## 1 Introduction
Nevertheless, one-dimensional, equivalent linear analyses are still the most commonly used in practice [1]. Two- and three-dimensional problems are more specific, while computer programs, such as DYNAFLOW [14], PLAXIS [15] or FLAC [16] for solving them are mostly proprietary and require significant qualifications. The majority of the algorithms assume homogeneous solid soil structure, while the subsea soils are porous and water-saturated in their nature. Few articles are devoted to modeling site response of porous saturated medium and, in particular, provide a characterization of the seafloor seismic motions [13,16,17,18]. It is commonly believed that the water content of the soils does not affect horizontal components of the ground motion significantly as water has no resistance to shear stress [19]. Conversely, porous soil structure and position of water table considerably affect the vertical response [20,21], especially in the high-frequency band (above 10 Hz) related to P-wave propagation [22]. In addition, some investigations based on seafloor seismic records analysis show that the vertical component of seafloor motions is significantly lower than those of onshore motions near the P-wave resonant frequencies caused by the water layer above the station [13,18,23]. Some approaches, such as [21,22], seem very perspective as they justify the applicability of the well-known codes, which are usually used for SH-wave, to P-wave propagation case. In this case, when solving the wave equation, the velocity of the P-wave is used instead of the S-waves. Furthermore, constrained moduli are used instead of shear moduli. However, for the P-wave case, no specific well-known code has been developed. Though SH-waves do not propagate in the soil's liquid phase, the pore space volume and fluid in it affects soil density value and, in turn, S-wave impedance, that is, the product of S-wave velocity and density, which is considered to be one of the most effective parameters used to study the ground motion amplification of soil deposits [24; 25]. Hence, the purpose of the present study is to introduce a procedure for modeling the nonlinear behavior of porous, moist soils during SH-wave propagation, to verify it and compare response for synthetic soil profiles with porous medium parameters specific for low moisture onshore and high moisture offshore sites with cohesive and non-cohesive soils. The well-known and approved NERA algorithm [6] was used as a basis and evolved to account for the Biot and Gassman models describing elastic waves propagation in a fluid-saturated porous soil [26; 27; 28], commonly used as a theoretical basis for similar tasks. The applicability of the presented approach was substantiated for integration into other well-known algorithms and compared results for NERA, DEEPSOIL (EQL) and DEEPSOIL (MKZ) cases. Obtained results showed good agreement between the simulated by different methods and observed spectra. The modeling also showed that the response of cohesive and non-cohesive soils with moisture specific both for onshore and offshore sites is explained by effects of resonances and effect of seismic amplitude saturation, which, in turn, depend on the corresponding value of the layer thickness and S-wave impedance for porous saturated soil layer. The solution to this problem can have significant practical usage for studying the effect of porous medium parameters on the seismic response of the moist soil deposits. In particular, the proposed approach makes it possible to use the analogy method in offshore site response analysis.