[65] Microseismic field variations were studied in the minute range of periods from data of 20 wide-band IRIS stations before four earthquakes: Neftegorskoe with magnitude M = 7.0 in 1995; Kronotskoe earthquake ( M = 7.8) in 1997; Hokkaido ( M = 8.3) in 2003; and Sumatra ( M = 9.2) in 2004.
[66] Several days before Kronotskoe, Neftegorskoe and Hokkaido earthquakes asymmetric impulse oscillations of several minutes duration were revealed that were broken by intervals of several tens of minutes. They were only manifested at stations located at the same seismically active zone where the earthquake occurred.
[67] Periodic oscillations were revealed before Kronotskoe earthquake with period of 20-60 minutes, Sumatra earthquake with period of 20-60 minutes and Hokkaido earthquake with period of 120-180 minutes. They appeared after foreshocks and remote strong earthquakes.
[68] Microseismic noise coherence was revealed at different stations before Kronotskoe earthquake with periods of more than 6 hours, Hokkaido with period more than 3 hours and Sumatra in the range of 2-60 minutes. Noise coherence measure increased if we chose stations located closer to the epicenter.
[69] These effects may appear and disappear several times in the process of earthquake preparation and fall in the category of phenomena characteristic of non-equilibrium systems dynamics.
[70] The nature of these effects is apparently associated with self-organization of seismic process in meta-stable lithosphere and synchronization of oscillations in the earth inner and outer spheres the dynamics of which comprise random and quasi-periodic components.
[71] To reveal their mechanism calls for further investigation with a greater length of observation series, an increased amount of stations and greater number of earthquakes under investigation.
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