Understanding the characteristics and long-term behavior of water-conducting fractures is important for the safety assessment of deep underground facilities in the plutons of an orogenic belt. This study describes the features of fractures and their fillings along with the long-term behavior of their hydrogeological structures in an underground environment based on the results of a geological investigation conducted at an underground facility in northern Kyushu. Fractures were classified into

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... ve groups on the basis of fracture orientation:A, B, C, D, and low-angle groups. The fractures in all groups are filled with epidote and quartz, which are formed at high temperature:from about 300 to 350 o C. Thus, the genesis of all fractures is the cooling process of granodiorite pluton. Almost all of the water-conducting fractures are included in the B group. Because a number of fracture fillings in the B group are filled by prehnite and crushed fragments of epidote and quartz, the fractures in this group alternated sealing and re-opening. The fracture characteristics in the B group are follow as:accompanying many alteration halos, long trace length, and cutting off other fractures. These results indicate that fractures in the B group have possibly functioned as pathways for groundwater flow in the long term.