Tohoku-Pacific Ocean Earthquake occurred in 2011 and caused many damages of various structures. At the dam at the distance of 266 km from epicenter, the upstream slope was deformed, and the effect on the safety was discussed. The peculiar unevenness of ripraps on the slope was observed. It was discussed whether the ripraps moved by the deformation of slope failure or by the dynamic behavior of each rock by seismic motion. It was, however, difficult to infer the true reason of unevenness of ripraps. In this study, the mechanism of displacement of ripraps was tried to examine by using DDA (Discontinuous Deformation Analysis) and NMM (Numerical Manifold Method). As a result, it was inferred that the ripraps at the lower part of the slope were moved by the inertial force by seismic wave, and that the ones at the upper part were moved by the failure of slope under the ripraps.

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