**3. Generation mechanisms and theoretical base of the electromagnetic precursors to earthquakes**

#### **3.1 Possible generation mechanisms of electromagnetic precursors**

This paper is not intended to present an exhaustive analysis of all of literature published in this field. Instead, we have tried to provide only some representative hypothetic mechanisms related to the electromagnetic precursors to earthquakes.

The theory of semiconductors launched by Freund (2000) is considered the most comprehensive one. According to this theory, the solid rocks (e.g. granites) begin to crack under the action of a stress exceeding their elasticity limit, what leads to a release of electric charges. These charges, carried by water moving through the rocks fissures, generate currents of high amperage that, in their turn, create disturbances of the magnetic field and, also, infrared signature in the bands of 8μm and 11μm, when the charges are neutralized at the surface of the Earth.

In accordance with this theory, to have seismo-electric signals it is necessary that a few compulsory conditions to be fulfilled:


The resistivity of porous rocks is changed as a function of compression and shearing (Brace et al., 1965) and may be measured by using passive experiments (magnetotelluric method), or experiments with active methods (Park et al., 1993).

Revol et al., (1977) have shown that the magnetic properties of rocks are changed depending on the applied stress and are associated to the changes of stress emphasized on fault rupture, what produces oscillations of the magnetic field of a few nT (Johnston, 1978, 1997); this mechanism is due to the piezomagnetic effect.

When a conductive fluid is forced to flow close to a surface with stationary electric charges, an electrokinetic effect appears generating currents that start to flow either through fluid, or

Sperner, B., the Collaborative Research Center [CRC] 461 Team, (2005), taking into consideration that the geometry of the subduction zone was not unequivocally defined, proposed four possible configurations for the Vrancea zone: (i) subduction beneath the suture zone; (ii) subduction beneath the fore deep area; (iii) two interacting subduction

Various types of slab detachment or delamination have been proposed to explain the present-day seismic images of the descending slab (Girbacea & Frisch, 1998; Gvirtzman,

Viscous flows due to the sinking seismogenic slab together with dehydration-induced faulting can be considered as possible triggering mechanism explaining the intermediate-

Stanica et al., (2004) show, on the base of the three-dimensional (3D) resistivity tomographic image carried out using magnetotelluric data, that the possible triggering mechanism of the intermediate-depth earthquakes in the Vrancea zone may be the rock response to the active torsion processes sustained by the descending asthenospheric currents and the irregular shape of the relic slab. In their opinion, this torque effect may generate the increase shear

**3. Generation mechanisms and theoretical base of the electromagnetic** 

This paper is not intended to present an exhaustive analysis of all of literature published in this field. Instead, we have tried to provide only some representative hypothetic

The theory of semiconductors launched by Freund (2000) is considered the most comprehensive one. According to this theory, the solid rocks (e.g. granites) begin to crack under the action of a stress exceeding their elasticity limit, what leads to a release of electric charges. These charges, carried by water moving through the rocks fissures, generate currents of high amperage that, in their turn, create disturbances of the magnetic field and, also, infrared signature in the bands of 8μm and 11μm, when the charges are neutralized at

In accordance with this theory, to have seismo-electric signals it is necessary that a few


The resistivity of porous rocks is changed as a function of compression and shearing (Brace et al., 1965) and may be measured by using passive experiments (magnetotelluric method),

Revol et al., (1977) have shown that the magnetic properties of rocks are changed depending on the applied stress and are associated to the changes of stress emphasized on fault rupture, what produces oscillations of the magnetic field of a few nT (Johnston, 1978, 1997);

When a conductive fluid is forced to flow close to a surface with stationary electric charges, an electrokinetic effect appears generating currents that start to flow either through fluid, or



that lead to disturbances of the magnetic field.

or experiments with active methods (Park et al., 1993).

this mechanism is due to the piezomagnetic effect.

**3.1 Possible generation mechanisms of electromagnetic precursors** 

mechanisms related to the electromagnetic precursors to earthquakes.

zones, and (iv) subduction beneath the suture, followed by delamination.

2002; Sperner et al., 2001; Wortel & Spakman, 2000).

depth seismicity in Vrancea (Ismail-Zadeh et al., 2000)

stress and drive faulting process within the rigid slab (Fig.2).

