This thesis deals with the model accuracy for seismic design of immersed tunnels, like the proposed underwater artery in Thessaloniki, Greece. The soil response to seismic waves is analysed in the frequency domain by means of the domain transformation method and the finite element method. The seismic response of an immersed tunnel and the damage in the gasket joints have been calculated with a closed form solution, a Winkler-type model, and a full three-dimensional continuum model. The Winkler model and the continuum model are applied in the time domain. Focus is especially given to the diverging results from the Winkler model, commonly used in seismic design, and the continuum model, which is considered to be more accurate. Through comparative analyses it is shown that the presented Winkler model is not able to model retroaction from the tunnel to the soil. This entails that the presented Winkler model is not suited for seismic design of an immersed tunnel with non-uniform cross section. Sensitivity analyses are performed to analyse the influence of the many parameters which must be determined for a seismic design. It is shown that the stratification and the soil parameters, together with the earthquake magnitude, influence significantly on the tunnel damage. On the other hand, the modelling of the immersed tunnel gasket joints has very little influence on the calculated gasket deformation. Finally, it is shown that the critical direction of wave propagation is an angle of approximately degrees to the tunnel axis.