Relationship between hot springs and geothermal fields in Japan

Abstract

Hot springs usually are situated in the volcanic belt of a convergent zone and are utilized as indicators of geothermal potential in an area. Characteristics of hot springs in Japan have been examined in order to understand the relationship between hot springs and geothermal fields. The characteristics examined include locations of hot springs, their temperature, density of distribution, flow rate, heat flow in the vicinity, and rate of volcanic activity in the area during last 200 years. Geochemical thermometers have been used to estimate subsurface temperatures in the area. The object of the study was to determine subsurface conditions in the vicinity of existing geothermal fields from these characteristics of hot springs. This examination shows that geothermal fields are situated near clusters of three or more hot springs with temperatures in the subsurface ≥150°C in the vicinity of these clusters, and many hot springs have a higher than average flow rate. The study also shows that higher subsurface temperatures and heat flow in the vicinity of the clusters of hot springs are not the result of significant volcanic activity in the area. The examination also shows that clusters of hot springs in Japan are located along the extension of zones which mark discontinuities in plate underthrusting. Measured heat flow in many of these zones is higher than average for volcanic areas. Such zones, with higher than average crustal temperatures and permeability, may be favorable for the development of productive geothermal fields if local geological and hydrological conditions permit.