**precursors to earthquakes** 

the surface of the Earth.

compulsory conditions to be fulfilled:

through the surrounding rock, what, in conditions imposed by real crustal parameters, may create surface magnetic fields of a few nT (Fenoglio et al., 1995).

Another theory supposes the generation of the magnetic signal either by conductive fluid flowing in presence of the magnetic field of the Earth, or by magnetohydrodynamic conversion of the seismic signal into an electric signal during the propagation through a conductive medium (Molchanov et al., 2001). While these mechanisms were proved in laboratory conditions, it is unclear, yet, how this process takes place in conditions rather similar to those specific to the Earth, owing to the lack of measurements in active fault zones.

#### **3.2 Theoretical base of the electromagnetic precursors**

At the Earth surface the vertical geomagnetic component (Bz) is entirely secondary field and its existence is an immediate indicator of lateral inhomogeneity. For a two-dimensional (2D) structure, the vertical geomagnetic component (Bz) is produced essentially by the horizontal geomagnetic component perpendicular (B) to geoelectric structure orientation and, consequently, the normalized Bzn function defined as:

$$\text{Bzn (f)} = \frac{\text{Bz}(\text{f})}{\text{B}\_{\perp}(\text{f})} \tag{1}$$

should be time invariant in non geodynamic conditions (Ward et al., 1970), but it becomes unstable due to the geodynamic processes and, therefore, it could be used as a precursory parameter of the intermediate depth seismic activity (Stanica and D.A. Stanica, 2010).

In order to explain cause (earthquake) - effect (anomalous Bzn) relationship, we introduce the following equations:

$$\text{pz (f)} = \frac{0.2}{\text{f}} \left| \frac{\text{E}\_{\parallel}(\text{f})}{\text{Bz}(\text{f})} \right|^2 \tag{2}$$

where: z is vertical resistivity [m = VmA-1], f is the frequency [Hz] and the E║ is the electric field parallel to strike [Vm-1], Bz is the vertical component of the magnetic induction [Tesla (T) = V s m-2].

Also, it is possible to write the relation:

$$\rho\_{\parallel\parallel}(\mathbf{f}) = \frac{0.2}{\frac{\mathbf{f}}{\mathbf{f}}} \left| \frac{\mathbf{E}\_{\parallel\parallel}(\mathbf{f})}{\mathbf{B}\_{\perp}(\mathbf{f})} \right|^{2} \quad \text{,} \tag{3}$$

where: ║ is the resistivity parallel to strike[m], B is the component of the magnetic induction perpendicular to strike

[Tesla (T) = V s m-2].

From the relations (2) and (3) we may estimate the normalized function Bzn (f), in terms of resistivities as follows:

$$\left| \text{Bzn (f)} \right| = \sqrt{\frac{\rho\_{||}(\text{f})}{\rho z \, \text{(f)}}} \tag{4}$$

This estimation of Bzn is in error for non two-dimensional geoelectrical structure.

Earthquakes Precursors 85

model (Fig.3, b.) is of interest in selecting the site (GOPS) for continuous monitoring of the geomagnetic field. The normalized frequency scale () is proportional to distance along the x-axis and inverse-proportional to penetration depth (δ1) in medium of resistivity 1. The

As we have seen in relation (1), Bzn could be used as precursory parameter of seismic event by measuring the vertical geomagnetic component (Bz) and horizontal component perpendicular to the strike (B) which have been collected at the Geodynamic Observatory Provita de Sus (GOPS), placed on the Carpathian electrical conductivity anomaly (CECA). This anomaly is delineated by the Wiese induction arrows, and it can represent a zone of partial melting or of hot highly-mineralized fluids in sedimentary layers, formed at the collisional limit between the both platforms (East European and Moesian) with Carpathian Alpine structures (Fig.4). It is also quit possible that these two varieties of fluid anomalies to co-exist and gradually flow one into another, as indicated by the fact that geoelectric parameters remain fairly constant throughout its entire length (Pinna et al., 1993,

