報告書番号

U97007

タイトル（和文）

１９９７年鹿児島県北西部地震における地震・被害の概要と斜面崩壊の特徴

タイトル（英文）

CHARACTERISTICS OF AND DAMAGES CAUSED BY 1997 KAGOSHIMAKEN-HOKUSEIBU EARTHQUAKE

概要 （図表や脚注は「報告書全文」に掲載しております）

１９９７年３月２６日に発生した鹿児島県北西部地震（Ｍ６．３）は，川内市，阿久根市，宮之城町で震度５強を記録し，斜面崩壊，液状化等により，道路・家屋の損傷，港湾岸壁の亀裂・陥没等の被害をもたらした。当所では地震被害の把握と地震動特性の解析を行うとともに，斜面崩壊に及ぼす地質的な影響を明らかにする目的で現地調査を行った。本震の震源深度は８ｋｍで，震源断層は左横ずれ断層と推定されるが，地表には，地震断層は出現しなかった。地震動特性は表層地質の影響を受け，厚い沖積層上の地震計は大きな加速度を記録した。道路・家屋の損傷，斜面崩壊等の被害は，震源直上の山地の他，特にシラス，マサ，沖積層等の未固結層分布地域で顕著であった。斜面崩壊は，余震と大雨が重なり，その被害を増し，砂岩・頁岩分布地域では，急勾配の東斜面で顕著で，トップリングが関与したと考えられる。阿久根港では，護岸構造によりその被害状況が異なった。

概要 （英文）

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