高知県谷の内地すべりにおける地形・地質構造発達過程と地下水流動構造  [in Japanese] Geomorphological and Geological Evolution Processes and their Relation to the Underground Water Flow in Tanino-uchi Landslide, Southwest Japan  [in Japanese]

Geomorphic and geological evolution processes and their relation to the distribution of groundwater flow were investigated in the Tanino-uchi Landslide in southwest Japan. The landslide consists of the entire landslide block moving toward southeast and the lower landslide block moving eastward. The lower landslide block is divided into a, b, c, and d blocks. Three geological units originating from the accretionary complex were formed as the bed rock of the area, namely 1) mudstone dominant layers with a thin basalt volcaniclastic bed, 2) alternative layers of sandstone and mudstone, and 3) massive chert and basalt volcaniclastic rocks. A rockslide occurred along the slip surface in the thin basalt volcaniclastic bed within the present entire landslide block area about 19,000 years ago. In the incident, the sliding mass climbed towards southeast along the slip surface of north dipping which was formed at the toe of rockslide due to the southern wing of synclinal structure. As a result, terrace-like topography was also formed in a and b blocks. Subsequently the lower landslide block started to move eastward following the deposition pattern of clayey volcaniclastic rock taking a ship bottom feature in the south part of the entire landslide block. To identify the distribution of groundwater flow on the landslide slope, we investigated the rise and fall characteristics of groundwater levels after rainfall using boreholes, as well as the distribution of groundwater drainage effect by well construction. Consequently, two flow routes were found. One is the southeastward flow from the upper part of the entire landslide block. It travels above the impermeable volcaniclastic bed and caused gradual groundwater level changes. Another is the eastward flow from the head part of the lower landslide block causing the prompt groundwater level changes. The confluence of these flows and the terrace-like topography induce groundwater concentration at a and b blocks. As above, present groundwater flow is strongly controlled by geomorphic and geological formative processes. Therefore understanding of geomorphological and geological formative processes is important for effective countermeasure planning.