## 4 Conclusions  
This paper carries out research on the seismic adaptability of a shield tunnel in a sea area under a complex foundation, supported by a shield tunnel water-intake project of a nuclear power plant in a sea area. The seismic analysis is carried out based on the response displacement method and the dynamic time-history method, and the following conclusions are drawn:  
(1) The fracture of the tunnel's joints under seismic force is mainly caused by the uneven settlement of the soil layer and the traveling wave effect of seismic waves. The rate of diameter change and the joint-stretching value of the lining structure in the shield tunnel can be used as control indexes of the lateral seismic performance. The inter-section joint-stretching value in the shield tunnel affects the water-tightness requirement inside the tunnel structure and can be used as an index of its longitudinal seismic performance.  
(2) The opening amount generated by the same joint at different section positions shows a similar change.  
(3) The rates of change of the diameter of lining segments at positions 4-12, where the vault and the bottom of the arch are relatively high, indicate that the tunnel is subjected to the lateral compression of the soil mass, resulting in a large deformation at the top and bottom of the lining segments. By comparing the two models, the rate of change of the joint model with consideration of the joint is greater than that of the model without consideration of the joint, so it should be taken as the control index for the analysis of the joint model with consideration.  
(4) Compared with the time-history analysis method, the displacement, internal force, rate of diameter change, and opening volume of the tunnel segments calculated by the reaction displacement method are not so rigorous; however, the rules are the same. Therefore, in order to solve the seismic response problems caused by shield tunnels in practical engineering, we should adopt the practice of combining the two methods, with the time-history analysis method used as the main method and the response displacement method used as the auxiliary method, which can make the results more reasonable.  
(5) Bolts link each segment in the shield tunnel, but they have nonlinear characteristics in practical engineering applications. In this paper, spring units are used to simulate the function of bolts in the processing of segment joints, which can only approximately reflect the deformation of the shield tunnel joints under the action of ground motion. Therefore, further research is needed in the simulation analysis of the joints.  
(6) In this study, only a corresponding numerical simulation analysis was carried out for the shield tunnel, and a shaking table test was not carried out. If the two kinds of analyses are combined for a comparative analysis and the actual test results are referenced, the obtained results will be more perfect and more referential and can thus be taken as a direction for further research.  
Conceptualization, J.Z. and C.W.; methodology, J.Z.; software, W.L.; validation, C.W.; formal analysis, C.W.; investigation, W.L.; resources, J.Z.; data curation, W.L.; writing--original draft preparation, C.W. and W.L.; writing--review and editing, C.W. and J.Z.; visualization, C.W.; supervision, J.Z.; project administration, J.Z.; funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.  
Key Project of National Natural Science Foundation of China [51738010]; Dalian Science and Technology Innovation Fund Project [2021]J13 SN83].  
Not applicable.  
The authors declare no conflict of interest.