2:30 PM - 2:45 PM
[HDS10-04] Estimation of subsurface structure in the linear depression spreading over the landslide crown
Keywords:Gravitational slope defomation, Accretionary complex, Surface wave exploration
In recent years, deep-seated catastrophic landslides caused by heavy rainfall have occurred one after another in Japan, and there is a need to clarify the relationship between landslides and depressions in slopes, which are known as precursors to collapses, in order to mitigate the damage. In the 2011 Kii Peninsula heavy rainfall disaster, linear depressions and small mountain- and valley-facing scarps remained characteristic of the upper region of the landslide, which can be closely related to the mechanism of landslide occurrence (Chigira et al. 2013; Arai and Chigira 2019). According to Yokoyama (2019), the subsurface structure within these linear depressions is prone to storing water, suggesting that a rise in the groundwater level within the depressions may affect landslide occurrence. However, there are very few studies that have empirically clarified the subsurface structure in the depressions (Nishii and Ikeda, 2013). In this study, we conducted a geological survey and a surface-wave exploration inside and outside the linear depressions in order to clarify the subsurface structure in the linear depressions adjacent to the landslide, which is considered to have a significant influence on the mechanism of deep-seated landslide.
The study area was the uppermost stream of the Kogouchi River along Kangyou-mine in northern Shizuoka Prefecture. This area is locally known as "Goshiki-gare", where green, red, and weathered brown rocks are conspicuous, and there spreading over clear linear depressions at the head of the landslide.
As a result of ortho-imaging from aerial photographs taken by UAV, the size of the leaf-shaped landslide was 200 m in length and 150 m in width. The geological survey revealed that the area in and around Goshiki-gare is dominated by greenstone with basaltic hyaloclastite, and red mudstone is partially incorporated. Blocked greenstone was present at the top of the landslide. In the middle to upper part of the area, fractured and clayed greenstone and red mudstone were observed. At the end of the slope in the sub-catchment including the landslide, slate toppled outcrops with a north-south orientation of 60° and eastward trending were observed, suggesting gravitational deformation. We also found a 20-30 m wide greenstone fracture zone on the landslide 150 m west of Goshiki-gare. This is considered to be a fault outcrop of the Sasayama Tectonic Line.
As a result of the surface wave exploration of two lines crossing the depression and one longitudinal line, the S-wave velocity was less than 160 m/s from the ground surface to a depth of about 3 m, 200-300 m/s at a depth of about 3-6 m, and 400 m/s at a depth of about 7 m or more. Comparing the results with the geological cross-section, it is inferred that they correspond to soil layers, weathered bedrock, and basement rock, respectively. The depth at which the S-wave velocity is 160 m/s for the concave and convex portions of the transverse measurement line was compared. As a result, the depth of the convex part was 1.6 m while that of the concave part was 2.8 m, indicating that the concave part was deeper. This trend was the same for the two measurement lines in the transverse direction. The depths at which the S-wave velocities in the concave longitudinal survey line were 160 m/s and 300 m/s were 1.4-2.5 m and 3.3-6.4 m, respectively, indicating that the velocities varied even in the concave area.
References: Chigira et al. (2013): Geomorphology, 201(1), 479-493. Arai and Chigira (2019): Engineering Geology, 260(3), 105236, Yokoyama (2019): Geomorphology, 352(1), 106977. Nishii and Ikeda (2013): J. of Geography, 122(4), 755-767.
The study area was the uppermost stream of the Kogouchi River along Kangyou-mine in northern Shizuoka Prefecture. This area is locally known as "Goshiki-gare", where green, red, and weathered brown rocks are conspicuous, and there spreading over clear linear depressions at the head of the landslide.
As a result of ortho-imaging from aerial photographs taken by UAV, the size of the leaf-shaped landslide was 200 m in length and 150 m in width. The geological survey revealed that the area in and around Goshiki-gare is dominated by greenstone with basaltic hyaloclastite, and red mudstone is partially incorporated. Blocked greenstone was present at the top of the landslide. In the middle to upper part of the area, fractured and clayed greenstone and red mudstone were observed. At the end of the slope in the sub-catchment including the landslide, slate toppled outcrops with a north-south orientation of 60° and eastward trending were observed, suggesting gravitational deformation. We also found a 20-30 m wide greenstone fracture zone on the landslide 150 m west of Goshiki-gare. This is considered to be a fault outcrop of the Sasayama Tectonic Line.
As a result of the surface wave exploration of two lines crossing the depression and one longitudinal line, the S-wave velocity was less than 160 m/s from the ground surface to a depth of about 3 m, 200-300 m/s at a depth of about 3-6 m, and 400 m/s at a depth of about 7 m or more. Comparing the results with the geological cross-section, it is inferred that they correspond to soil layers, weathered bedrock, and basement rock, respectively. The depth at which the S-wave velocity is 160 m/s for the concave and convex portions of the transverse measurement line was compared. As a result, the depth of the convex part was 1.6 m while that of the concave part was 2.8 m, indicating that the concave part was deeper. This trend was the same for the two measurement lines in the transverse direction. The depths at which the S-wave velocities in the concave longitudinal survey line were 160 m/s and 300 m/s were 1.4-2.5 m and 3.3-6.4 m, respectively, indicating that the velocities varied even in the concave area.
References: Chigira et al. (2013): Geomorphology, 201(1), 479-493. Arai and Chigira (2019): Engineering Geology, 260(3), 105236, Yokoyama (2019): Geomorphology, 352(1), 106977. Nishii and Ikeda (2013): J. of Geography, 122(4), 755-767.