Here, we investigated the topographic characteristics of slopes where a rainfall-induced landslide had occurred after the Iwate-Miyagi Nairiku Earthquake on June 14, 2008. We compared shaded relief maps, contour maps, and cross sections from LiDAR data measured immediately and 3 months after the earthquake. The landslides were caused by rainfall less than those observed in usual rainfall-induced landslide occurrences. From a comparison of the maps, we observed that prior to landslide occurrence, oval or horseshoe-shaped gentle slopes had formed on the slope after the earthquake and cracks had appeared on these slopes. Similar micro-topographies were observed and checked by field survey on neighborhood slopes without landslide occurrences. From the results of a dynamic cone penetrometer and soil cross section, we determined that the weathering layer of the base rocks (Nc value of 10-30) was relatively thick on the slopes where the cracks had appeared. Further, it became clear that a weak layer (Nc value below 5) had formed in the weathering layer or near the border with the lower hard layer (Nc value over 40). Therefore, we consider that the weak layer became a slip plane and subsequently, landslide occurred despite lesser rainfall than that normally observed after an earthquake.
A unique upheaval occurred with a primary landslide in Horomoe, Hokkaido. The upheaval mass occurred at the toe of the landslide was rare in terms of its dissymmetric box fold having both hinges in the front and the rear of the flat crest and the left echelon fold developed inside it. Through the analysis of geological structure and the use of Balanced Cross Section Method, this paper reveals 3 points : 1) Upheaval formation mechanism, 2) Echelon fold formation mechanism and 3) Relationship between landslide moving direction and fold. This upheaval mass is a nontectonic fault-propagation fold associated with its flat-lamp structure formed in parallel strata at a dip slope. Since the flat-lamp boundary line becomes deeper toward the south side in a staircase pattern on the same horizon, the echelon fold is formed with axes parallel to strike of the bedding plane. The structure of these folds decides the landslide moving direction.
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