Global Seismic Noise Entropy

Abstract

Data of continuous records of low-frequency (periods from 2 to 1,000 min) seismic noise on a global network of 229 broadband stations located around the world for 23 years, 1997–2019, are analyzed. The daily values of the entropy of the distribution of the squares of the orthogonal wavelet coefficients are considered as an informative characteristic of noise. An auxiliary network of 50 reference points is introduced, the positions of which are determined from the clustering of station positions. For each reference point, a time series is calculated, consisting of 8,400 samples with a time step of 1 day, the values of which are determined as the medians of the entropy values at the five nearest stations that are operable during the given day. The introduction of a system of reference points makes it possible to estimate temporal and spatial changes in the correlation of noise entropy values around the world. Estimation in an annual sliding time window revealed a time interval from mid-2002 to mid-2003, when there was an abrupt change in the properties of global noise and an intensive increase in both average entropy correlations and spatial correlation scales began. This trend continues until the end of 2019, and it is interpreted as a feature of seismic noise which is connected with an increase in the intensity of the strongest earthquakes, which began with the Sumatran mega-earthquake of December 26, 2004 (M = 9.3). The values of the correlation function between the logarithm of the released seismic energy and the bursts of coherence between length of day and the entropy of seismic noise in the annual time window indicate the delay in the release of seismic energy relative to the coherence maxima. This lag is interpreted as a manifestation of the triggering effect of the irregular rotation of the Earth on the increase in global seismic hazard.

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

“…The article continues the studies carried out in (Lyubushin, 2014;Lyubushin, 2015;Lyubushin, 2018;Lyubushin, 2020a;Lyubushin, 2020c;Lyubushin, 2021c) to investigate the correlation and coherence properties of low-frequency seismic noise on a global scale covering the entire planet. Since 1997, the total number of broadband seismic stations from various networks has become quite large (229 stations), and their location provides satisfactory coverage of the earth's surface.…”

“…We assume that the noise is concentrated mainly in variations at the first highest frequency level of detail. Seismic noise entropy (1) was used in (Lyubushin, 2014;Lyubushin, 2015;Lyubushin, 2017;Lyubushin, 2018;Lyubushin, 2020a;Lyubushin, 2020b;Lyubushin, 2020c;Lyubushin, 2021a;Lyubushin, 2021b;Lyubushin, 2021c) to analyze seismic noise for different regions of the Earth and at the global level as a standalone tool. By its design, entropy (1) is also multiscale, like the entropy proposed in (Costa et al, 2003;Costa et al, 2005) for studying the properties of random signals.…”

“…These data were converted to 1-min time series by calculating averages for successive 60-s time intervals. Estimates of spectral power densities after coming to 1-min time step were presented in (Lyubushin, 2020c). The analysis is restricted by vertical component only because behaviors of multi-fractal and entropy properties of the noise which are used in the paper are rather typical for all components of seismic noise in the investigated range of periods exceeding 2 min.…”

“…It turned out that the behavior of average correlations for all properties has a common qualitative feature: before 2003, a decrease is observed, but after 2003, a rapid increase in correlation begins. In (Lyubushin, 2020a;Lyubushin, 2020c), it was hypothesized that the break in the average correlation trends in 2003 is due to a high-frequency anomaly in the Earth's rotation regime, which can also be a trigger for an increase in the intensity of the strongest seismic events after the Sumatran mega-earthquake on 26 December 2004. The kink in the trend of correlations in 2003 is accompanied by a kink in the trends of the values of the seismic noise properties being analyzed: after 2003, the average widths of the carrier of the singularity spectrum support width and DJ-index decrease, and the average values of the noise entropy increase.…”

Low-Frequency Seismic Noise Properties in the Japanese Islands

“…The minimum entropy (1) was proposed in [ 11 ]. In works [ 3 , 5 , 6 , 11 , 12 , 13 ], it was used to analyze the predictive properties of seismic noise. Entropy (1) has common features with the multiscale entropy considered in [ 14 , 15 ].…”