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

#### **4.1 Electromagnetic data collection**

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, Rokityansky & Ingerov, 1999).

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 determined by the magnitude of conductivity gradient.

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 through CECA.

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

Earthquakes Precursors 87

Zxx Zyy Zxy Zyx 

Using single site magnetotelluric impedance tensor decomposition technique (Bahr, 1988), it was possible to identify the following two parameters: skewness and strike orientation. The skewness is a dimensionality parameter of the impedance tensor, defined as:

The tensor impedance from relation (7) can be rotated to obtain the strike orientation of the

<sup>4</sup> 2 2 ( Zxx - Zyy - Zxx + Zyx )

Fig. 5. Skewness parameter versus frequency (Hz): pink arrow delineates the frequency

The MAPROS software packages include all these mathematical relations presented above and have been applied to MT sounding carried out at GOPS. This program performs the

This parameter should be less than 0.3 to interpret the structure as 2D.

<sup>1</sup> 2Re(Zxy + Zyx)(Zxx - Zyy) <sup>α</sup> = arctan

2D geoelectrical structure using the relation:

Where: (strike) is rotation angle [0].

range for 2D structure

following basic tasks:

(8)

(10)

Zxx + Zyy Skew = Zxy + Zyx (9)

GMS-06 (METRONIX - Germany). This geophysical system has 5 channels (two electric Ex, Ey and three magnetic Bx, By, Bz components), 24 bit resolution, GPS, two frequency ranges (LF: 4096sec.-1kH; HF=0.5kH-10kH) and for data processing "MAPROS" software packages.

Fig. 4. Carpathian electrical conductivity anomaly and Wiese induction arrows on a tectonic sketch map: 1) main boundaries and fractures (over thrusts) of regional structures; 2) Neogene volcanic rocks; 3) Carpathian fore-deep; 4) Carpathian flysch nape system; 5) Piena and Marmarosh cliff belt; 6) Carpathian electrical conductivity anomaly (CECA); 7) Wiese induction vectors magnitude; 8) seismic active Vrancea zone (intermediate-depth earthquakes); 9) Geodynamic Observatory Provita de Sus (GOPS) used for the electromagnetic data collection (Modified after Rokityansky & Ingerov, 1999)

It is well known that the magnetotelluric (MT) method is a passive technique that involves measuring fluctuations of natural electric (E) and magnetic (B) fields in orthogonal directions at the surface of the Earth (Kaufman, & Keller, 1981). The orthogonal components of the horizontal electric (Ex, Ey) and magnetic (Bx, By) fields are related by the complex impedance tensor, Z:

$$
\begin{pmatrix} \mathbf{Ex} \\ \mathbf{Ey} \end{pmatrix} = \begin{pmatrix} \mathbf{Zxx} & \mathbf{Zxy} \\ \mathbf{Zyx} & \mathbf{Zyy} \end{pmatrix} \begin{pmatrix} \mathbf{Bx} \\ \mathbf{By} \end{pmatrix}, \text{ or } \mathbf{E} = \mathbf{ZB} \tag{7}
$$

Where: Zxx, Zxy, Zyx, Zyy are elements of the impedance tensor [VA-1] For a 2D structure, in which the conductivity varies along one horizontal direction as well as with depth, the following relations are fulfilled:

GMS-06 (METRONIX - Germany). This geophysical system has 5 channels (two electric Ex, Ey and three magnetic Bx, By, Bz components), 24 bit resolution, GPS, two frequency ranges (LF: 4096sec.-1kH; HF=0.5kH-10kH) and for data processing "MAPROS" software packages.

Fig. 4. Carpathian electrical conductivity anomaly and Wiese induction arrows on a tectonic sketch map: 1) main boundaries and fractures (over thrusts) of regional structures; 2) Neogene volcanic rocks; 3) Carpathian fore-deep; 4) Carpathian flysch nape system; 5) Piena and Marmarosh cliff belt; 6) Carpathian electrical conductivity anomaly (CECA); 7) Wiese

It is well known that the magnetotelluric (MT) method is a passive technique that involves measuring fluctuations of natural electric (E) and magnetic (B) fields in orthogonal directions at the surface of the Earth (Kaufman, & Keller, 1981). The orthogonal components of the horizontal electric (Ex, Ey) and magnetic (Bx, By) fields are related by the complex

For a 2D structure, in which the conductivity varies along one horizontal direction as well as

**,** orE = ZB(7)

induction vectors magnitude; 8) seismic active Vrancea zone (intermediate-depth earthquakes); 9) Geodynamic Observatory Provita de Sus (GOPS) used for the electromagnetic data collection (Modified after Rokityansky & Ingerov, 1999)

> Ex Zxx Zxy Bx <sup>=</sup> Ey Zyx Zyy By

Where: Zxx, Zxy, Zyx, Zyy are elements of the impedance tensor [VA-1]

