Abstract
The records of seismic noise in Japan for the period of 1997–2020, which includes the Tohoku seismic catastrophe on 11 March 2011, are considered. The following properties of noise are analyzed: The wavelet-based Donoho–Johnston index, the singularity spectrum support width, and the entropy of the wavelet coefficients. The question of whether precursors of strong earthquakes can be formulated on their basis is investigated. Attention is paid to the time interval after the Tohoku mega-earthquake to the trends in the mean properties of low-frequency seismic noise, which reflect the constant simplification of the statistical structure of seismic vibrations. Estimates of two-dimensional probability densities of extreme values are presented, which highlight the places in which extreme values of seismic noise properties are most often realized. The estimates of the probability densities of extreme values coincide with each other and have a maximum in the region: , . The main conclusions of the conducted studies are that the preparation of a strong earthquake is accompanied by a simplification of the structure of seismic noise. It is shown that bursts of coherence between the time series of the day length and the noise properties within annual time window precede bursts of released seismic energy. The value of the lag in the release of seismic energy relative to bursts of coherence is about 1.5 years, which can be used to declare a time interval of high seismic hazard after reaching the peak of coherence.
Highlights
IntroductionPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
It is shown that bursts of coherence between the time series of the day length and the noise properties within annual time window precede bursts of released seismic energy
Based on a bursts of coherence between length of day (LOD) and the first principal component of the daily median retrospective analysis of seismic catalogs in [57], it was concluded that a magnitude 9 properties of seismic noise can, on average, outpace the release of seismic energy is of earthquake off the coast of Japan should not have come as a surprise
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The article investigates three properties of seismic noise: Entropy, determined through the distribution of the squares of orthogonal wavelet coefficients, the multifractal singularity spectrum support width, and the Donoho–Johnstone (DJ) index, defined as the proportion of the total number of wavelet coefficients that can be considered as "information carriers." These properties are estimated daily from seismic noise records at a network of stations. The choice of these parameters is due to the fact that their changes reflect the complication or simplification of the noise structure. The correlation function between bursts of coherence and the release of seismic energy turned out to be significantly shifted by time delays corresponding to the advance of the coherence maxima to strong earthquakes
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