| 講演要旨(和文) | | 地熱地帯における反射法弾性波探査の有効性は複雑な地質，急峻な地形や弾性波エネルギー伝播等の問題により疑問視する傾向が強い。しかし，秋ノ宮地熱地域で実施の反射法弾性波探査は十分解釈出来る結果を得ている。発振間隔が20m，受振間隔が10m，CDP間隔が5mである空間的に高分解能な測定や大型バイブレーター3台を導入したことが功を奏している。断裂からの反射波は地熱地帯の断裂モデルに合致した特徴を示し，断裂形成と共に外部から熱水流入，断裂面に沿って薄板状に熱水変質が起き岩石均質化し，その結果反射波は反射強度の低下，均質化している。反射法弾性波探査データ解釈結果は東西圧縮場の地質構造形成モデルで説明出来る。深部固結マグマを切りほぼ地表に至る高傾斜角の南西−北東走向左横ずれ断層は浅部にフラワー構造を形成しこれらが一つの断裂帯を形成している。調査地域では６つの断裂帯が確認できる。 |
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| | 講演要旨(英文) | | 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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