Interpretation of gravity data to delineate underground structure in the Beppu geothermal field, central Kyushu, Japan

Study region The area of the Beppu geothermal field is located in southeast Japan (33°15′ to 20′ N, 131°27′ to 32′ E). Study focus The Beppu geothermal field, which is one of the largest hot-spring resorts, is located in the eastern end of the Beppu-Shimabara Graben, in central Kyushu, Japan. Details of the subsurface structure are necessary for hydrothermal modeling and numerical simulation. This study aims to delineate the shallow underground fault and basement structure using gravity analysis. New hydrological insights for the region A high Bouguer anomaly is detected in the southern part of this area, which corresponds to the distribution of the Kankaiji andesite. The results of the edge-detection filter of gravity data indicate that the northern edge of the high Bouguer anomaly corresponds to the Asamigawa Fault in the southern part of the study area, but deviates 1 km southeast of the Kankaiji hot spring to the north. In this area of high value, which indicates the fault, three hot spring areas, Horita, Kankaiji and Hamawaki, are located. The distribution of the depths of the three-dimensional gravity basement enables the delineation of the interface between the volcanic basement rocks and the fan deposit. The conspicuous, steep slope of the gravity basement is detected along the Asamigawa Fault and the southern hot-spring area. However, the northern hot-spring area is distributed on the uplift of the gravity basement. The results of the gravity analysis indicate that the structure of the hot-spring water path differs between the southern and northern hot-spring areas.