Because landslides with long-travel distance (so-called long-travelling landslides) can cause severe damages, it is important for disaster-prevention works to find slopes prone to long-travelling landslide in advance. In this study, we collected 77 landslides induced by snowmelt, and conducted topographic analyses of source and transfer-deposition areas of the landslides to find features correlated with occurrence of long-travelling landslides. The topographic analyses using 10-m DEM revealed that long-travelling landslides, which traveled longer than their slope lengths, tended to occur at the slopes with following three features : 1) preexisting landslide slope (reactivation), 2) coupling with steep valley topography (more than 0.15m m-1 or 9 degrees), and 3) sharp inflow angle (less than 70 degrees) of sliding mass to the valley topography. Then, simple discriminant test using the features was developed. By using all the three features, 12cases of 14 long-travelling landslides (86％ of positive predictive value) and 58 cases of 63 normal landslides (92％ of negative predictive value) were correctly distinguished, and correctness of the test attained 91％, suggesting the above three topographic features were effective to find slopes prone to long-travelling landslide.
Heavy rainfall associated with the typhoon T1326 (Wipha) of 2013 triggered large-scale shallow landslides and long-runout of the displaced mass at outer slopes of the caldera rim, resulting severe disaster at Motomachi district, Izu Oshima Island. To prevent such disasters in volcano regions, thickness distributions of pyroclastic fall layers should be known as potential source of landslide materials. Thickness distribution of alternated tephra and loess layers on slopes were estimated based on data from the study on eruptive history of the Izu Oshima Volcano (Koyama and Hayakawa, 1996) using a kriging method, and compared with actual values measured in field. Results showed that the measured tephra thickness and accumulated thickness varied from 0.2 to 2.2 times and from 0.8 to 1.9 times of the estimation, despite the similarity in fall out amount and stratigraphy of tephra (within the Okane-zawa Catchment : 1.2km2)
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