## 3 Modeling Results and Discussion  
### The Response of Soft Clay Layer at the Black Sea Offshore Site  
There is much less OBS data worldwide than obtained at onshore sites. These are mostly collected in the framework of temporary observations with the local networks or arrays, except a few long-term cabled systems. In addition, detailed geotechnical information, including velocity profiles and porous medium parameters of the soils, are usually unknown for OBS networks. Especially it concerns downhole array data at offshore sites. Onshore downhole array data are commonly used to validate earthquake response simulation techniques. However, the offshore data are usually not freely available and mostly comes from vertical seismic profiling, which is an expensive engineering survey. In the present study, we used seismic signals of the 2011 Kutahya earthquake obtained by three OBS located on the Black Sea shelf. The main purpose of this temporary local network monitoring was local microseismic events acquisition, so the Kutahya earthquake was the only one to make it possible to neglect the effects of intermediate wave paths. The assumption of no relative movement between fluid and solid at low frequencies leads to using the mean density of the porous soils calculated with Equations (6) and (10) in time-domain equations of motion. The maximum shear modulus \(G_{max}\) and, as a consequence (Equation (13)), \(V_{s}\) also depends on the void ratio. Hence, mean density and shear wave velocity are only parameters, which explicitly depend on the porous structure of \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Moisture Coefficient,** & **Density of Solid Grains,** & **Density of Solid Matrix,** & **Density of Water,** & **Mean Density, \(\rho\)** & **Void Ratio, \(e\)** & **Saturation Factor, \(S_{r}\)** & **S-wave Velocity, \(V_{s}\)** \\ \(W\) & \(\rho_{\text{s}}\) (g/cm\({}^{3}\)) & \(\rho_{m}\) (g/cm\({}^{3}\)) & \(\rho_{f}\) (g/cm\({}^{3}\)) & (g/cm\({}^{3}\)) & & **(m/s)** \\ \hline
0.75 & & & & 1.54 & 2.08 & 1 & 80 \\ offshore & 2.71 & 0.88 & 1 & offshore & offshore & offshore & offshore \\
0.25 & & & & 2.02 & 0.68 & 1 & 102 \\ onshore & & & & onshore & onshore & onshore & onshore \\ \hline \end{tabular}
\end{table} 
Table 4: Input parameters for the case of soft clay layer underlain by the flysch half-space. Figure 7: Input soil parameters used in the study: (**a**) shear moduli degradation curves; (**b**) damping ratio curves. the soil. Here, the porous parameters were built-in equations of motion of the NERA code explicitly (see Equation (12)). However, the same reasoning, described in Sections 2.1-2.3, can be applied for other codes, either equivalent-linear or nonlinear. Hence, the calculated mean densities and shear wave velocities were used as input parameters also for DEEPSOIL code running in equivalent-linear (EQL) and time-domain nonlinear cases (MKZ). In Figure 8a, the response spectra of the horizontal component of the S-wave are presented both from modeled and experimental records (OBS site with soft clay layer of 5.5 m thickness). The synthetic and observed spectra are largely consistent with each other. The curves for NERA and EQL cases are very similar and slightly overestimated, while the response obtained by the MKZ model is underestimated. Nevertheless, the position of resonant frequency obtained by the MKZ model is more precise. The similarity of the curves for NERA and EQL cases can be due to small strains and near-linear stress-strain dependency while loading (Figure 8b). At the same time, two different constitutive models for nonlinear cases (IM-model for NERA and MKZ-model for DEEPSOIL) gave a more pronounced difference. There is a clear spectral spike corresponding to a frequency of 3-4 Hz for both synthetic and observed records. The existence of this spike is explained by resonance due to the presence of a layer with rather contrasting S-wave impedance. The impedance of the flysch is 3120 [m*g/s/cm\({}^{3}\)], while the impedance of the soft clay is 123 [m*g/s/cm\({}^{3}\)].