Descriptions of meso- and microscopic structures of fault zone rocks obtained from tunnel penetrated across the Mozumi-Sukenobe fault, central Japan

Tanaka, Hidemi*; Ito, Tanio*; Nohara, Tsuyoshi   ; Ando, Masataka*

The Mozumi-Sukenobe fault, running ENE-WSW and dipping almost vertically, is one of the Atotsugawa right-lateral active faults in central Japan. A 500 m long horizontal tunnel was excavated across the Mozumi-Sukenobe fault, utilizing the pre-existing tunnel of the Kamioka Mine for geological and geophysical integrated research of the fault zones. In order to clarify the fault zone architecture and rock distribution, we have investigated the distribution across the tunnel and obtained samples directly from the tunnel. The following results were obtained by meso-and microscopic observations and mineralogical examination of fault rocks; (1) Two fracture zones (referred to as zones A and B) are distinguished. They are 15 and 50 m in thickness, respectively, composed of foliated fault breccia in a damaged zone and foliated fault gouge in the fault cores. The core-zones contain an 8 cm slip layer for zone A and several thin (10 cm) slip layers for zone B. (2) Most of the fault rocks obtained from the fault cores of the two fracture zones show a distinct foliated texture, which is clearly a product of cataclastic flow deformation. Very thin, ultra-fine grained slip layers, generated by seismic rapid slip, are also detected from fault core of zones A and B. The cross-cutting relationship between foliated and rapid slip textures indicate a repeated process of slow and rapid slips in zones A and B. (3) The results of mineral assemblage analysis show that these fault rocks are dominated by smectite clay minerals as well as by mica minerals and chlorite, all of which could be potential candidates for the main reason of the stable slip. Combining all these data, we conclude that the Mozumi-Sukenobe fault is activated by extremely slow frictional-viscous creep which releases crustal strain energy. However, stress would also be released by earthquakes based on the existence of earthquake slip surfaces in the fault cores of the Mozumi-Sukenobe fault.