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| | Abstract | | Seismic surveys have not generally been applied for geothermal explorations due to mainly their complex subsurface geology. In the Akinomiya geothermal area in the Akita prefecture in northern Japan, the high resolution seismic survey using vibroseis as seismic source was acquired and processed in 2000. Its seismic interpretation leads us to understand satisfactorily the assessment of fractures charcterized in the geothermal area and subsquently of the regional subsurface geology. Each fracture, generally thin with its width of 25 m at most, can be picked in the seismic section by its anomalous weaker seismic reflections than those of adjacent rocks. These weak seismic reflectivities are interpreted as associated with either thermally altered rocks by steam and/or hot water, or intrusive rocks, or both. Upper parts of these fractures seem often to be of closed nature, possibly due to swelling of clay minerals formed through thermal alteration. The series of left-lateral strike-slip faults with their dip of about 80 degrees, which originate from deep situated consolidated magma as deep as -3,000 masl to or near the surface, are interpreted as formed in the area under the active compressions of the Pacific Plate. Each strike-slip fault is inducing to build up corresponding flower structures in its shallow part, which formulate a zone of fractures. Six fracture zones are recognized in the seismic section with approximately 500 m apart. |
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