## 1 Introduction  
An undersea tunnel has been planned and constructed on a large scale due to its obvious advantages of direct access, great convenience, large traffic volume, and small climate impact [1]. It is, however, located in a complex marine environment, leading to a huge investment cost and long-term construction period. Additionally, due to the complicated circumstances, the undersea tunnel's seismic safety becomes a crucial problem that engineering designers should pay considerable attention to. At present, seismic research on underground structures under land is relatively mature; however, there are few research studies on submarine tunnels, and they mainly focus on theoretical analyses, experimental research, and numerical simulations. Of these avenues, the response displacement method does not need to carry out a finite element time-history analysis of the structure of the continuous soil layer, which simplifies the analysis and research workload and has been widely used in the seismic codes of underground structures [2; 3]. In addition, research involving subsea tunnel seismic response analyses mostly concentrates on numerical simulations with the increasing development of computer science and technology, which makes the analysis results more reasonable and reliable. [PERSON] et al. [4] studied the seismic response of tunnel structures in complex foundations by changing the seismic peak. The influence of seismic force on the tunnel structures was obtained by studying the internal forces of the tunnel segments. [PERSON] [5] synthesized the dynamic characteristics of structure, soil, and water and established a numerical analysis model of tunnel structure. By doing so, she concluded that the maximum internal force of a tunnel structure was directly proportional to the foundation modulus ratio, lining thickness, and water depth and that the softer the soil was, the more remarkable the structural inertia force would be. [PERSON] et al. [6] constructed a fluid-solid coupling numerical model of a shield tunnel under the condition of the seepage flow of segment joints and analyzed the tunnel's mechanical deformation mechanism and surface settlement rule in accordance with distinctive factors, including the seepage of different joints, seepage flow, joint stiffness, and waterproof performance. [PERSON] et al. [7] built a finite element model of the dynamic interaction between seawater, the seabed, and a tunnel and studied the seismic response rule of a submarine tunnel with different ground motion inputs, seismic excitation directions, and overlaying water depths. [PERSON] [8] systematically researched the seismic response rule of underwater tunnels under different conditions through the use of analytical methods and revealed the dynamic fluid-solid coupling mechanism of the water-soil structure system during an earthquake. [PERSON] et al. [9] proposed a transient seismic structure-water-sediment-rock interaction model to evaluate the marine spatial seismic response of ocean structures experiencing oblique incidence earthquakes and studied the influence of sediment properties such as the thickness, porosity, and permeability of the sinking tube tunnel as well as the incident angle of the seismic wave on the dynamic response of structures. The marine geological environment which the subsea tunnel faces is complex. Compared with a land tunnel, apart from the differences in the physical and mechanical properties of the surrounding submarine rock-soil mass, there exists overlying sea water exerting an impact on the seabed and the structures within the seabed [10]. Therefore, the impact of the coupling effect of the overlying sea water and an earthquake on the response of an undersea tunnel should be taken into account when conducting seismic research on undersea tunnels. Through numerical examples, [PERSON] et al.