Records obtained at the IRIS world-wide system of broadband seismic stations before strong earthquakes were investigated with a purpose of detecting hidden periodicities, multiple coherence effects and seeking for asymmetric impulses within a minute range of periods. The stations are located at different distances from the epicenters of earthquakes. The initial realizations consisted of discrete measurements with a sampling rate of 20nbsp;Hz and the total volume of analyzed data exceeded 25nbsp;Gb. We used various programs of processing and analyzing time series: revealing of hidden periodicities in the sequences of peak values at a given level; wavelet analysis of microseisms flow; search for multiple coherence effects based on Fourier and wavelet approaches; estimates of spectral coherence measures evolution of variations of multi-fractal singularity indexes and others. Asymmetric pulses about 3-10 min long did arose several days before the Kronotskoe 05.12.1997 nbsp;M = 7.8, Neftegorskoe 27.05.1995 nbsp;M = 7.0, and Hokkaido 25.09.2003 nbsp;M = 8.5 earthquakes. Intervals of a stable manifestation of several periods tens of minutes of pulses were found before the Kronotskoe and Hokkaido events. Synchronization of microseismic oscillations at different stations was detected starting several days before Hokkaido and Sumatra 26.12.2004 nbsp;M = 9.2 earthquakes. Comparison of records obtained at different stations allows estimating the regional and local peculiarities of the anomalies. It is assumed that the nature of these phenomena is related to self-organization properties of the seismic process. The periodic vibrations, asymmetric pulses and synchronization intervals are indicators of the unstable state of a seismically active region and could be regarded as earthquake precursors.
IRIS world-wide system, microseismic oscillations, wavelet analysis, earthquake precursors.
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