Geothermal Fluid Flow Model in Shimabara Peninsula

The Unzen Scientific Drilling Project (USDP) had been conducted by the Science and Technology Agency (FY1999-2000) and the Ministry of Education, Culture, Sports, Science and Technology, Japan (FY2001-2004). In this project, one of the objectives of our study was to construct a comprehensive hydrothermal model of beneath Unzen Volcano by using a numerical simulation. Four large geothermal systems are known in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure source locations (“Sources A”, “B” and “C” from shallow to the deep) were determined by geodetic data during the 1990-95 eruption. Source C is located at about 8 km deep at WUHTB, and is considered to be a magma reservoir. We attempted to explain the existence mechanism of the four geothermal systems from the large-scale structures (the topography of the Shimabara Peninsula and Unzen Graben) and the various heat sources. We first set a heat source around Source C and changed its position and size. This numerical model produced the upflow zones at the Obama and Shimabara hot springs and WUHTB; however the Unzen fumarolic field became a recharge area. This result indicated that it would be difficult to develop the Unzen fumarolic field only by Source C ; therefore, we set another heat source just beneath the fumarolic field. Consequently, two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. Especially, the heat source beneath the Unzen fumarolic field is essential to generate heat discharges at the fumarolic field.