**2. A proposed strategy for the study of MHz and kHz EM precursors**

This chapter concentrates, in an appropriately critical spirit, on asking 3 crucial questions:


We shall attempt to approach the above mentioned questions in the simplest and most intuitive way, rather than emphasize mathematical rigor. In any case, the readers should be aware that this attempt refers to a *snap-shot* of a rapidly moving field.

One wonders whether necessary and sufficient criteria, have yet been established, that permit the characterization of an EM anomaly as a real EM precursor. One of the main purposes of this contribution is to suggest a procedure for the designation of observed kHz / MHz EM anomalies as seismogenic ones.

As it is said, an important feature, observed both at laboratory and geophysical scale, is that the MHz radiation precedes the kHz one [25, 28, 29 and references therein]. The remarkable asynchronous appearance of these precursors indicates that they refer to different stages of EQ preparation process. Moreover, it implies a different mechanism for their origin. Scientists ought to attempt to link the available various EM observations, which appear one after the other, to the consecutive processes occurring in Earth's crust.

The following *two stage model of EQ generation by means of pre-fracture EM activities* has been proposed: The pre-seismic MHz EM emission is thought to be due to the fracture of the highly heterogeneous system that surrounds the family of large high-strength entities distributed along the fault sustaining the system, while the kHz EM radiation is due to the fracture of the aforementioned large high-strength entities themselves [e.g.,28-30,32-36,39]. In the frame of the above mentioned two stage model, the identification of MHz and kHz EM precursors requires different methods of analysis.

roughness of fracture surfaces has been interpreted as a universal indicator of surface fracture, weakly dependent on the nature of the material and on the failure mode [27-30,35,36 and references therein]. Such universal structural patterns of fracture and faulting process should be included into an EM precursor which is rooted in the activation of a single fault. Therefore, an important pursuit is to examine whether universal patterns of fracture and faulting are

Are There Pre-Seismic Electromagnetic Precursors? A Multidisciplinary Approach 221

EQ's occur on a fractal structure of faults. An active crack or rupture, can be simulated by a "radiating element" [32]. The idea is that a fractal geo-antenna can be formed as an array of line elements having a fractal distribution on the focal area as the critical point is approached. The recently introduced Fractal Electrodynamics [44, 45], which combines fractal geometry with Maxwell's equations, offers a new possibility for the exploration of the kHz EM

EQ's preparatory process has various facets which may be observed before the final catastrophe. The science of EQ prediction should, from the start, be multidisciplinary. A candidate preseismic kHz-MHz EM activity should be consistent with other EM precursors (SES [46], EM precursors rooted in lithosphere-atmosphere-ionosphere coupling [47]) and precursors which are imposed by data from other disciplines such as: Seismology, Infrared Remote Sensing [48], Synthetic Aperture Radars Interferometry [49]. The sequential appearance of different precursors in a relative short time interval supports the seismogenic origin of each of them, increases the probability that a significant EQ is coming, and leads to higher estimation accuracy of its parameters, namely, magnitude, time and position (see Section 8). The EQ generation is a cooperative phenomenon and its prediction needs the

The field of study of complex systems holds that their dynamics is founded on universal principles that may be used to describe various crises [50,51]. The presence of common pathological symptoms in candidate kHz EM precursors on one hand and other catastrophic events (e.g., epileptic seizures, magnetic storms and solar flares), which clearly distinguish the catastrophic event from the corresponding normal state, strongly supports the seismogenic

The burden of this section was to describe a plausible scenario for the study of kHz EM precursors, without obvious internal inconsistencies and without violating the laws of physics. In the next sections we present results gained from previous studies applying the

In natural rocks at large length scales there are long-range anti-correlations, in the sense that a high value of a rock property, e.g. threshold for breaking, is followed by a low value and

**3. The precursory MHz EM activity as a second order phase transition**

**2.2.4 The science of EQ prediction should, from the start, be multi-disciplinary**

hidden in the observed candidate kHz EM precursors (see Section 5).

**2.2.3 Analysis by means of fractal electrodynamics**

anomalies (see Section 7).

cooperation of scientists!

**2.2.5 Analysis in terms of complex systems**

framework of analysis described above.

**phenomenon**

origin of the detected kHz EM anomalies (see Section 8.4).

#### **2.1 Focus on MHz EM precursors**

Fracture process in heterogeneous materials *can be attributed to phase transition of second order* [40,41,42]. This crucial property should be hidden in a seismogenic MHz EM activity [28,29,34,39]. The temporal evolution of a MHz EM precursor, which behaves as a second order phase transition, reveals transition from the phase from non-directional almost symmetrical cracking distribution to a directional localized cracking zone that includes the backbone of strong asperities (*symmetry breaking*) [29]. The identification of the time interval where the *symmetry breaking* is completed indicates that the fracture of heterogeneous system in the focal area has been obstructed along the backbone of asperities that sustain the system: *The siege of strong asperities begins [29]. However, the prepared EQ will occur if and when the local stress exceeds fracture stresses of asperities*. Consequently, the appearance of a really seismogenic MHz EM anomaly does not mean that the EQ is unavoidable (see Section 3).

