Structural Evaluation of Geothermal Fields in Japan

Abstract

Hot springs usually are situated in the volcanic belt of a convergent zone and are used as indicators of possible geothermal potential in an area. This paper discusses a study which examined the locations ·of hot springs (i. e., with temperatures > 90°C) in Japan, along with such other ~haracteristics as Na/K ratio, flot,.,· rate, and rate of volcanic activity in the area, to determine the influence that plate underthrusting exerts in determining the location's of these hot springs. The results indicate that bot springs in Japan are situated in clusters in the volcanic areas along the extension of zones which mark discontinuities in plate underthrusting. Geothermal fields that have been exploited to date are located in the vicinity of these clusters of hot springs. Heasured heat flow in many of these zones is higher than average for volcanic areas. Such zones, with higher than average crustal temperatures, may be favorable for the development of productive geothermal fields if local geological and hydrological conditions permit.