Microstructure and origin of faults in siliceous mudstone at the Horonobe Underground Research Laboratory site, Japan

Ishii, Eiichi   

Origin and scaling of faults in a massive siliceous mudstone at the Horonobe Underground Research Laboratory site, Japan, were studied by outcrop observations to estimate approximate sizes of the faults from borehole data and to construct a fracture-flow model in and around the Ventilation Shaft in studies related to radioactive waste disposal. Very thin shear or compacted shear bands with weak internal foliations were observed near and along the faults and at and beyond the fault tips. This suggests that the faults originated as such thin bands nucleated by ductile deformation were immediately overprinted by slip surfaces. Such descriptions of faults in mudstone are comparatively rare. Furthermore, good scaling between fault rock thickness and displacement was observed for fault rock thickness less than 8 cm. Although the scaling for fault rock thickness larger than 8 cm was not observed, the scaling of thicker faults was estimated using the cumulative distribution of displacements calculated from fault rock thickness in drill core. A fracture-flow model in and around the shaft was constructed by interpolation/extrapolation from nearby boreholes based on the developed scaling method in this study. This model includes hydraulic pressure responses observed in nearby boreholes associating with inflow into the shaft.