I.R. Stakhovsky - Dr.Sc. (Phys.-Math.), Leading Research Scientist Institute of Physics of the Earth O.Yu. Schmidt of Russian Academy of Sciences E-mail: direction@ifz.ru

The term «earthquake nidus» signifies directly not observable source of rock macrofracture in Earth\'s bowel (a source of a seismic wave) but there is a great variety of opinions about what the nature of this source may be. The existing models of shallow earthquake nidus in spite of a formal diversity are united by common conceptual approach to the item of modeling: they interpret the original as a material system in which the rules of classic physics are valid and the metric is enounced in terms of Euclidian geometry. Meanwhile the experimental investigations of last decades educe the new fundamental feature of rocks which in principle can not be explicated in the framework of classic physics - scale invariance of disjunctive structures in rocks. This feature is the specific inherency of strong non-equilibrium dissipative systems which evolve far beyond the applicability limits of classic physics. Scale invariance can not be modeled by parametric correction of classic physics rules. The preparation of earthquake takes place in a strong non-equilibrium medium, so any model of earthquake nidus must proceed from this fact and straightforwardly take it into account. This paper contains a review of experimental and theoretical investigations of rock fracture which lead to the definition of shallow earthquake nidus in terms of fractal geometry. The concept of «geometrical phase transition» is introduced (geometrical phase transition - avalanche-like coalescence of microcracks distributed in space as scale invariant sets). The methods of modeling of realistic microcracks ensembles in Earth\'s bowel (with the help of multiplicative cascade procedure) are discussed. Today we have all the reasons to consider that macroruptures in lithosphere are formed in consequence of critical transitions in fractal sets (clusters) of microcracks. These microcracks appear as a result of intermolecular bond fracture due to energy fluctuations in crystal lattice of minerals. Fractal sets of microcracks can be defined as the dissipative structures of seismo-generating system or as the product of its self-organization. Key factors of macrorupture genesis in Earth\'s bowel (i.e. earthquakes) proved to be non-equilibrium state of medium and scale invariance of microcrack sets. The process of earthquake preparation is the process of microcrack accumulation up to achievement of the critical value of fractal dimension of microcrack set what leads to avalanche-like coalescence of microcracks (geometrical phase transition). The system producing earthquake nidus is i) - open, ii) - non-linear, iii) - non-equilibrium, iv) - self-organized, v) - scale-invariant. Thus we must define earthquake nidus as a fractal cluster of microcracks generated by thermodynamic entropy variations in seismo-generating system. This cluster accumulates microcracks during tectonic deformation up to the geometrical phase transition (earthquake). It can be said that the process of shallow earthquake preparation is clearly multifractal process.

 

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