Topographic features and stratified soil characteristics of a hillslope with fissures formed by the 2016 Kumamoto earthquake

Abstract

We examined topographic and stratified soil characteristics of a hillslope with fissures formed by the 2016 Kumamoto earthquake in Japan. Numerous fissures were developed on mountainous areas and concentrated along convex topography of ridgelines in addition to the occurrence of many landslides. Within a 6 × 20 m plot, depth of fissures ranged from 0.3 to 0.4 m near the ridgeline and from 0.6 to 0.8 m in the lower part of hillslope adjacent to a landslide of the same depth on the lower slope. Width of fissures ranged from 0.4 to 2.2 m (mean = 1.4 m; standard deviation = 0.6 m). Between fissures, soil blocks rotated downslope and remained on the slope. The soil matrix within the plot consisted of the four sequences of tephra with various thicknesses. Andisol layers with higher organic matter (16 ± 6%) were found between tephra deposits with lower organic matter (11 ± 2%), particularly at shallow depths (0.5 and 1.5 m). Soil bulk density was low in most andisol layers (0.55 ± 0.15 g/cm3), while mean bulk density of tephra deposits was slightly higher (0.66 ± 0.14 g/cm3). Bulk density was negatively correlated to organic matter content. The wet conditions (133 ± 37% gravimetric moisture) in the andisol layer (≈1.0 m depth), which corresponds to the landslide failure plane, are related to higher organic matter content and lower bulk density compared to the tephra layers situated above and below this layer. Differences in the depths of fissures along the ridgeline suggested that the development of fissures was associated with combinations of topographic factors and soil sequences. Findings from detailed topography and soil surveys provide insights into fissure formation processes and slope stability assessment.