**4.2 Results**

90 Earthquake Research and Analysis – Statistical Studies, Observations and Planning

7/17/2009 0:00:37 22.869 0.22 42.367 1.852595 1.852561 0.000281

**(1day data) STDEV** 

**Date Time [s] B[T] B║[T] Bz[T] Bzn Bzn average** 

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.

7/17/2009 0:01:37 22.869 0.22 42.367 1.852595 7/17/2009 0:02:37 22.869 0.22 42.367 1.852595 7/17/2009 0:03:37 22.868 0.22 42.367 1.852676 7/17/2009 0:04:37 22.868 0.22 42.367 1.852676 7/17/2009 0:05:37 22.868 0.221 42.367 1.852676 7/17/2009 0:06:37 22.869 0.22 42.367 1.852595 7/17/2009 0:07:37 22.868 0.22 42.367 1.852676 7/17/2009 0:08:37 22.868 0.221 42.367 1.852676 7/17/2009 0:09:37 22.868 0.22 42.367 1.852676 7/17/2009 0:10:37 22.868 0.22 42.367 1.852676 7/17/2009 0:11:37 22.869 0.221 42.368 1.852639 7/17/2009 0:12:37 22.868 0.22 42.368 1.85272 7/17/2009 0:13:37 22.868 0.221 42.368 1.85272 7/17/2009 0:14:37 22.869 0.221 42.368 1.852639 7/17/2009 0:15:37 22.868 0.22 42.367 1.852676 7/17/2009 0:16:37 22.868 0.221 42.367 1.852676 7/17/2009 0:17:37 22.869 0.221 42.367 1.852595 7/17/2009 0:18:37 22.868 0.221 42.367 1.852676 7/17/2009 0:19:37 22.869 0.221 42.367 1.852595 7/17/2009 0:20:37 22.869 0.221 42.368 1.852639 7/17/2009 0:21:37 22.869 0.221 42.368 1.852639 7/17/2009 0:22:37 22.869 0.221 42.368 1.852639 7/17/2009 0:23:37 22.869 0.221 42.368 1.852639 7/17/2009 0:24:37 22.87 0.221 42.368 1.852558 7/17/2009 0:25:37 22.869 0.221 42.368 1.852639 7/17/2009 0:26:37 22.869 0.221 42.367 1.852595 7/17/2009 0:27:37 22.869 0.221 42.367 1.852595 7/17/2009 0:28:37 22.869 0.221 42.367 1.852595 7/17/2009 0:29:37 22.869 0.221 42.367 1.852595 7/17/2009 0:30:37 22.869 0.221 42.368 1.852639 7/17/2009 0:31:37 22.869 0.221 42.367 1.852595 7/17/2009 0:32:37 22.869 0.222 42.367 1.852595 7/17/2009 0:33:37 22.869 0.222 42.367 1.852595 7/17/2009 0:34:37 22.868 0.222 42.367 1.852676 7/17/2009 0:35:37 22.869 0.222 42.367 1.852595 7/17/2009 0:36:37 22.868 0.221 42.367 1.852676 7/17/2009 0:37:37 22.869 0.221 42.367 1.852595 7/17/2009 0:38:37 22.869 0.221 42.367 1.852595 7/17/2009 0:39:37 22.869 0.221 42.367 1.852595 7/17/2009 0:40:37 22.868 0.221 42.367 1.852676 7/17/2009 0:41:37 22.869 0.221 42.367 1.852595 7/17/2009 0:42:37 22.868 0.222 42.367 1.852676

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 in pre-seismic conditions.

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 is shown in Fig. 9.

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 25) and M5.0 (May 11).

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 orientation of the focal mechanism.

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 earthquake of M4.2 occurred.

Figures 11 and 12 depict results of Bzn distribution observed at GOPS on the two intervals May 28 - August 26, 2009 and the whole September month, 2009.

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

Earthquakes Precursors 93

the occurrence of M5.1 earthquake (July 24). In the last anomalous domain (August 11- August 26), the normalized function Bzn has values between 1.853 -1.454 and could be associated with the two earthquakes of M4.1 occurred on August 17 and 26. Here, the pre-

Figure 12 illustrates the Bzn distribution for September 2009, where similar pre-seismic characteristics are observed. Thus, enhanced values of Bzn are correlated with the increased

Fig. 10. Bzn and STDEV distributions at the GOPS, within the interval February01– March 31, 2010; vertical arrows are earthquakes and ratio 4.2/110 is the magnitude/hypocenter depth of earthquake in [km]; dashed red line is threshold limit between normal trend and

anomalous behaviour of the normalized function Bzn

seismic occurrence interval is of about 7 days.

values of earthquake magnitudes and decreased foci depth.

characterized by enhanced values of Bzn comprised between 1.852 and 1.854, and may be related to the superposition effect generated by the two earthquakes of M3.9 (June 20) and M4.0 (June 27). Pre-seismic increased values of Bzn are extended on 16 days interval for this group of earthquakes.

