## 3 Data Analysis
### Synchronization Signals in Noise Parameter Changes  
Synchronization signals allocated by increased values of the spectral coherence of noise parameters \(\
u(\tau,\,\omega)\geq 0.3\) are shown in the time-frequency diagrams in Figures 5 and 6 for the time interval 2011-2021, including the moments of all five strong earthquakes (Table 3). Such diagrams were constructed based both on the data of the entire network of stations and on individual groups of stations. According to data from the entire network of stations as well as from non-volcanic and continental stations (Figure 5a-c), the most pronounced synchronization signals appeared during the period from six months to one year before three earthquakes with \(M_{\mathrm{up}}\geq 7.5\) (No. 1, 3, 5 in Table 3). The maximum amplitudes, \(\
u(\tau,\,\omega)\geq 0.45\), were recorded before the strongest Sea of Okhotsk earthquake (No. 1 in Table 3, \(M_{\mathrm{up}}=8.3\)) for half a year. Before the two considered earthquakes with \(M_{\mathrm{up}}<7.5\), similar signals of spectral coherence growth either did not appear (Zhupanovskoe, No. 2 in Table 3, \(M_{\mathrm{up}}=7.2\)), or were much less pronounced (Uglovoye Podnyatiye, No. 4 in Table 3, \(M_{\mathrm{up}}=7.3\)). Increased values of spectral coherence also appeared during the aftershock stages of all considered earthquakes. After the Sea of Okhotsk earthquake, the duration of postseismic synchronization was about one year and no more than 1-2 months after other earthquakes. The diagrams in Figure 5d,f show the \(\
u(\tau,\,\omega)\geq 0.3\) distributions according to the data (from top to bottom) for the northern, central, and southern groups of stations. According to the data from the northern stations (Figure 5d), increased values of spectral coherence manifested themselves during the 7-9 months before the Near Islands Aleetian (No. 3 in Table 3) and Uglovoye Podnyatiye (No. 4 in Table 3) earthquakes, as well as at their aftershock stages. Both of these two earthquakes occurred in the northern part of the region. Other manifestations of increased \(\
u(\tau,\,\omega)\) values are difficult to associate with strong earthquakes. Perhaps they reflect regional movements associated with geodynamic activity on the periphery of strong earthquake preparation areas, or they are random due to local features of seismic noise at the northern group of stations. According to the data from the central group of stations (Figure 5e), increased \(\
u(\tau,\omega)\) values with amplitudes up to 0.45-0.60 were most pronounced 6-9 months before the Sea of Okhotsk earthquake. Figure 5f shows the spectral coherence distribution diagram for the southern stations. In this diagram, noise synchronization before the Sea of Okhotsk and North Kuril earthquakes with \(M_{w}\geq 7.5\) (No. 1, 5 in Table 3) is very weak. This may be due to the fact that 67% of the stations in the southern group (six stations out of nine, Table 2) are coastal, and seismic records from them are noisy due to sea waves. In addition, the Ebeko volcano, located 6 km from the SKR station, has been erupting since 2019, and volcanic microseisms could mask the North Kuril earthquake preparation. However, it should be noted that Figure 5f shows an increase in the synchronization of seismic noise parameters during all 11 years of observations. The most pronounced increase in synchronization has manifested itself over the past year and a half, from mid-2020 to the end of 2021. This may indicate an increased danger of strong earthquakes in the southern part of the region under consideration. This assumption is consistent with the spatial distribution of noise parameters on maps for 2019-2021 (Figure 4, maps on the right), showing the increased danger of strong earthquakes in the southern part of the region. On the diagrams constructed from coastal and volcanic stations, the distribution of \(\
u(\tau,\omega)\geq 0.3\) values is mainly mosaic and non-systematic (Figure 6a,b).