Investigation of the Global Seismic Noise Properties in Connection to Strong Earthquakes

Abstract

The global seismic noise, recorded on a network of 229 broadband seismic stations located around the globe for 25 years, from the beginning of 1997 to the end of 2021, has been investigated. To study the noise properties a set of statistics estimated daily have been used: the multifractal singularity spectrum support width, the minimum entropy of the squared wavelet coefficients, and the wavelet-based Donoho-Johnstone (DJ) index. It is shown that the time points of local extrema of the average values of the analyzed noise properties (minima for singularity spectrum support width and DJ-index and maxima for entropy) tend to occur before strong earthquakes. The time interval from the middle of 2002 to the middle of 2003 is determined, when the trend of decreasing the average coherence of the noise properties in the auxiliary network of 50 reference points changed to an increase. Along with an increase in the average coherence, there is an increase in the radius of the spatial maximum coherences of noise properties. Both of these trends continue until the end of 2021, which is interpreted as a general sign of an increase in the degree of criticality of the state of the planet and, as a result, an increase in global seismic danger. After two mega-earthquakes close in time: 27 February 2010, M=8.8 in Chile and 11 March 2011, M=9.1 in Japan, there was an increase in the spatial scales of the strong coherence of noise parameters, which is a sign of an increase in the critical state. The response of seismic noise properties to variations in the length of the day (LOD) has been studied. An estimate of the correlation function between the mean values of the response to LOD and the logarithm of the released seismic energy in a time window of 1 year indicates a delay in energy release with respect to the maxima of the response of noise properties to LOD with a delay time of about 500 days. In connection with this result, an additional intrigue is the extreme increase in the average value of the response to LOD in 2021.

Spatial Correlations of Global Seismic Noise Properties

“…The choice of 2011 as a boundary time mark is connected, as will be seen from the following presentation, with the fact that the correlation properties of seismic noise changed dramatically after 2 mega-earthquakes: February 27, 2010, M=8.8 in Chile and March 11, 2011, M=9.1 in Japan. Previously, this change was already detected in [3,18]. Increased attention to the areas where the minimum values γ are most often realized, as was already written when discussing Fig.…”

“…3), and is also reflected in an increase in entropy (1) and a decrease in the width of the multi-fractal singularity spectrum support width ("loss of multifractali-ty"). These changes in noise properties are indicators of the transition of a seismically active region (including the whole world) to a critical state [3,17].…”

“…It is of interest to identify those reference points in the vicinity of which strong correlations most often occur. As before, for this purpose we use estimates (3)(4) of the probability density of the distribution over space of the maximum correlation values for the time intervals before and after 2011, as well as the distribution histograms of the numbers of control points in which the maximum correlation values occurred.…”

“…Figure 9 shows a graph of the time series of the length of the day for the time interval 1997-2022. Further, the LOD time series is used as a kind of "probing signal" for the properties of the seismic process [3,[15][16][17][18]. To study the relationship between the properties of the seismic process and the irregularity of the Earth's rotation, the squared coherences maxima between the LOD and the properties of seismic noise at each reference point were estimated.…”

“…Information about seismic noise gives possibility to study processes preceding strong earthquakes [1, 2,3]. Atmosphere and ocean are the main sources for the energy of seismic noise [4][5][6][7][8][9][10].…”