 

with depth, the following relations are fulfilled:

impedance tensor, Z:

$$\begin{aligned} \text{Zxx} &= -\text{Zyy} \\ \text{Zxy} &\neq -\text{Zyx} \end{aligned} \tag{8}$$

Using single site magnetotelluric impedance tensor decomposition technique (Bahr, 1988), it was possible to identify the following two parameters: skewness and strike orientation. The skewness is a dimensionality parameter of the impedance tensor, defined as:

$$\text{Skew} = \begin{vmatrix} \text{Zxx} & + & \text{Zyy} \\ \hline \text{Zxy} & + & \text{Zyx} \end{vmatrix} \tag{9}$$

This parameter should be less than 0.3 to interpret the structure as 2D.

The tensor impedance from relation (7) can be rotated to obtain the strike orientation of the 2D geoelectrical structure using the relation:

$$\mathbf{a} = \frac{1}{4} \arctan \frac{2 \text{Re}(\mathbf{Z} \mathbf{x} \mathbf{y} + \mathbf{Z} \mathbf{y} \mathbf{x})(\overline{\mathbf{Z} \mathbf{x} \mathbf{x} - \mathbf{Z} \mathbf{y} \mathbf{y}})}{\left( \left| \mathbf{Z} \mathbf{x} \mathbf{x} \cdot \mathbf{Z} \mathbf{y} \mathbf{y} \right|^{2} \cdot \left| \mathbf{Z} \mathbf{x} \mathbf{x} + \mathbf{Z} \mathbf{y} \mathbf{x} \right|^{2} \right)} \tag{10}$$

Where: (strike) is rotation angle [0].

Fig. 5. Skewness parameter versus frequency (Hz): pink arrow delineates the frequency range for 2D structure

The MAPROS software packages include all these mathematical relations presented above and have been applied to MT sounding carried out at GOPS. This program performs the following basic tasks:

Earthquakes Precursors 89

Fig. 7. Monitoring system of the geomagnetic components (Bx, By and Bz) and real time data transfer : acquisition module MAG-03DAM (a); computer (b) for data storage; monitor (c) for real time geomagnetic data display (d); data transfer program (e) ; wireless connection (f)

Fig. 8. Geomagnetic time series (Bperp and Bz) recorded at the GOPS for 7 days interval (April19- April 25, 2009); Bperp is B; red star is earthquake of M5.0; pink ellipse marks a pre-seismic disturbance of the vertical component (Bz) of geomagnetic field (lead time is

about 6 days before the earthquake).


Thus, on the base of MT results, a 2D geoelectrical structure has been identified on the frequency range less than 1.66 E-2 Hz where skewness < 0.3 (Fig.5) and average strike orientation is N960E (Fig.6). This frequency range is also associated with the intermediatedepth earthquakes interval (70-180km) where EM precursors are generated.

These results confirm, once more, that the CECA's geoelectrical structure is of 2D type with strike orientation approximately east-west, and forms not only a tectonic boundary between Moesian Platform and Carpathian Alpine structures, but also represents a peculiar conducting channel extended to the seismic active Vrancea zone (Fig.4).

Fig. 6. Strike orientation (degrees) versus frequency (Hz): blue rectangle delineates the frequency range for 2D structure with average strike orientation of N960E

The next step in our study was to realize a continuous monitoring of the geomagnetic components (B, B║, Bz) using the acquisition module MAG-03 DAM (Bartington-England), with 6 channels, 24 bit resolution and three axis magnetic field sensor MAG-03 MSL (frequency range: DC - 1kHz). In order to obtain B component of the geomagnetic field, one of the horizontal components of the three axis magnetic sensor must be orientated perpendicular to strike. The parameters of the data acquisition card are under software control and additional program collects information at each five seconds and stored them, every 60 seconds (Table.1), on the PC HD. Using the wireless connection, all the data are transferred from GOPS to the central unit, placed at the Institute of Geodynamics in Bucharest, for real-time data processing and analysis (Fig.7).


Thus, on the base of MT results, a 2D geoelectrical structure has been identified on the frequency range less than 1.66 E-2 Hz where skewness < 0.3 (Fig.5) and average strike orientation is N960E (Fig.6). This frequency range is also associated with the intermediate-

These results confirm, once more, that the CECA's geoelectrical structure is of 2D type with strike orientation approximately east-west, and forms not only a tectonic boundary between Moesian Platform and Carpathian Alpine structures, but also represents a peculiar

Fig. 6. Strike orientation (degrees) versus frequency (Hz): blue rectangle delineates the

The next step in our study was to realize a continuous monitoring of the geomagnetic components (B, B║, Bz) using the acquisition module MAG-03 DAM (Bartington-England), with 6 channels, 24 bit resolution and three axis magnetic field sensor MAG-03 MSL (frequency range: DC - 1kHz). In order to obtain B component of the geomagnetic field, one of the horizontal components of the three axis magnetic sensor must be orientated perpendicular to strike. The parameters of the data acquisition card are under software control and additional program collects information at each five seconds and stored them, every 60 seconds (Table.1), on the PC HD. Using the wireless connection, all the data are transferred from GOPS to the central unit, placed at the Institute of Geodynamics in

frequency range for 2D structure with average strike orientation of N960E

Bucharest, for real-time data processing and analysis (Fig.7).


skewness and strike, etc).


depth earthquakes interval (70-180km) where EM precursors are generated.

conducting channel extended to the seismic active Vrancea zone (Fig.4).

