Numerical Modelling of the Hydrothermal System in Unzen Volcano, Japan

Abstract

We have been conducted geothermal investigations on Unzen Volcano since 1999 as part of the Unzen Scientific Drilling Project. Using the results of our investigations, we constructed a conceptual model of Unzen Volcano. This model shows that permeating rainwater makes dominant downflow and cools the body of the lava dome - except for a part of the conduit which is the pathway for high temperature gas - and that there is no strong hydrothermal circulation near the dome. On the other hand, there is another conceptual hydrothermal model based on self-potential (SP) data, which suggests the existence of hydrothermal circulation near the lava dome. Therefore, we constructed a 3-D numerical model of the Mt. Fugen area to investigate which conceptual model is reasonable. A computer program called HYDROTHERM Version 2.2 was used for this numerical modelling. As a result of the numerical modelling of this study, we conclude that (a) there are two dominant flows, the upflow of gas in the conduit and the downflow of permeating rainwater, (b) the deeper part of the conduit of Unzen Volcano maintains a hot temperature, and (c) there is little possibility of geothermal fluid convection at a scale of a few hundred meters in Unzen volcano in 13 years since the commencement of the 1990-95 eruption.