16:00 〜 18:00
[HDS07-P01] Landslide induced by weathering controlled by geologic structure: an example from the eastern part of Hakkaido, northern Japan
キーワード：地すべり、内部摩擦係数、変形バンド、風化、スメクタイト、粘土鉱物による透水性の減少
Landslide is obviously caused in rocks where the rock strength has been significantly reduced due to some reasons. In particular, since the mechanism of landslide is frictional sliding, mechanically anisotropic and weak plane with a low coefficient of friction such as fault, joint and bedding plane in geological bodies plays an important role in the generation of landslide (e.g. Chigira, et al., 2013; Kojima et al., 2015). Further, it has been also found that the failure surface of landslide is often coated by clay minerals, in particular, smectite with a low coefficient of internal friction (c. 0.1), which has been proposed to be the most important cause of landslide (e.g. Torii, et al., 2006). We have recently found that the formation of clay minerals by weathering is favored in those anisotropic planes, and thus the cites of weathering are controlled by geologic structures, which could trigger landslide.
We studied a small-scale landslide (c. 50 m wide, and c. 60 m long) which developed in the Middle Eocene Urahoro Group, Shiranuka-town, eastern Hokkaido. We found that deformation bands (e.g. Fossen et al., 2007) less than 1 mm wide pervasively developed in bedrocks consisting of sandstones beneath the failure surface of the small-scale landslide. We further found that the landslide developed in the axial part of a regional scale NNE-SSW trending flexure. We analyzed microstructures of deformation bands using the image analysis software (ImageJ), and found that detrital grains in sandstones are fractured into the sizes ranging between one half to one fifth of the original size (i.e. cataclasic bands). Also, based on XRD analyses, we found that authigenic clay minerals such as smectite and kaolinite increase from little deformed sandstones, protolith (5 wt.%, one sample) through those with deformation bands (13-27 wt.%, three samples) to failure surface (23-34 wt.%, four samples). It is clearly observed using SEM-BSE that these authigenic clay minerals formed in pore spaces created by fracturing of detrital grains. In particular, the growth of smectite in pore spaces caused by cleavage fracturing in biotite detrital grains is notable. Furthermore, detrital biotite grains, which are aligned along deformation bands (i.e. phyllosilicate bands), are altered to vermiculite, and further to kaolinite by weathering.
It has been recently reported that conjugate deformation bands with a thrust sense of displacement formed in the axial part of flexure similar to the occurrence of the present landslide, where the compressional stress is concentrated (Ballas et al., 2014; Braathen et al., 2020). In the present landslide, the failure surface, which is parallel to the bedding plane and accompanied by mudstones and coal layers other than sandstone, dips ESE at c. 15 degrees. Therefore, based on a traditional idea, it can be interpreted that the dip slope structure is simply responsible for the occurrence of landslide. However, as clarified in the present study, in addition to the dip slope structure, the occurrence of numerous deformation bands, which became the preferred site for growth of authigenic clay minerals with very low friction by weathering, could be the most important cause for the commencement of landslide. In the presentation, we further stress another role of clay minerals, which can greatly reduce the permeability beneath the failure surface, thus leading to the increase in pore-fluid pressure to assist landslide.
We studied a small-scale landslide (c. 50 m wide, and c. 60 m long) which developed in the Middle Eocene Urahoro Group, Shiranuka-town, eastern Hokkaido. We found that deformation bands (e.g. Fossen et al., 2007) less than 1 mm wide pervasively developed in bedrocks consisting of sandstones beneath the failure surface of the small-scale landslide. We further found that the landslide developed in the axial part of a regional scale NNE-SSW trending flexure. We analyzed microstructures of deformation bands using the image analysis software (ImageJ), and found that detrital grains in sandstones are fractured into the sizes ranging between one half to one fifth of the original size (i.e. cataclasic bands). Also, based on XRD analyses, we found that authigenic clay minerals such as smectite and kaolinite increase from little deformed sandstones, protolith (5 wt.%, one sample) through those with deformation bands (13-27 wt.%, three samples) to failure surface (23-34 wt.%, four samples). It is clearly observed using SEM-BSE that these authigenic clay minerals formed in pore spaces created by fracturing of detrital grains. In particular, the growth of smectite in pore spaces caused by cleavage fracturing in biotite detrital grains is notable. Furthermore, detrital biotite grains, which are aligned along deformation bands (i.e. phyllosilicate bands), are altered to vermiculite, and further to kaolinite by weathering.
It has been recently reported that conjugate deformation bands with a thrust sense of displacement formed in the axial part of flexure similar to the occurrence of the present landslide, where the compressional stress is concentrated (Ballas et al., 2014; Braathen et al., 2020). In the present landslide, the failure surface, which is parallel to the bedding plane and accompanied by mudstones and coal layers other than sandstone, dips ESE at c. 15 degrees. Therefore, based on a traditional idea, it can be interpreted that the dip slope structure is simply responsible for the occurrence of landslide. However, as clarified in the present study, in addition to the dip slope structure, the occurrence of numerous deformation bands, which became the preferred site for growth of authigenic clay minerals with very low friction by weathering, could be the most important cause for the commencement of landslide. In the presentation, we further stress another role of clay minerals, which can greatly reduce the permeability beneath the failure surface, thus leading to the increase in pore-fluid pressure to assist landslide.