Fig. 9. Bzn and STDEV distributions at the GOPS, within the interval January 16 – April 30, 2009; vertical arrows are earthquakes and ratio 5.0/109 is the magnitude/hypocenter depth of earthquake in [km]; dashed red line is threshold limit between normal trend and anomalous behaviour of the normalized function Bzn

The second anomalous domain, with an average value of Bzn of about 1.855, is extended on the interval July 14 – August 7 and reflects also superposition effect of the two earthquakes of magnitude 5.1 and 5.2, occurred on July 24 and August 5, respectively. The pre-seismic superposition effect of the Bzn started on July 15 and is developed in 9 days interval up to

characterized by enhanced values of Bzn comprised between 1.852 and 1.854, and may be related to the superposition effect generated by the two earthquakes of M3.9 (June 20) and M4.0 (June 27). Pre-seismic increased values of Bzn are extended on 16 days interval for this

Fig. 9. Bzn and STDEV distributions at the GOPS, within the interval January 16 – April 30, 2009; vertical arrows are earthquakes and ratio 5.0/109 is the magnitude/hypocenter depth

The second anomalous domain, with an average value of Bzn of about 1.855, is extended on the interval July 14 – August 7 and reflects also superposition effect of the two earthquakes of magnitude 5.1 and 5.2, occurred on July 24 and August 5, respectively. The pre-seismic superposition effect of the Bzn started on July 15 and is developed in 9 days interval up to

of earthquake in [km]; dashed red line is threshold limit between normal trend and

anomalous behaviour of the normalized function Bzn

group of earthquakes.

the occurrence of M5.1 earthquake (July 24). In the last anomalous domain (August 11- August 26), the normalized function Bzn has values between 1.853 -1.454 and could be associated with the two earthquakes of M4.1 occurred on August 17 and 26. Here, the preseismic occurrence interval is of about 7 days.

Figure 12 illustrates the Bzn distribution for September 2009, where similar pre-seismic characteristics are observed. Thus, enhanced values of Bzn are correlated with the increased values of earthquake magnitudes and decreased foci depth.

Fig. 10. Bzn and STDEV distributions at the GOPS, within the interval February01– March 31, 2010; vertical arrows are earthquakes and ratio 4.2/110 is the magnitude/hypocenter depth of earthquake in [km]; dashed red line is threshold limit between normal trend and anomalous behaviour of the normalized function Bzn

Earthquakes Precursors 95

The local variation of earthquakes energy (Es) carried out for the analyzed interval in 2009 year is shown on Fig. 12. The relationship between earthquake magnitude and energy in

Fig. 13. Variation of earthquakes energy (in foci) on the interval January 01– September 30, 2009: vertical red dashed lines represent lead time of the pre-seismic anomalous behaviour

It is quite obvious that the pre-seismic anomalous behaviour of Bzn observed on March 9 (Fig.9) and July 18 (Fig.10) respectively, may be correlated with the increased seismic energy reflected by Fig. 13, as follows: more than 15 days interval for the earthquake of M4.0 (March 21) and, about 7 days for earthquakes of M5.1 (July 24). This seismic activity support a possible generation mechanism of the electromagnetic precursors based on the stress generation followed by dehydration of rocks and fluid migration through the faulting system, which may produce concentration of induced currents in highly conductive elongated structure such as Carpathian electrical conductivity anomaly. These induced currents may cause distortion of the vertical geomagnetic component (Fig. 8) which is reflected by increased values of Bzn (Fig.9). The pre-seismic lead time of the normalized function Bzn is between 7days and 15 days for all the data presented in this

The results carried out in this paper are based on the hypothesis according to the preseismic conductivity changes, due to the fluid migration through faulting system, may generate increased values of the normalized function Bzn proportionally with the intensity

Where: ES is energy [Erg], M is earthquake magnitude.

of the normalized function Bzn

paper.

**5. Conclusions** 

of anomalous current concentrations through CECA.

logEs = 11.8 + 1.5 M (11)

foci is:

Fig. 11. Bzn and STDEV distributions at the GOPS, within the interval May 28– August 26, 2010; vertical arrows are earthquakes and ratio 5.1/140 is the magnitude/hypocenter depth of earthquake in [km]

Fig. 12. Bzn and STDEV distributions at the GOPS, within the interval 01February – March 31, 2010; vertical arrows are earthquakes and ratio 4.2/150 is the magnitude/hypocenter depth of earthquake in [km]

Fig. 11. Bzn and STDEV distributions at the GOPS, within the interval May 28– August 26, 2010; vertical arrows are earthquakes and ratio 5.1/140 is the magnitude/hypocenter depth

**Superposition effect** 

Fig. 12. Bzn and STDEV distributions at the GOPS, within the interval 01February – March 31, 2010; vertical arrows are earthquakes and ratio 4.2/150 is the magnitude/hypocenter

of earthquake in [km]

depth of earthquake in [km]

The local variation of earthquakes energy (Es) carried out for the analyzed interval in 2009 year is shown on Fig. 12. The relationship between earthquake magnitude and energy in foci is:

$$\text{logEs} = 11.8 + 1.5 \,\text{M} \tag{11}$$

Where: ES is energy [Erg], M is earthquake magnitude.

Fig. 13. Variation of earthquakes energy (in foci) on the interval January 01– September 30, 2009: vertical red dashed lines represent lead time of the pre-seismic anomalous behaviour of the normalized function Bzn

It is quite obvious that the pre-seismic anomalous behaviour of Bzn observed on March 9 (Fig.9) and July 18 (Fig.10) respectively, may be correlated with the increased seismic energy reflected by Fig. 13, as follows: more than 15 days interval for the earthquake of M4.0 (March 21) and, about 7 days for earthquakes of M5.1 (July 24). This seismic activity support a possible generation mechanism of the electromagnetic precursors based on the stress generation followed by dehydration of rocks and fluid migration through the faulting system, which may produce concentration of induced currents in highly conductive elongated structure such as Carpathian electrical conductivity anomaly. These induced currents may cause distortion of the vertical geomagnetic component (Fig. 8) which is reflected by increased values of Bzn (Fig.9). The pre-seismic lead time of the normalized function Bzn is between 7days and 15 days for all the data presented in this paper.
