Abstract
Based on geological field survey on granitoids distributed in the Chubu Ryoke Belt, orientation and occurrence of macroscopic (outcrop scale) fractures were analyzed to characterize the anisotropic fracture network system in the study area. Variation of P-wave velocity was measured to identify preferred orientation of weak planes developed in the granitic rocks. In northeast part of the Inagawa granite, parallelepiped block structure composed from fractures with three preferred orientation was observed. The structure is inferred to be derived from tensile stress field in the granitic body. On the contrary, in southwest part of the Inagawa granite and the old Ryoke granitic rocks, complex sheared fractures were dominant. Predominant orientations of those macroscopic fractures were close to the directions of major and representative active fault systems of the Chubu Ryoke area such as the Median Tectonic Line (NE -SW strike) and the Atera fault (NW-SE strike).
P-wave velocity measurement revealed that anisotropy of the P-wave velocity was closely related to the distribution of parallelepiped fractures, however, shear fractures had no clear relation to variation of the P-wave velocity.
P-wave velocity measurement has an advantage on prediction of major orientations of parallelepiped joint system developed in granitic rocks, however in the case of highly stressed rock mass, better approximation on preferred orientations of fractures is likely to be achieved with consideration for occurrence of major faults and tectonic stress regime distributed in the area.
