Abstract The effects of amplitude and phase difference of an earthquake motion on the seismic response of long earth dams were studied. The dam was modeled as a two-dimensional shear beam whereas the seismic input, applied at the base of the dam, assumed various forms to reflect the variation of amplitude and phase of the ground motion. A computer program, based on the finite element method, was developed to compute the dam response to each of the assumed base motions. It was noted that the length-to-height ratio of the dam has a marked effect on the dynamic characteristics of the dam and on its seismic response. It was also found that the dam response can be sensitive to the assumed spatial variation of ground motion along its base.