Three-Dimensional Thermal Structure in the Tohoku District, Japan Estimated from the Activity Index

Abstract

Subsurface three-dimensional thermal structure in the Tohoku District, Japan was estimated from the shallow-depth hydrothermal data in order to evaluate exploitable potentials of the next-generation geothermal energy. One of the future targets of geothermal development in Japan is supercritical geothermal reservoirs which are expected immediately beneath the brittle-plastic transition (BPT). The temperature profile of the well WD-1a in Kakkonda encountered a temperature inflection point at 380 ˚C and 3100 m depth. The temperature inflection point at 380 ˚C is considered to denote the BPT on granitic crust in contraction tectonic fields. Strong correlation between the 380 ˚C isothermal depth and the bottom of seismicity clouds is demonstrated in this study. This result indicates reliability of the subsurface three-dimensional thermal structure in the Tohoku District, Japan providing an important indicator for the accessible next-generation geothermal energy beneath the BPT.