Monitoring of a preseismic phase from its electromagnetic precursors.

Fracture in disordered media is a complex problem for which a definitive physical and theoretical treatment is still lacking. We view earthquakes (EQ's) as large-scale fracture phenomena in the Earth's heterogeneous crust. Our main observational tool is the monitoring of the microfractures, which occur in the prefocal area before the final breakup, by recording their kHz-MHz electromagnetic (EM) emissions, with the MHz radiation appearing earlier than the kHz. Two fundamental questions (unanswered yet) that scientists in this field ought to address are as follows. (i) Is there a way of estimating the time to global failure? (ii) Is the evolution towards global failure irreversible after the appearance of distinguishing features in the preseismic EM time series? We attempt to put forward physically powerful arguments with regard to answering these two basic questions. Our approach will be in terms of critical phase transitions in statistical physics, drawing on recently published results. We obtain two major results. First, the initial MHz part of the preseismic emission, which has anti-persistent behavior, is triggered by microfractures in the highly disordered system that surrounds the essentially homogeneous "backbone asperities" within the prefocal area and could be described in analogy with a thermal continuous phase transition. However, the analysis reveals that the system is gradually driven out of equilibrium. Considerations of the symmetry-breaking and "intermittent dynamics of critical fluctuations" method estimate the time beyond which the process generating the preseismic EM emission could continue only as a nonequilibrium instability. Second, the abrupt emergence of strong kHz emission in the tail of the precursory radiation, showing strong persistent behavior, is thought to be due to the fracture of the high-strength "backbones". The associated phase of the EQ nucleation is a nonequilibrium process without any footprint of an equilibrium thermal phase transition. The family of asperities sustains the system. Physically, the appearance of persistent properties may indicate that the process acquires a self-regulating character and to a great degree the property of irreversibility, one of the important components of predictive capability. We address the role of the order of material heterogeneity on the transition from anti-persistent to persistent behavior.