Most of experimental studies on soil-structure interaction have been focused on the evaluation of impedance functions that represents the relationship between the dynamic response of rigid foundation and external forces. However, it is important to obtain the foundation input motions as well as the impedance functions in order to evaluate properly the seismic response of soil-structure system. This paper presents an approximate but practical method to evaluate the foundation input motions based on sparsely observed earthquake ground motions. The characteristics of the foundation input motions are also discussed. The comparison between the results obtained by the proposed method and the numerically calculated results shows good coincidence in lower frequencies. The method can be applied to practical purpose to evaluate observationally the foundation input motions for structures before construction.
