## 4 Results
### HVSR Modeling Results  
Figure 6 shows the HVSR time-frequency plots for the OBS records obtained at the St3 and Typ2 sites. The pink arrows indicate the parts of the spectrograms that are characterized by a reduced level of HVSR in the low-frequency band of 0.2-2 Hz, which makes the high-frequency peaks above 2 Hz more pronounced than in the parts of the spectrograms indicated by the black arrows.  
The low level of HVSR in the low-frequency band of 0.2-2 Hz is observed during the period of time when the sea was free of ice. When there is ice on the sea, the signal level becomes flatter over the entire time and frequency range. However, a clear HVSR maximum at 10 Hz is observed throughout the entire observation period (indicated by the white arrows). At the same time, a less pronounced constant HVSR maximum at 3 Hz is observed only in the spectrogram corresponding to the St3 site (indicated by the red arrow). The spectrogram corresponding to the Typ2 site has weak permanent maxima at 8 and 15 Hz that appear after December 8 (indicated by the gray arrows). The parts of the spectrograms below 0.2 Hz for the St3 site and below 0.6 Hz for the Typ2 site are not representative.  
Figure 5: Demonstration of the correlation between smoothed amplitudes of the sea level fluctuation in the 30 s-5 min spectral periods range obtained by the wave recorder RBR virtuoso3 and its spectrogram, smoothed averaged amplitudes of the horizontal component of ground velocity, obtained by the OBS at the Typ2 site, and the wind speed obtained from the ERA5 reanalysis [26]. Semi-transparent colored numbered rectangles indicate the specific time periods, described in the text.  
Figure 6: HVSR time-frequency plots for: (**a**) the OBS records obtained at the St3 site; (**b**) the OBS records obtained at the Typ2 site. Colored arrows indicate the features of the spectrogram, described in the text.  
Figures 7 and 8 show the examples of the time window selection for HVSR modeling and the results of the HVSR modeling, correspondingly. For the HVSR modeling, we selected 6-8 h long OBS records obtained at the St3 and Typ2 sites on 25 October, 25 November and 25 December 2019. The time periods were chosen in such a way as to obtain separate results for three different months, because they are characterized by different weather and ice conditions.  
In Figure 8, two characteristic frequency ranges are distinguished on the HVSR curves for the St3 site: 2-5 Hz with a maximum at 3 Hz (pink rectangle 1) and 6-15 Hz (gray rectangle 2) with a maximum at 9-10 Hz. On the HVSR curves for the Typ2 site, there is only one high-frequency characteristic frequency range 5-15 Hz with a maximum at 9-10 Hz.  
The maximum of the curves at 9-10 Hz is most likely due to the coupling effect, a distortion of high frequencies of records due to parasitic oscillations in the system \"station-ballast-soft sediments\" [41], which is usually observed in the high-frequency range of 5-10 Hz. This issue will be covered in more detail in the Discussion section.  
Therefore, the resonant peak associated with the presence of a contrast reflective boundary under the seabed surface can be considered at a maximum of 3 Hz for the St3 site, which is more pronounced for the time period with an ice-free sea. Accordingly, there is no such resonance peak at all in the considered frequency range for the Typ2 site.  
Figure 7: The example of the time window selection for HVSR modeling for (**a**) the OBS records obtained at the St3 site; and (**b**) the OBS records obtained at the Typ2 site. The input parameters for the windowing are presented in Table 6.