The seismic response of local sites is one of hot topics in the field of earthquake engineering. Currently, there have been many researches on the seismic effect of single basin site or local depression sites. However, studies about the basin-depression coupled sites are quite few. Actually, local lakes or deep foundation pits can be frequently found in the basins. So the studies of ground motion amplification characteristics of such a coupled terrain are of significance for earthquake prevention and disaster reduction in corresponding areas. In this paper, a numerical simulation method combining high-precision spectral element method and multiple transmission boundary is firstly introduced, which are used for the wave motion simulation of the interior nodes and boundary nodes, respectively. The accuracy of the method is validated by comparison with the analytical solution for seismic response of a hemisphere basin under SV wave incidence. Then, by using this method, the seismic responses of coupled three-dimensional depression terrain in sedimentary basins under vertical S-wave incidence are investigated considering different input waves, and the impacts of dimensionless width （defined as the ratio of depression width to basin’s radius, abbreviated as κ in the following） on the amplification effect of ground motion are analyzed. The distributions of peak ground displacement （PGD） and the corresponding amplification coefficients （defined as the ratio of the PGDs of basin-canyon coupled model to the single basin model）, the displacement time histories along characteristic profiles and their amplification factor distributions are used for the study. Finally, influence of the depression shapes （three-dimensional rectangle and three-dimensional trapezoid） on the seismic amplification features is comparatively studied. The results indicate that:

1） When no concave is contained in the basin, the incident waves on the semisphere surface are focused at the center of the basin, resulting in the strongest shaking in this region, which is the so called “focusing effect” . But the focusing areas varies significantly with the input wave, with the largest area for the Ninghe wave, and the smallest area for the pulse wave.

2） The dimensionless width of the depression significantly changes the distribution characteristics, intensities, and locations of strong ground motions inside the basin. For the pulse wave incidence case, excepting for the κ＝0.4 model, the ground motion amplitudes of the concave-coupled terrain are always larger than those of the only-basin model. While for the Ninghe and Kobe wave incidence case, the opposite happens.

3） The scattering effect of concave terrain on incident wave components with wavelengths equivalent or close to their sizes is more significant, and the strongest ground motion is mainly located in the top area of the concave. In contrary, the ground motion amplification effects are insignificant for incident waves with wavelengths greatly longer than the size of depression, when the strong ground motion are still located at the center area of the basin.

4） Compared to a single basin terrain, the concave terrain within the basin can cause amplification of ground motion in local areas adjacent to the top of the depression or at a certain distance from it, with amplification coefficients ranging from 1.1 to 1.3. Its position changes with the input wave and the dimensionless width, but the bottom of the depression is always a ground motion reduction zone. The amplification effect of the ground motions near the top of the concave or the basin edge is probably caused by the interference between the basin-edge induced surface wave, the directly input wave and the scattering wave on the concave surface.

5） The dimensionless width of the depression and the polarization direction of the incident wave jointly affect the distribution characteristics of the peak ground displacement and amplification coefficient. In this study, the input ground motion is polarized in the plane for the left-right sides, and out-of-plane for the front-back sides. Then different wave mode conversion phenomenon occurred when waves impinge on the basin basement. The interference with the directly input wave gives rise to the significant difference in wave propagation behavior and ground motion distribution features along the two axis of the basin, which cannot be considered for the two-dimensional model. In addition, the amplification coefficient curve on the profile parallel to the polarization direction fluctuates more violently.

6） The shape of the depression has certain influence on the distribution of simulated amplification coefficient. The ground motion distribution characteristics of trapezoidal depression on slope surface and bottom of depression are significantly different from those of rectangular depression. The effect of ground motion intensity on the depressed slope is greater than that on the top of both slopes when the slope angle is small. The PGD and amplification coefficient of the three-dimensional trapezoid concave site are a little smaller than the results of three-dimensional rectangle models. But the general distribution feature is comparable, the most affected area is always concentrated in the region of depression and the area adjacent to the edge of it.

Therefore, for the basin-depression coupled site, the ground motion amplification effect caused by the canyon should be considered, especially when the predominant wavelength of the input wave is close to or smaller compared to the canyon scale. If the predicted ground motion values are obviously larger than the designed basic seismic acceleration of the corresponding area, the seismic fortification level of important engineering structures in the area should be appropriately raised. The results of this study can provide theoretical reference for the seismic damage defense work of sedimentary basin-depression coupling sites.