2004年10月23日に発生した新潟中越地震により，芋川中流部の東竹沢と寺野地区において地すべりが発生し，天然ダムが形成された。この二つの天然ダムに対して，決壊による二次災害を未然に防止するために，直ちに緊急対策が取られ，順次応急対策及び恒久対策が進められた。天然ダム堤体の内部構造を調査し，より信頼性の高い天然ダム安定性評価手法を開発するため，東竹沢天然ダムに対して，高精度表面波探査および微動アレイ調査を併用し，ダム堤体のS波速度構造を調べた．また，得られたダム堤体のS波速度構造から，地すべりの運動・堆積過程についても考察した。

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  On October 16, 2013, Typhoon No.26 (Typhoon Wipha) passed through Izu-Oshima Island, approximately 100km south of Tokyo. On this day, the Motomachi district, on the northwestern slope of Mt. Mihara, experienced its heaviest rainfall in recorded history. The 24-h precipitation reached 824mm, inducing many shallow landslides and mudflows, particularly in the Okane-zawa river basin. Recent studies have shown that most of the landslides occurred along a boundary between high-permeability layers consisting of scoriaceous ash and low-permeability layers consisting of loess deposits. Many shallow landslides also occurred in the Omiyasawa river basin, which lies towards the south of the Okane-zawa river basin. However, the reason for these landslides and subsequent processes, such as mudflows, had not previously been clarified. In this study, we estimated the areas of the landslides and erosion and deposition sites from altitude changes determined using digital elevation models (DEMs) developed before and after the typhoon. Through field surveys, we also found that the topographies and processes of the shallow landslides in the upper Omiya-sawa basin differed from those in the Okane-zawa basin. The landslides in the Omiya-sawa basin occurred along a boundary between scoriaceous ash layers and low-permeability layers consisting of lava and lahar deposits.

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