Numerical model of the hydrothermal system beneath Unzen Volcano, Japan

Our study also aimed to use a numerical simulation to construct a comprehensive hydrothermal model beneath Unzen Volcano. There are four large geothermal systems in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure sources (“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 body. We attempted to explain the existence 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 indicates that another heat source is required to explain the Unzen fumarolic field and that two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula.