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Abstract

Numerical simulation is a useful technique for studying the complex behavior of volcanic hydrothermal systems. Most previous simulations have used highly simplified structures, and few studies have considered the natural heterogeneity of the permeability distribution which significantly affects fluid flow within volcanoes. This is mainly because it is difficult to estimate the subsurface permeability structure. In this study, as a first step towards establishing a comprehensive scheme for constructing a realistic permeability structure, we attempted to numerically reproduce the steady-state hydrothermal system of a volcano using a rough permeability structure based on the three-dimensional resistivity structure. The volcano studied was the Kusatsu-Shirane Volcano (KSV) in central Japan. Several scenarios with different permeability structures were prepared to determine what structures significantly affect hydrothermal flow. In each scenario, saline fluids were injected from the bottom of the domain to simulate hydrothermal circulation within the volcanic edifice. In all scenarios, the fluid that ascended to around the summit area flowed down toward the eastern and western foot of the volcano and discharged mainly along the valleys, reproducing the actual distribution of hot springs around KSV. However, the observed discharge rates were reproduced only when a seal zone surrounding the conduit and low-permeability basement rocks were assumed. Although the constructed permeability structure was relatively simple, the simulation results that considered interpretations of the resistivity model closely reproduced several observed data. This suggests that incorporating information on the resistivity structure can significantly reduce the uncertainty of the hydrothermal fluid flow simulations.