Hydrothermal System After the 1990-95 Eruption Near the Lava Dome of 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 result of our investigations, we constructed a conceptual model for a hydrothermal system after the 1990-95 eruption near the lava dome 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 lava dome. On the other hand, there is another conceptual hydrothermal model based on self-potential data presented by another group, 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 modeling. As a result of the numerical modeling of this study, we concluded that (a) there are two dominant flows, the upflow of high temperature 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.