and degree of closure of the silica sealing layer determined the pressure distribution within the region that the layer enclosed, and were, thus, responsible for the low resistivity of the subvertical conductors commonly found beneath volcanoes. The permeability structure used in this study was simple but systematically constructed, and simulations based on this permeability distribution successfully reproduced the key observations. The knowledge obtained from the numerical model of KSV can be used to explain the resistivity distribution of other active volcanoes, including higher- and lower-temperature systems. The results suggest the validity and potential broad applicability of the proposed modeling scheme.