The relationship betweeen geological structure and permeability is studied to investigate the movement of the underground water in the Hanawa granite body, distributed in the southwesten part of the Abukuma Plateau.
In the Hanawa granite body, many minor structures are developed, which are mylonitic structure, dykes of pegmatite and aplite, quartz vein, micro-crack and fractures (fault, joint). Among the structures mentioned above, the fractures are significant in the sence of the flow of the underground water, especially in a deeper part.
From the observation in the field, these fractures can be divided into six series clearly, namely A, B, C, D, E and F in the order of formational age, based on the relationships among the fractures. The A (A1 and A2), C (C1 and C2) and F (F1 and F2) series are composed of conjugated sets of faults. On the basis of the geometrical relationship of these conjugate sets of faults, the A series are deduced to be formed under the strike-slip fault type stress field with NW-SE compressional principal stress axis. And both of the C and F series are also deduced to be formed under the N-S tensional stress field.
Statistical analysis of the trends of the fractures and topographic lineaments clarified that the trend is concentrated in NW-SE to NNW-SSE, and it corresponds to the direction of A1, D and F1 type faults.
Taking acount of the geological history from the Cretaceous to the Tertiary age in the Tanakura Shear Zone and adjacent area, the A series were possibly activated during the left lateral movement stage (Cretaceous to Paleogene) of the Tanakura Shear Zone (Koshiya, 1986). And C and F series may be formed in the Miocene.
From the analysis of stress field and period of the activation of the faults, the low permeability in the A1, A2, D and F1 type faults is concluded, and high permeability in the C1, C2 and F2 type faults. The above conclusions is confirmed by the simulation model of FEM and the boring survey performed by the Tohoku Regional Agricultural Administration Office.