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A considerable number of rock bodies with varying percentages of supercritical fluid exist around the brittle–ductile transition (BDT) zone at a depth of several kilometers from the surface of the Earth, in northeastern Japan. As the BDT zone in the granitic basement of the continental crust is estimated to occur at about 380 °C, the identification of the depth corresponding to 380 °C is important to utilize the thermal energy inside the “supercritical geothermal systems”. In this study, we focused on an estimation method to determine the depth of the isothermal layer corresponding to 380 °C, using the activity index (AI) obtained from the maximum-temperature data of the geothermal wells and hot springs. The thermal profiles of deep and hot exploration boreholes and the hypocentral distribution of natural earthquakes were used to evaluate the characteristics and accuracy of the deep thermal structure, using the activity index. The estimated depth corresponding to 380 °C tended to be higher than the actual depth, with a maximum possible estimation error of approximately 9.7 km. Distribution maps showing the depth of the isothermal layer corresponding to 380 °C were created for six major geothermal fields in northeastern Japan, using the results from this study.

Validation and Evaluation of an Estimation Method for Deep Thermal Structures Using an Activity Index in Major Geothermal Fields in Northeastern Japan