Sloping seabeds often exist in offshore areas with complex structures. It is difficult to accurately analyze the seismic response characteristics of sloping seabeds based on the assumptions used for horizontal seabeds. The degree of saturation in nearly saturated soil notably affects the seismic response of seabeds. Therefore, we developed an analytical solution for the seismic response of a sloping, nearly saturated multilayer seabed. Using this solution, we analyzed the effects of the slope angle and soil saturation degree on the seismic response of the seabed. The results show that the seafloor inclination may has a minor impact on the seismic motion at a specific point, but it has a very significant effect on the overall site. A weak interlayer refers to a layer of material that has lower strength and/or permeability compared to the surrounding soil or rock. This layer can reduce the natural frequency of the seabed and increase the amplitude of long-period seismic components. In addition, the presence of a weak interlayer can lead to increased pore water pressure, decreased effective stress, and increased susceptibility to shear failure. These factors combine to reduce the stability of submarine slopes, highlighting the importance of understanding and managing the effects of weak interlayers in geotechnical engineering and coastal defense projects.

Wave-induced submarine slope instability and its subsequent submarine landslide pose a huge threat to the coastal communities and offshore infrastructure. This study conducted a wave flume experiment to understand the effect of low-permeability layer on the excess pore water pressure response and the instability of the layered submarine clayey slope under wave actions. The experiment captured the whole process of soil progressive liquefaction and instability of submarine clayey slope. When the wave propagates from the toe to the crest of the slope, the wave shoaling results in the soil at the slope crest above the low-permeability layer liquefy and then slide down due to the wave oscillation and scouring. The low-permeability soil layer leads to a delay in the accumulation of excess pore water pressure. However, once the excess pore water pressure is accumulated, this layer restrains the dissipation of excess pore water pressure resulting in significant liquefaction potential of the soils below this layer. Due to the capping effect of the low-permeability soil layer, there was no significant sliding of the soil mass below it. Our findings might provide an implication and guiding significance for offshore site selection and the coastal engineering safety.