Abstract
It is impossible to distinguish slip surface clay from tectonic fault gouge based on only geological observation of surface of a drilled core. Then, half cut drilled core specimens prepared based on the resin fixing method were used to observe the slip surface clay and fault gouges in high-quality drilled cores in the landslide located in the Chichibu Belt in the Shikoku Central Mountain Region. Borehole inclinometers have detected cumulative displacements in this landslide. The results revealed fine structures such as composite planar fabric and asymmetric structures in finely crushed parts around the slip surface. In parts presumed to be slip surface clay, normal-fault sense was confirmed by the fine structure. Whereas in parts outside the slip surface, reverse-fault sense was confirmed by the composite planar fabric. Normal-fault sense in slip surface clay and reverse-fault sense in the other parts is consistent with those senses expected from the mechanisms of formation of a slip surface and tectonic faults in accretionary prism. Therefore, observing the fine composite planar fabric of a half cut core to evaluate the shear sense will be an effective method to distinguish slip surface clay from a fault gouge, which was formerly difficult to do simply by visually examining a drilled core surface.
