## 5 Calculation of Water Column Reduction Spectra Using the OBS Records Obtained in the Arctic  
Analysis of the seismic response of soils for water areas has peculiarities. Some studies based on the analysis of seismic records obtained on the seafloor show that the vertical component of seafloor ground motions is significantly lower than that of on-land ground motions near the P-wave resonant frequencies caused by the water column above the seismograph [20; 21; 22]. To demonstrate this effect, records of local earthquakes obtained by ocean bottom seismographs in the Laptev Sea in 2019-2020 were used. Figure 12 shows the location of two OBSs deployed during the AMK-78 cruise (October 2019) and dismantled during the AMK-82 cruise (October 2020) in the Laptev Sea aboard the R/V Akademik Mstislav Keldysh. A series of scientific cruises to the Laptev Sea  
Figure 11: Residuals of response spectra (damping 5%) of modeled and recorded accelerators (components EW, NS, UD; spectral acceleration—SA, spectral velocity—SV, spectral displacement—SD) calculated for the surface of soil layers at Kik-net sites TCGH15 and KSRH10. and East-Siberian Sea were organized by the V.I. Ilichov Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences and the IO RAS with the participation of a number of other institutions. Since 2018, the program of seismological research has been included in these expeditions and has aimed to obtain the seismic and seismotectonic characteristics of the Laptev Sea region in the context of the relationship of the tectonic processes with the discharge of bubble methane from the bottom by registering local microearthquakes, remote teleseismic events and ambient seismic noise on the shelf and the continental slope of the Laptev Sea [45,46]. Figure 12: (a)—OBS deployment sites on the outer shelf of the Laptev sea (operation time period: October 2019—January 2020); (b)—schematic view of the OBS deployed in the Laptev sea with indication of sea depths. The two OBSs were located near, but not exactly within, large methane seep fields [14]. The central part of the outer Laptev Sea shelf is characterized by scattered areas of sub-bottom permafrost and talks [47; 48]. The soil layers in such areas have a multiphase structure: solid soil grains, pore water, ice and gas mixture. It turned out that the OBS recording capabilities on the Arctic shelf and the upper slope are highly dependent on the level of ambient seismic noise, which, in turn, is influenced by the wind waves. A strong noise level, at least in the autumn ice-free time period, practically leads to the impossibility of obtaining high-quality records of earthquakes. Ice-cover smooths out wind waves and significantly reduces noise levels [45; 46]. In the ice-covered time period, the OBS deployments resulted in a significant number of signals from both large remote and weak local earthquakes in a broad frequency range. Figure 13 shows the V/H curves, i.e., the ratio of the FFT spectra of the vertical to the horizontal component, for 10 local earthquakes of similar strength and epicentral distance, obtained at two points (St3 and Typ2--see Figure 12) located at a considerable distance from each other. A clear drop in the V/H curves is seen at frequencies of 3-10 Hz and for both OBSs. The depth of the sea above the St3 and Typ2 sites is 51 and 61 m, respectively. The theoretical estimate of the resonant frequencies of P-waves by Equation (37) is 5-7 Hz for these depths. In addition, the theoretical reduction spectra due to the water column above the sites are shown in Figure 13. The following values were used for calculation of theoretical reduction spectra by Equations (36) and (37): \(c_{w}=1500\) m/s, \(H=51\) m, \(\alpha=2\) (Figure 13a); \(c_{w}=1500\) m/s, \(H=61\) m, \(\alpha=1\) (Figure 13b). The speed of sound in water \(c_{w}\) is assumed to be typical. The thicknesses of water column \(H\) for both sites were obtained from ship echo sounder data.