#### **2.2 Focus on kHz EM precursors**

It has been suggested that the lounge of the kHz EM activity shows the fracture of asperities sustaining the fault [28,29,32-36]. This fracture is characterised by a non-equilibrium instability, thus acquiring a self-regulating character and to a great degree the property of irreversibility. The latter, is one of the most important components of prediction reliability. An associated fracto-EM precursor should show persistent behaviour and evolve as a phase transition far from equilibrium without any footprint of an equilibrium phase transition. Two questions effortlessly arise:


What follows concentrates on the above aforementioned two questions.

#### **2.2.1 Statistical analysis of the kHz candidate EM precursors**

An anomaly in a recorded time series is defined as a deviation from normal (background) behaviour. Concerning the development of a quantitative identification of kHz EM precursors, tools of information theory and concepts of entropy rooted in extensive and nonextensive statistical mechanics can be used in order to identify changes in the statistical pattern. A significant change is expected in the time series of the EM precursor, namely the appearance of entropy "drops" or information "peaks", revealing that the underlying fracto-EM mechanism is characterized by a high order of organization. The catastrophic fracture of asperities should be also characterized by a positive feedback mechanism. This means that the kHz EM precursors should show persistent behaviour (see Section 5.2.1).

#### **2.2.2 Analysis in terms of universal structural patterns of fracture and faulting**

From the early work of Mandelbrot [43], the aspect of self-affine nature of faulting and fracture is widely documented from field observations, laboratory experiments, and studies of failure precursors on the small (laboratory) and large (EQ) scale. The activation of a single fault should behave as a "reduced image" of the regional seismicity, and a "magnified image" of the laboratory seismicity. Moreover, fracture surfaces were found to be self-affine following the fractional Brownian motion (fBm) model over a wide range of length scales, while, the spatial roughness of fracture surfaces has been interpreted as a universal indicator of surface fracture, weakly dependent on the nature of the material and on the failure mode [27-30,35,36 and references therein]. Such universal structural patterns of fracture and faulting process should be included into an EM precursor which is rooted in the activation of a single fault. Therefore, an important pursuit is to examine whether universal patterns of fracture and faulting are hidden in the observed candidate kHz EM precursors (see Section 5).

### **2.2.3 Analysis by means of fractal electrodynamics**

4 Will-be-set-by-IN-TECH

Fracture process in heterogeneous materials *can be attributed to phase transition of second order* [40,41,42]. This crucial property should be hidden in a seismogenic MHz EM activity [28,29,34,39]. The temporal evolution of a MHz EM precursor, which behaves as a second order phase transition, reveals transition from the phase from non-directional almost symmetrical cracking distribution to a directional localized cracking zone that includes the backbone of strong asperities (*symmetry breaking*) [29]. The identification of the time interval where the *symmetry breaking* is completed indicates that the fracture of heterogeneous system in the focal area has been obstructed along the backbone of asperities that sustain the system: *The siege of strong asperities begins [29]. However, the prepared EQ will occur if and when the local stress exceeds fracture stresses of asperities*. Consequently, the appearance of a really seismogenic

It has been suggested that the lounge of the kHz EM activity shows the fracture of asperities sustaining the fault [28,29,32-36]. This fracture is characterised by a non-equilibrium instability, thus acquiring a self-regulating character and to a great degree the property of irreversibility. The latter, is one of the most important components of prediction reliability. An associated fracto-EM precursor should show persistent behaviour and evolve as a phase transition far from equilibrium without any footprint of an equilibrium phase transition. Two

An anomaly in a recorded time series is defined as a deviation from normal (background) behaviour. Concerning the development of a quantitative identification of kHz EM precursors, tools of information theory and concepts of entropy rooted in extensive and nonextensive statistical mechanics can be used in order to identify changes in the statistical pattern. A significant change is expected in the time series of the EM precursor, namely the appearance of entropy "drops" or information "peaks", revealing that the underlying fracto-EM mechanism is characterized by a high order of organization. The catastrophic fracture of asperities should be also characterized by a positive feedback mechanism. This means that the kHz EM precursors should show persistent behaviour (see Section 5.2.1).

From the early work of Mandelbrot [43], the aspect of self-affine nature of faulting and fracture is widely documented from field observations, laboratory experiments, and studies of failure precursors on the small (laboratory) and large (EQ) scale. The activation of a single fault should behave as a "reduced image" of the regional seismicity, and a "magnified image" of the laboratory seismicity. Moreover, fracture surfaces were found to be self-affine following the fractional Brownian motion (fBm) model over a wide range of length scales, while, the spatial

MHz EM anomaly does not mean that the EQ is unavoidable (see Section 3).

(i) *How can we recognize an observed kHz EM anomaly as a seismogenic one?*

**2.2.2 Analysis in terms of universal structural patterns of fracture and faulting**

What follows concentrates on the above aforementioned two questions.

(ii) *How does it indicate that the impending EQ is unavoidable?*

**2.2.1 Statistical analysis of the kHz candidate EM precursors**

**2.1 Focus on MHz EM precursors**

**2.2 Focus on kHz EM precursors**

questions effortlessly arise:

EQ's occur on a fractal structure of faults. An active crack or rupture, can be simulated by a "radiating element" [32]. The idea is that a fractal geo-antenna can be formed as an array of line elements having a fractal distribution on the focal area as the critical point is approached. The recently introduced Fractal Electrodynamics [44, 45], which combines fractal geometry with Maxwell's equations, offers a new possibility for the exploration of the kHz EM anomalies (see Section 7).
