Seismic Response of Nearly Saturated Ocean Sediment Layer in Ocean Foundation under P-Wave Incidence

Utilizing the theories of nearly saturated porous media and mixture continuum mechanics, this study formulates a mathematical model to describe P-wave oblique incidence on a seawater–nearly saturated ocean sediment layer–bedrock system. Theoretical analysis and numerical simulations are conducted to examine how incident frequency, sediment layer thickness, saturation, and porosity influence the seismic response of ocean foundation sites. The results indicate that P-wave incidence angle and frequency have a pronounced effect on seismic response, with horizontal displacement peaking near 65° and increasing significantly with frequency. At 50 Hz, the horizontal displacement amplitude is approximately 56 times greater than that at 1 Hz. Changes in saturation lead to significant quantitative differences: under fully saturated conditions, the maximum vertical displacement can be about 20 times larger than under nearly saturated conditions. Sediment thickness also plays a critical role: an increase in thickness can result in up to a 43% rise in maximum horizontal displacement and more than a hundredfold increase in maximum vertical displacement. Additionally, porosity exhibits a nonlinear effect on seismic response, wherein increased porosity amplifies horizontal movement, whereas reduced porosity exacerbates vertical vibrations. These findings provide both theoretical and quantitative insights for the seismic evaluation and design of oceanic foundations, highlighting the importance of accounting for the dynamic properties of nearly saturated sediment layers.