The prediction of strong ground motion with high resolution is a challenging task. In this article, the authors propose a multiscale analysis based on a singular perturbation and a new finite-element method with a hybrid of structured and unstructured elements, for a full 3D numerical simulation of earthquake wave propagation from fault to surface, including soft surface layers. The multiscale analysis refines a solution of lower resolution by considering effects of ground structures on wave propagation, and the hybrid grid radically reduces the amount of numerical computation. Several numerical experiments are carried out to show the validity and usefulness of the finite-element method with the hybrid grids, by comparing the results with those obtained using existing methods. An exemplary problem of wave propagation is solved using the proposed method. The potential usefulness of the proposed method is discussed, with particular attention paid to the accuracy of computing strong ground motion with higher spatial resolution.