# Spatial Correlations of Global Seismic Noise Properties
## 7 Conclusions  
The results of a detailed analysis of the properties of seismic noise based on the use of the Donoho-Johnston index, defined as the proportion of the maximum values of the moduli of the wavelet coefficients separating the \"noise\" coefficients, was presented. The main results of the conducted research were as follows:  
1. Three characteristic time intervals for the behavior of seismic noise were identified: 1997-2003, 2004-2015 and 2016-2022. 2. For these time intervals, there was a parallel decrease in the average values of the DJ-index (interpreted as a simplification of the structure of seismic noise and an approximation of its properties to white noise) and an increase in average spatial correlations. According to general ideas about the behavior of complex systems, this means that the system is approaching a critical state, i.e., an increase in the probability of strong earthquakes. 3. After 2011 (after the Tohoku mega-earthquake in Japan on 11 March 2011), there was an explosive increase in the maximum distances at which strong correlations occur. Such behavior is known in the theory of critical phenomena as \"an increase in the radius of fluctuation correlations\" [26], which also heralds a sharp change in the state of complex systems. 4. The estimation of the correlation function between the logarithm of the released seismic energy and the average response of seismic noise to the irregularity of the Earth's rotation shifted towards negative time delays, which corresponds to the shift of the coherence maxima towards the peaks of the seismic-energy release. The maximum correlation fell on a time shift of 534 days. This estimate assumes that the burst of LOD response corresponding to the data analysis in 2020-2022 precedes a strong earthquake with an average delay of 1.5 years. 5. From the estimation of the change in time of the synchronization measure of the seismic-noise response to the irregularity of the Earth's rotation (Figure 13), one can also see a growth of correlations, similar to the growth of the initial correlations in Figure 6.  
It should be noted that the noise level in real observations of the seismic background is inevitably extremely high. Transition to the low-frequency part of the spectrum allows the elimination of most of the man-made noise. Moreover, this transition makes it possible to study that part of the spectrum that is in the intermediate, little-studied region of periods between traditional seismology and gravimetry. The author's previous studies give hope that it is in this region of periods that the effects preceding strong earthquakes can be detected. This work is presented as a further development of these methods. The complexity of the data and processes underlying the formation of low-frequency seismic noise in the Earth's crust does not allow direct use of conventional statistical procedures for estimating confidence intervals, standard deviations and other measures of uncertainty. It is possible that further study of the statistical structure of low-frequency seismic noise will enable us to build such models that will make it possible to estimate the measures of uncertainty of the obtained statistical inferences, for example, using the bootstrap technique. This research received no external funding. **Data Availability Statement:** Open-access data were used from the sources: [[http://www.iris.edu/](http://www.iris.edu/)]([http://www.iris.edu/](http://www.iris.edu/)) mda/_CSN, [[http://www.iris.edu/mda/G](http://www.iris.edu/mda/G)]([http://www.iris.edu/mda/G](http://www.iris.edu/mda/G)), [[http://www.iris.edu/mda/GE](http://www.iris.edu/mda/GE)]([http://www.iris.edu/mda/GE](http://www.iris.edu/mda/GE)), [[https://hpiers.obspm](https://hpiers.obspm)]([https://hpiers.obspm](https://hpiers.obspm)). fr/iers/eop/eopc04/eopc04.1962-now and [[https://earthquake.usgs.gov/earthquakes/search/](https://earthquake.usgs.gov/earthquakes/search/)]([https://earthquake.usgs.gov/earthquakes/search/](https://earthquake.usgs.gov/earthquakes/search/)).