The aim of this paper is to assess numerically the recorded response of the Aratozawa dam to the Iwate-Miyagu Nairiku earthquake in June 14 of 2008. For this purpose, a 2D Finite Element model of the dam was considered. An elastoplastic multi-mechanism model was used to represent the soil behaviour. A mechanical-equivalent fully drained decoupled analysis is performed to simulate the behaviour of the core and the upstream rock-fill. In addition, a “half-space bedrock’s boundary condition” is used to simulate the borehole condition found at the gallery level. Finally, the obtained results show that the two conditions assumed to define the initial state of all materials could be used as a first approach to simulate the dam behaviour.