Issues of predominant period of ground motion and derived underground velocity structure model were investigated in the coastal plains affected by the soft sedimentary layer after the last ice age. Specifically, it is found that two predominant periods due to the shallow soft sediments and deep sedimentary layers over the seismic bedrock created by the tectonic movement after the quaternary period are close in a small plain such as the Tottori Plain, Japan as an example. It was shown by the analysis of underground velocity structure derived from H/V spectrum ratio of earthquake ground motions with the diffuse wave field theory. It is feared that the interaction of close predominant periods due to the different layer boundaries with high contrast may amplify the seismic motion in the period range that affects building structures in the small plains in coastal area.
