The paper presents the response of unsaturated sand to cyclic loading followed by water infiltration to study possible slope failures in earthquake areas. Measurement of shear modulus (Gmax) and Young’s modulus (E) are included and considered as the key parameters to assess the dynamic behavior of the soils. The cyclic triaxial tests and dynamic measurements were carried out on Edosaki sand, a silica sand taken from a natural slope in Chiba prefecture in Japan. The specimens were first consolidated in anisotropic condition with a principal stress ratio (K = 2), and then cyclically loaded under un-drained conditions. After cyclic loading, water infiltration was applied to the specimens until failure after different periods of waiting time. During the tests, the elastic wave velocities were continuously measured. Test results clearly show the variation of stiffness during cyclic loading and infiltration attributed to changes in relative density. Test results also confirm that there is a possible threshold value for the cyclic amplitude, depending on the relative density of the soil material, causing a rearrangement of the soil skeleton giving rise to higher failure resistance during water infiltration after reconsolidation. In short, it is very useful to monitor the stiffness and pore water pressure in a slope after an earthquake in areas with high and frequent rainfall.