Subsurface Structure in Oita Hot Spring Area Inferred from Gravity Data, Eastern Kyushu, Japan

摘要

The sustainability of hot springs in Japan has not been thoroughly explored. Only a few geochemical and geophysical surveys were conducted to interpret the subsurface structure of hot spring sites because of financial restrictions set by hot spring owners. This results in the depletion or drawdown of the hot spring water. For sustainable utilization of hot springs, it is important to understand the hydrothermal systems governing the hot spring activity. The hot spring area in the Oita plain, eastern Kyushu, Japan, contains many non-volcanic low-temperature hot spring wells. Some of them were formed by fluid extracted in the subduction of the Philippine Sea Plate. Such hot springs are rare in Japan; therefore, their hydrothermal system is not well known. Details of the subsurface structure are necessary to understand the hydrothermal system and to construct a hydrothermal model. This study outlines the shallow underground fault system and basement structure of the Oita hot spring area. Gravity surveys were conducted to understand the relationship between the low-temperature hydrothermal systems and the geophysical data of the area. Using data of dense gravity surveys of the Oita plain, we obtained a detailed Bouguer anomaly map and modelled the subsurface structure. Inversion analysis was performed using three-dimensional gravity analysis. The location and dip of the faults were clearly identified and the subsurface model was obtained. Based on our results and those of previous studies, a hot springs conceptual model of the hydrothermal system of the Oita plain hot springs was constructed.