2011年紀伊半島大水害で発生した宇宮原深層崩壊の地質・地形学的背景について

Abstract

Typhoon Talas induced the deep-seated catastrophic landslides (DCL) in the Kii Mountains between 2 and 5 September 2011. The Uguhara landslide is one of the DCLs that occurred on the W-facing slope called Nigoridani with a first-order Kumano River. This area is underlain by the Cretaceous accretionary complex, consisting of mixed rock and broken formation. This landslide debris that rushed into the swollen Kumano River caused a hydraulic bore to propagate upstream and destroyed a hydropower station and a house 1 km upstream. This study investigated the geological and geomorphological causes of the Uguhara landslide by detailed geological mapping and GIS analysis using 1m DEMs before and after the landslide. As a result, the landslide had sliding surfaces with wedge-shaped discontinuities that consisted of NW-dipping faults with incohesive fault rocks and cataclasite, SW-dipping faults. Newly formed scarps observed above the main scarps suggest the occurrence of landslides by heavy rain or earthquakes in the near future.