This review paper provides an overview of the existing research papers on landslide slip surface with specific focus on the mechanism and shear strength. For the slip surface mechanism, the author emphasizes the following elements. (1) Geological differences in source rocks in reactivating landslides, (2) Characteristics of earthquake-induced landslides such as those in the 2004 Niigata Chuetsu Earthquake or those in the 2008 Iwate-Miyagi Nairiku Earthquake, (3) Mechanism for which smectite clays coat the slip surface in the mineral-rich landslides, (4) Mechanism of positive and negative pore water pressure, (5) Mechanism of water pressure transmitted along the surface for providing uplift pressure. For the slip surface shear strength, the author focuses on the following elements. (1) Role of clay minerals affecting the values of residual strength, (2) Characteristics of two different methods of measuring residual strength values (Ring shear test and multi-reversal direct shear test) , (3) Data set compiled from the previously reported shear strength tests, (4) Characteristics of earthquake-triggered-rapid landslides, (5) Influence of rate dependent shear strength test. At the end, the author addresses the emerging issues and potential research topics including; (1) detailed slip surface mechanism in the geological areas of metamorphic rocks and the Mesozoic to Paleozoic layer, (2) needs for developing a new testing method such as dynamic testing involving slip surface as shear surface, (3) influence of temperature dependency of shear strength values in snowy region.
One ancient large-scale landslide, as well as recent small-scale and ancient middle-scale landslides in the Ikutahara-Minami landslide prevention area can be classified as hydrothermal alteration zone landslides (HAZLs) based on their bedrock geology, and as hydrothermally altered rockslide, and as debris or earth slide, respectively, based on the landslide body. The large-scale HAZL occurred on the hydrothermally argillized zone composed of the smectite-analcite subzone of early-stage regional hydrothermal alteration and the illite-interstratified illite/smectite mineral subzone of early-stage neutral-type vein-related alteration in the Kitano-oh fossil hydrothermal system. The middle-and small-scale HAZLs occurred on the smectite-clinoptilolite subzone of the early-stage regional hydrothermal alteration and the K-feldspar, illite, and interstratified illite/smectite mineral zones of the early-stage neutral-type vein-related alteration. The bedrock of the K-feldspar and illite zones, in particular, contains characteristic swelling clay minerals such as interstratified illite/smectite minerals and smectite. The HAZL occurrence, therefore, is closely related to the swelling clay minerals-bearing hydrothermal alteration zones, regardless of the size of the landslides.