An Examination of the Stability of an Earthquake-Induced Landslide and Landslide Dam

Abstract

The Masuzawa landslide was triggered by the Iwate–Miyagi Earthquake in 2008. The landslide ran almost 30 m and crushed the opposite side of the slope. The lower part of the sliding mass covered the riverbed and formed a landslide dam that was 23 m high. If the dam body eroded, the landslide could possibly become destabilized and move again. A heavy rainfall or large seismic aftershocks could make the landslide unstable.Two important points must be considered regarding the stability problem. One problem is how to estimate the deformation process of the dam, and the other is how much seismic force the landslide might experience during a possible aftershock. Three numerical methods were used for these estimates: a one-dimensional riverbed calculation model for the dam-erosion process, a three-dimensional finite difference method (FLAC®) for the estimation of seismic forces acting on the slide, and a Newmark method to estimate the sliding distance.Following the two-stage seismic design used in Japan, level-1 and level-2 earthquakes were tested. The landslide was stable during a level-1 earthquake, which has a typical magnitude of shaking. Also, in the future, the landslide might become unstable during a level-2 earthquake, which represents the strongest possible shake. The Newmark method was used to estimate the sliding distance during the level-2 quake. The sliding distance was calculated as 0.1 m, which was too small to trigger a big slide.