| Abstract |
Hydrothermal veins and minor faults associated with geothermal activities in the Kakkonda and Oyasu geothermal fields, northeastern Honshu, Japan, are investigated to clarify the developmental process of fractures and the relationship between fracture systems and path of geothermal water. Many of hydrothermal veins are characterized by the combined structure of shear fractures and tensile fractures. That is jog structure. This structure is used to determine the sense of shear along the shear fracture and to reconstruct the associated stress field. In the Kakkonda geothermal field, these fractures are classified into four systems on the basis of their dominant orientation and sense of shear, which are NNW-SSE to NW-SE strike and moderate dip system (NW-moderate dip system), NE-SW to E-W strike and steep dip system (NE to E-W system), and horizontal system. The NE to E-W system consists of veins and faults with lateral and normal displacement , both of which were formed by the stress field with horizontal minimum principal axis trending NNE-SSW to ENE-WSW though fractures belonging to the moderate dip and NW-steep dip systems were also formed by lateral of normal fault type stress field. The present paths of geothermal water are controlled by the NE to e-W and horizontal systems. Fractures developed in the Oyasu geothermal field are also classified into four systems, which are NNW-SSE through N-S to NNE-SSW strike and steep dip system (N-S system), N-W strike and steep dip system (NW system), NNE-SSW through E-W to WNW=ESE strike and steep dip system (NE to E-W system), and horizontal system. Lateral slip type fractures are dominant in the former three systems. The NE to E-W system and horizontal system in this field are considered to play an important role on the present paths of geothermal water. The geothermal reservoir in both fields are characterized by lateral slip type fractures accompanied with horizontal fractures, though the regional stress field in northeastern Honshu is believed to be a reverse fault type. |