抄録
Hachijojima, a cocoon-shaped island located along the volcanic front of the Izu-Ogasawara arc, is composed of two volcanoes, Nishiyama and Higashiyama. Although there is no geothermal signature on the surface or in the shallow subsurface, systematic exploration has confirmed a high potential for vapor flow and geothermal resources in the southern Higashiyama area. The high-temperature and high-pressure geothermal resource develop in vertical fractures of upheaval zones within the Tertiary stystem, which is overlain by Quaternary formations. The project site was selected above the dome-like cap rock of the upheaval zones. The cap rock was produced by the self-sealing alteration of the Tertiary rocks by hydrothermal fluids ascending along the vertical fractures. It is likely that gerthermal fluid has formed as a result of the mixing of meteoric water with seawater at ratios of 1:1 to 1:2, and with a small addition of volcanic gas. Tokyo Electric Power Co.(TEPCO) is now constructing a 3, 300 kW geothermal power plant, which is scheduled to begin power production in March 1999.
