[2] Numerous oscillating fields of different nature affect the earth in extremely wide range of periods. When this happens certain types of energy partially transform to other types. For example, magnetic waves energy coming to the earth from the outside causes elastic oscillations because of inverse piezoelectric and seismoelectric effects; elastic stresses in the earth appear with the coming heat owing to thermoelastic coupling coefficients and others. The intensity of external effects may be small as compared to the forces acting inside the earth but the extent of their influence depends on the energy saturation of rocks and cannot be explained by linear effects.
[3] The appearance of rhythms under the effects caused by internal and external sources that is synchronization was discussed in geophysical papers long ago. Solar activity, earth tides, and climate are known to influence seismicity.
[4] The problem remains to be solved of the external effect threshold that is sufficient to synchronize the process caused by more powerful forces. A system open in terms of energy and sensitive to minor external effects apparently is in metastable state. As the system approaches instability, the efficient external effect threshold lowers. The earth, however, is continuously affected by noise from natural and artificial sources. Therefore the threshold of effective influence that can be detected (including trigger mechanism) is apparently of finite value exceeding the noise level.
[5] The effect of hidden periodic oscillations in weak earthquakes and microseisms series discovered in [Sobolev, 2003, 2004] falls into the class of phenomena under discussion. In principle, it can be considered in terms of self-organized criticality concept (SOC) [Bak et al., 1989; Sornette and Sammis, 1995], which attaches much importance to the emergence of remote correlation of seismic events (collective behavior). However the physical mechanism of the possible remote correlation in the context of seismology is not clear yet; general theories of catastrophes and phase transitions in open-energy systems invite more detailed studies for heterogeneous environments.
[6] From the end of the 1990s and after the global system of wide-band seismic stations was established, seismic noise was studied in the range of 102-103 seconds. The authors [Tanimoto et al., 1998] believed that oscillations in the solid earth appeared under the influence of atmospheric pressure variations. The authors of alternative hypothesis [Kobayashi and Nishida, 1998] assume that oscillations are caused by numerous weak earthquakes of energy below seismic stations sensitivity. These studies as well as others show that minute range oscillations are actually permanent including time intervals free of strong earthquakes. However sequences of individual impulses divided with intervals of their missing evidently were not revealed. It may be related to the fact that most researchers used Fourier spectral analysis, which is not intended for research in non-stationary process comprising bursts of different amplitudes and duration. The use of the program complex described below including wavelet analysis appears to be more promising.
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