11:45 〜 12:00
[HDS09-09] Rock avalanches controlled by a thrust fault and river incision in an accretionary complex of the Shimanto Belt
キーワード：rock avalanche, thrust, river incision, accretionary complex, gravitational slope deformation, rain
The purpose of this study was to clarify the geological and geomorphological background of rain-induced rock avalanche occurred in the Shimanto accretionary complex. We performed the detailed geological survey for the area of 5.4 km2 in the middle of the Kii Mountains in Japan. In this area, the two large rock avalanches (Akatani and Akatani-east, both have the volume over 106 m3) induced by the heavy rain of Typhoon Talas in 2011. Using the 1 m DEMs made before and after the landslides, we performed the detailed geological survey and geomorphological analysis of deep-seated gravitational slope deformation. We investigated the distribution of the thrust faults with the incohesive brittle facture zone in the mountain slopes and the inner structures of these thrusts. And, we performed the mineralogical analysis, permeability test and direct shear test to know the material features of the gouge. Around the Akatani landslide we investigated the faults distribution in the slope by boring data.
As a result, we found out that the two large rock avalanches have the main sliding surfaces corresponding to the same thrust (we named the Kawarabi thrust) with the clayey brittle fracture zone with the maximum width of 6 m. Two landslides slid under the condition of wedge failure consisting of the Kawarabi thrust and some high-angle faults. It is estimated that the gravitational deformations started at the timing when the depth of the Kawarabi thrust under the slope toe reached the threshold value. The Kawarabi thrust has the clayey crushed materials to the surrounding rock mass, and form the discontinuity of ground water, and prevent the ground water flows, and builds up the pore water pressure in the heavy rain.
As a result, we found out that the two large rock avalanches have the main sliding surfaces corresponding to the same thrust (we named the Kawarabi thrust) with the clayey brittle fracture zone with the maximum width of 6 m. Two landslides slid under the condition of wedge failure consisting of the Kawarabi thrust and some high-angle faults. It is estimated that the gravitational deformations started at the timing when the depth of the Kawarabi thrust under the slope toe reached the threshold value. The Kawarabi thrust has the clayey crushed materials to the surrounding rock mass, and form the discontinuity of ground water, and prevent the ground water flows, and builds up the pore water pressure in the heavy rain.