Induction arrows are vector representations of the ratio of vertical to horizontal magnetic field components. Since vertical magnetic fields are generated by lateral conductivity gradients, induction arrows map can be used to infer the presence, or absence of lateral variation of conductivity/resistivity. In the Wiese convection (Wiese, 1962) the vectors point away from the conductivity anomaly generated by anomalous internal concentrations of current, while in the Parkinson convection (Parkinson, 1959), the vectors point towards anomalous internal concentrations of current. Thus, insulator-conductor boundaries extended through a 2D geoelectrical structure (like CECA) give rise to induction arrows that orientate perpendicular to their geoelectrical strike, and have magnitude proportional to the intensity of anomalous current concentration (Jones & Price, 1970), which are in turn

In our methodology, it was also supposed that pre-seismic conductivity changes, due to the fluid migration through faulting system, may generate changes of the normalized function Bzn, having magnitude proportional to the intensity of anomalous current concentrations

The Geodynamic Observatory Provita de Sus (Fig.1) is located at about 100 km towards south-west of seismic active Vrancea zone and the criteria of selection as monitoring site are:



In order to select the frequency range where the relation (1) is valid (i.e., existence of a 2D geoelectrical structure and its strike orientation), as a first step in our EM methodology, at the GOPS we made a magnetotelluric sounding using the magnetotelluric (MT) equipment

<sup>1</sup> <sup>10</sup><sup>ρ</sup> [Ωm] <sup>1</sup> <sup>δ</sup> [km] = <sup>1</sup> <sup>2</sup><sup>π</sup> f[Hz] (6)

electromagnetic skin depth or penetration depth is given by:

**4. Electromagnetic (EM) methodology and results** 

determined by the magnitude of conductivity gradient.


condition for a 2D type geoelectric structure is fulfilled (Fig. 4);

**4.1 Electromagnetic data collection** 

Rokityansky & Ingerov, 1999).

through CECA.

Bucharest).

Fig. 3. Bzn distribution versus normalized frequency for sloping interface (a.) and vertical contact (b.): =a/δ<sup>1</sup>

The relation (4) demonstrates that normalized function Bzn could be linked to the resistivity/conductivity variation along the faulting systems acting as high sensitive path (represented by the Carpathian electrical conductivity anomaly) through the lithosphere and its right part lead to the normalized resistivity defined as:

$$\rho \mathbf{u}(\mathbf{f}) = \frac{\rho \mathbf{z}(\mathbf{f})}{\rho \mathbf{z}(\mathbf{f})} \tag{5}$$

Approximate field solutions were computed for two simple 2D geoelectric structures to illustrate the robustness of the relation (1). Solutions for the sloping interface and vertical contact models were obtained using finite element code (Wannamaker et al., 1986) and the results are presented in Fig. 3. These models represent two extremes in dipping angle of the interface and similarity in the properties of the normalized function Bzn obtained for the

Fig. 3. Bzn distribution versus normalized frequency for sloping interface (a.) and vertical

its right part lead to the normalized resistivity defined as:

<sup>ρ</sup> (f) <sup>ρ</sup>n(f) = <sup>ρ</sup>z(f)

The relation (4) demonstrates that normalized function Bzn could be linked to the resistivity/conductivity variation along the faulting systems acting as high sensitive path (represented by the Carpathian electrical conductivity anomaly) through the lithosphere and

Approximate field solutions were computed for two simple 2D geoelectric structures to illustrate the robustness of the relation (1). Solutions for the sloping interface and vertical contact models were obtained using finite element code (Wannamaker et al., 1986) and the results are presented in Fig. 3. These models represent two extremes in dipping angle of the interface and similarity in the properties of the normalized function Bzn obtained for the

(5)

contact (b.): =a/δ<sup>1</sup>

model (Fig.3, b.) is of interest in selecting the site (GOPS) for continuous monitoring of the geomagnetic field. The normalized frequency scale () is proportional to distance along the x-axis and inverse-proportional to penetration depth (δ1) in medium of resistivity 1. The electromagnetic skin depth or penetration depth is given by:

$$\left\{\delta\_{\rm I}[\rm km] = \frac{1}{2\pi} \sqrt{\frac{10\rho\_{\rm I}[\rm \Omega m]}{\rm f[\rm Hz]}}\right. \tag{6}$$
