Abstract
Shallow landslides densely occurred on tephra-covered slopes in the northeastern part of Aso caldera during the July 2012 northern Kyushu heavy rainfall. We estimated a thickness distribution of tephra deposits above the potential rupture surface of shallow landslide using spline interpolations of an isopach map, which was developed based on previous studies on the eruptive history of Aso Volcano, and compared it with the actual thickness measured at 11 sites. The results showed that ratios of the measured thickness to the estimated one (named residual ratio, Rr) ranged from 0.12 to 0.31, and had decreasing trends with increasing slope angle and curvature. Infinite slope stability analysis based on these characteristics captured the locations of approx. 84％ of the landslides induced by the July 2012 rainfall event (total 309 head scarps) at its maximum, demonstrating the accuracy of Factor of safety (Fs) calculated by this analytical method. On the other hand, the total area of landslide-prone sites, where the Fs was estimated to be lower than10.0, accounted for approx. 67％ of the analyzed area, which was higher than the area ratio of landslides in a single rainfall event (approx. 4-15％).
