## 3 Modeling Results and Discussion
### The Response of Soft Clay Layer at the Black Sea Offshore Site
The Response of Synthetic Soil Deposits With Porous Medium Parameters Specific for Low Moisture Onshore and High Moisture Offshore Sites  
In Figure 11, Figure 12, Figure 13, the results are shown for input modeling data described in Section 2.5. In each case, the spectra of the seismic signal at the bedrock are compared with the surface spectra for low moisture onshore and high moisture offshore sites. Corresponding surface and bottom PGA values and acceleration amplification curves are also presented. Figure 11: Modeling results for the case of clayy silt layer: (**a–c**) response spectral acceleration \(SA\) (\(T\), 5%); (**d**) PGA values; (**e**) acceleration amplification curves for offshore case; (**f**) acceleration amplification curves for the onshore case. Figure 12: Modeling results for the case of soft clay layer: (**a–c**) response spectral acceleration \(SA\) (\(T\), 5%); (**d**) PGA values; (**e**) acceleration amplification curves for offshore case; (**f**) acceleration amplification curves for the onshore case. In Figure 11a-c, it is seen that for clayey silt, the clear resonance is observed due to interactions of the predominant period of input motion and natural site period. At the same time, Figure 11d shows that the PGA curve for offshore cases starts to flatten in contrast to the PGA curve for onshore cases. It can be due to the smaller value of the S-wave impedance for soft clay for the offshore case. Therefore, acceleration amplitudes for offshore cases saturate faster. Figure 11e-f shows that the acceleration amplification values generally decrease with an increase of seismic load at bedrock, while resonant periods shift to the long-period range. A decrease in amplification and shift in resonant periods are more pronounced for the offshore case. In the case of soft clay (Figure 12), the situation is almost the same. However, the effect of amplitude saturation occurs faster. Soft clay is such a weak soil that even with a relatively small seismic load at bedrock, the amplitude of surface accelerations for \(W=0.7\) is already in a state of saturation--it is slightly lower than the amplitude of surface accelerations for \(W=0.25\). In Figure 13a-c, e-f it is seen that the spectra for sand for onshore and offshore cases differ insignificantly. There are no clear resonances. It can be due to the very low value of the natural site period for the layer thickness of 2.2 m (T = 0.03 s or f = 33 Hz), which is close to the Nyquist frequency (sampling rate is 100 Hz) and corresponding slope of the spectra curve. However, Figure 13d shows that acceleration amplitudes for the onshore case saturate faster because the S-wave impedance for the onshore case is lower. The difference in the behavior of the spectra for sandy and clayey soils can be explained by the fact that for clayey silt and soft clay soils, there is a significant drop of the S-wave impedance of the soil with increasing moisture. The boundary between flysch and clayey silt or soft clay becomes more contrasting, and resonance is observed. Thus, the response of cohesive and non-cohesive soils with moisture specific both for onshore and offshore sites is explained by effects of resonances and the 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.