Fig. 7. Monitoring system of the geomagnetic components (Bx, By and Bz) and real time data transfer : acquisition module MAG-03DAM (a); computer (b) for data storage; monitor (c) for real time geomagnetic data display (d); data transfer program (e) ; wireless connection (f)

Fig. 8. Geomagnetic time series (Bperp and Bz) recorded at the GOPS for 7 days interval (April19- April 25, 2009); Bperp is B; red star is earthquake of M5.0; pink ellipse marks a pre-seismic disturbance of the vertical component (Bz) of geomagnetic field (lead time is about 6 days before the earthquake).

Earthquakes Precursors 91

The geomagnetic time series recorded at the GOPS for 7 days interval (April19- April 25, 2009), including the occurrence time of the earthquake of M5.0 (April 25), are presented in Fig. 8. The pre-seismic disturbance of the vertical component (Bz) occurred about 6 days before earthquake. But, as we have seen later on, this disturbance is masked by the

In this paper, daily mean distribution of the normalized function Bzn and its standard deviation are performed in the frequency range less than 1.666E-2 Hz, where 2D structural condition is fulfilled. The concept of this analysis is based on the idea that signal associated with solar-terrestrial origin is constant, according to relation (1), while lithospheric origin signal from the underground current flowing along the CECA is considered to have a vertical component (see fig.8). With the other words, the normalized function Bzn shows a small and certain value for its normal trend (in non seismic condition) and increased values

To assess the robustness of the presented methodology, some examples of Bzn distribution acquired in a span of about two years (2009 -2010) are shown in correlation with the intermediate depth earthquakes, with magnitude (Mw) higher than 4.0 (Richter scale), selected from the catalogue issued by National Institute of the Earth Physics-Bucharest. The first particular case of the Bzn distribution correlated with the both standard deviation (STDEV) and intermediate depth earthquakes, within the interval January 16 – May 11, 2009

The Bzn distribution emphasizes two domains, the first one, with normal values of about 1.842 on the interval January16 - March 8 and second one, on March 9- May 11 interval, having values between 1.850-1.856, and all earthquakes are marked by vertical arrows. Average value of 1.842, associated with earthquakes of M< 3.3 occurred on the interval January 16 – March 8 represents the threshold limit between the so called "normal trend" of Bzn and its second anomalous domain, which started on March 9, which may represent a superposition effect of the four earthquake of M4.0 (March 21), M4.1 (April 12), M5.0 (April

The earthquake of magnitude 5.0 was triggered in the Vrancea zone, at 109 km depth, on April 25 at 20:18:48 (local time), being felt in Bucharest and over a large area extended from the epicentral zone towards NE and SW directions, corresponding with the fault plane

Similar results have been obtained in the Bzn distribution (Fig.10) on the interval February 1–March 31, 2010, where the threshold limit of about 1.842 separates also two domains, one with normal trend (earthquakes of M<3.4) extended on the interval February 01- February 18, and anomalous one, on the interval February 21- March 31, having Bzn values between 1.850- 1.855. The last interval could be correlated with the superposition effect produced by the two earthquakes of M4.2, and the pre-seismic lead time is about 10 days before the first

Figures 11 and 12 depict results of Bzn distribution observed at GOPS on the two intervals

Figure 11 reveals three anomalous domains of Bzn which may be related to 5 earthquakes with magnitude larger than 4. First domain, extended on the interval June 4 – July 10, is

May 28 - August 26, 2009 and the whole September month, 2009.

superposition effect started on March 9, 2009.

**4.2 Results** 

in pre-seismic conditions.

is shown in Fig. 9.

25) and M5.0 (May 11).

orientation of the focal mechanism.

earthquake of M4.2 occurred.


Table 1. Geomagnetic time series B, B║ and Bz recorded on July 17th 2009 (42 minutes record): Bzn average is computed for 1day data; STDEV of the Bzn average.

The geomagnetic time series recorded at the GOPS for 7 days interval (April19- April 25, 2009), including the occurrence time of the earthquake of M5.0 (April 25), are presented in Fig. 8. The pre-seismic disturbance of the vertical component (Bz) occurred about 6 days before earthquake. But, as we have seen later on, this disturbance is masked by the superposition effect started on March 9, 2009.
