Non-linear elastic phenomena

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The experimental in situ study of non-linear effects by means of artificial sources (vibrators) has confirmed earlier results indicating that the propagation of seismic waves is governed by non-linear laws even at amplitude substantially lower (strain levels well below 10-5) than previously thought Devices were constructed to record low-level seismic noise in prescribed frequency windows in the range 10 Hz - 2 KHz. In situ (on the ground surface and in boreholes) noise measurements were performed and it was found that the geologic medium constancy emits seismic (acoustic) signals in the entire frequency range studied. Despite displaying considerable spatio-temporal variability, the emission can often be correlated with factors such as earth tides, distant strong earthquakes and nuclear explosions. Laboratory tests with rock samples subjected to mechanical excitation with properly scaled amplitude-frequency characteristics support the findings Also studied were triggering of earthquakes and nuclear explosions. In the Mediterranean area a statistically significant increase in seismicity in the 5-10 days following strong (M>5.5) distant earthquakes was detected. In Middle Asia triggering of M>1.5 shocks by distant nuclear explosions was observed Fractal and multifractal techniques were developed to analyze the fine structure of seismicity and its spatio-temporal variations. Application to seismic zones of northern Greece revealed that their seismicity is multifractal. If confirmed, the multifractal nature of seismicity strongly suggests that seismicity and its driving mechanism, faulting, are compatible with a percolation process. Another important implication is that the process responsible for faulting and seismicity is non-linear, since non-linear systems and phenomena often display (multi) fractal behaviour. This is consistent with the view that the real geologic medium differs radically from the idealized models representing it as a linearly-elastic continuum in dynamic equilibrium. Rather it is an energy-saturated non-linear fractured medium in a state of dynamic non-equilibrium.

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