Although the effective rainfall method has been widely used for analyzing variations in the groundwater level (GL) in a landslide site, few previous studies have applied the method to snow-cover season. This study estimated the snowmelt water without using large-scale observation facilities and calculated the meltwater and/or rainwater (MR), which can affect the GL variation. Then, the half-life of the effective MR (EMR), delay days, and coefficients of a regression line were optimized to reproduce the GL variation the best. The results showed that EMR is a significant explanatory variable of the observed GL, suggesting that the method used in this study is very effective to analyze the GL variation in a landslide site in snow-cover season, where large-scale observation facilities are difficult to install. Properties of the GL response were different between two periods probably due to the effects of pore air, snow distribution property in the last stage of snowmelt period, horizontal drainage boring, and evapotranspiration. Individual analyses for the two periods improved the accuracy of the GL reproducibility. The parameter values reported in previous studies and this study showed no clear relationship with landslide properties such as geology and the depths of GL variation, suggesting that more cases are needed to predict parameter values from landslide properties.
The Hokuyo A-160 Slide is one ancient greenstone landslide occurred at the Hokuyo-Toyomi landslide-prone area in Kitami City, northeastern Hokkaido, Japan, and is a large scale having approximately 250 m in width, approximately 300 m in length, and approximately 30 m in thickness. The slide is a weathered and hydrothermally altered rockslide, and the slide body consists primarily of weathered and hydrothermally altered rocks of accretionary greenstone and small amounts of their debris and earth. The inferred slip surface clay at depth of 21.25 m in core of a borehole excavated in the middle part of the slide body is tan in color, has shear planes, and is primarily composed of smectite and vermiculite, with lesser albite, laumontite, and goethite. Because this tan thin clay layer rich in swelling clay minerals such as smectite and vermiculite is considered to be the hydrothermal alteration and weathering products of the fault gouge in the crush zone based on the mode of occurrence and properties, this suggests a close relationship between the slide and the hydrothermal alteration and weathering of the fault gouge and fault breccia in the crush zone of greenstone.