The seismic ground motion topographic effect, as an important research content in the field of seismic engineering, the study on the mechanism of the effect of complex terrain on the ground motion characteristics can provide basis for engineering seismic defense. A large number of post-earthquake site investigations have shown that the complexity of local terrain has a significant impact on the distribution of seismic damage, especially the irregular terrain can change the intensity and spectral characteristics of ground motion. As a type of locally irregular topography widely existing in nature, the unique geometric shape of river terraces can cause complex scattering and diffraction of seismic waves, resulting in variation of ground motion in its local areas, and then affecting the seismic damage degree of surrounding buildings.

Based on on-site seismic damage investigation data of the river terraces during Wenchuan M_S8.0 earthquake in 2008, buildings in the area with thicker alluvial deposits at the front edge of the terrace suffered severe damage, while those in the area with thinner slope deposits at the rear edge of the terrace suffered relatively mild damage. In general, the seismic damage at the front edge was significantly greater than that at the rear edge. At the same time, in order to study the mechanism of this seismic damage feature, the river terraces were selected as the research object, and the finite difference software FLAC3D is used to establish three-dimensional river terrace analysis models with different thicknesses of overburden soil layers, and then simulate and calculate the ground motion response under impulse loading, thus further revealing the influence law and internal mechanism of river terrace topography on ground motion characteristics and the distribution of seismic damage to buildings.

For the same order terrace, the peak values of the horizontal and vertical acceleration and the 90% energy duration all show an upward trend with the increase of the overlying soil thickness, reaching the maximum value at the front edge of the terrace and the transition point with the steep slope. Meanwhile, the ground motion level at the front edge of each terrace is significantly higher than that at the rear edge. As the terrace order decreases, the peak values of the horizontal and vertical acceleration and the 90% energy duration at the corresponding area also gradually decrease. Similarly, the trend of the Fourier spectrum amplitude and ratio at different monitoring points on the same order terrace is basically consistent, but the amplitude and ratio increase gradually as the monitoring point approaches the edge of the terrace facing the steep slope and the front edge of the area with increased soil thickness. As the terrace order increases （the terrain height rises）, the natural frequency of the structure increases accordingly, thereby significantly enhancing the amplification effect of low-frequency ground motion. The characteristic period T_g, platform value, and platform amplification coefficient β in the standard response spectrum of seismic acceleration for different monitoring points are all affected by the terrace order and overlying soil thickness. The T_g value decreases gradually as the terrace order decreases; the platform value increases as the overlying soil thickness at the front edge of the terrace increases; however, the platform amplification coefficient β decreases as the terrace order increases, and the amplification coefficient at the front edge of the terrace is significantly larger than that at the rear edge.

River terraces have a significant impact on the propagation of ground motion and the seismic damage degree to buildings. The change in the thickness of the overlying soil layer leads to different distributions of building damage by affecting the amplification of ground motion, while the terrace orders exacerbate or mitigate the seismic damage by affecting the spectral characteristics and overall level of ground motion. Therefore, the terrace orders and overlying soil thickness are the key factors affecting ground motion response, and the amplification effect of ground motion is stronger in high terrace and thick covering soil layer, which leads to severe damage of buildings.