THE GEOMAGNETIC METHOD ON PRECURSORY PHENOMENA ASSOCIATED WITH 2004 SIGNIFICANT INTERMEDIATE-DEPTH VRANCEA SEISMIC ACTIVITY I.A. MOLDOVAN 1, A.S. MOLDOVAN 2, C.G. PANAIOTU 3, A.O. PLACINTA 1, GH. MARMUREANU 1 1 National Institute for Earth Physics, Bucharest, Romani, iren@infp.ro 2 AZEL Designing Group Ltd., Bucharest, Romania 3 Bucharest University, Bucharest, Romania Received May 7, 2008 The paper tries an association between an unexplained, slow evolving and significant amplitude geomagnetic anomaly and the Vrancea (Romania) October 2004 intermediate-depth earthquake of moderate-to-high magnitude (Mw=6.0) followed by few weaker earthquakes (Mw<3). The fact that previous studies of Vrancea seismogenic zone indicate that observable precursory anomalies in the geomagnetic impedance might precede intermediate Vrancea earthquakes of moment magnitudes Mw 4.0 encouraged us to continue the researches in this direction and to improve both the research and the monitoring system. Key words: geomagnetic precursory anomaly, intermediate earthquakes. 1. INTRODUCTION During the last 20 years there have been reported facts that confirm the interrelations between the tectonic activity and the anomalous changes of the geophysical, geochemical and geohydrological parameters characterizing the Earth s lithosphere (Contadakis and Biagi, 2003, Pulinets and Boyarchuk, 2004). These changes may reflect certain modifications in the crustal and subcrustal state of stress and may indicate that critical stress has been reached. As a consequence, different methods of geosciences are trying to monitor tectonic activity, with the purpose of detecting regions and phases of critical stress on the base of precursory phenomena connected with earthquakes. The prediction of the complex systems evolution is possible just in circumstances where local specific information may be identified. The success related to the implementation of a system which enables us to get information concerning the imminence of an earthquake is strictly determined by its adaptation to the particularities of the zone taken into consideration (Stanica, 2003). Rom. Journ. Phys., Vol. 54, Nos. 1 2, P. 249 261, Bucharest, 2009
250 I.A. Moldovan et al. 2 In Romania, the association between geomagnetic anomalies and Vrancea earthquakes of moment magnitudes 3.7 Mw 5 was first surveied by studying the local magnetic and seismic data in the period 1998 2003 (Enescu et al., 2001, 2002 and 2004). This finding was extended in 2004 to a broader magnitude range 3.7 Mw 6.0. Vrancea [45.3N-46.0N and 26.0E-27.3E] is the most active intermediatedepth seismic area in Romania, where the last devasting earthquake with Mw=7.4 and h=94km occurred in March, 1977. Since then, there have been recorded three earthquakes with Mw>6.0, in August 1986, Mw=7.1 and in May 1990, Mw=6.9 and Mw=6.4. During the geomagnetic monitoring period (1998-2007), there were only two earthquakes with 5<Mw<5.4, one in April 1999, Mw=5.3 and the other in May 2005, Mw=5.2, h=147km. The October 27th 2004, Mw=6.0 earthquake offered us the first opportunity to investigate possible connections between the geomagnetic field behavior and the local intermediate-depth seismicity with moment magnitude larger than 5.5 and to study the nature of a large geomagnetic anomaly prior the occurrence of the earthquake. 2. THE DATA AND RECORDING SYSTEM While Vrancea earthquakes had never exceeded Mw=5 for nearly seven years, they did so in October 2004, culminating with a seism of Mw=6.0 on October 27, 2004. The hypocentral coordinates of the Vrancea earthquake of October 27, 2004, 23:34:36 (local time) are: 45.8 N, 26.7 E and h=100 km. Its macroseismic intensity was Io=VII as a maximum (on the Mercalli scale) and I= VI in Bucharest. Further data with respect to Vrancea seismic activity in the geomagnetic monitoring period were taken from the seismic catalogue of the National Institute for Earth Physics. In Romania, at present, is operating only one magnetic observatory in the Vrancea epicentral area, at Muntele Rosu (MLR Table 1), in an L-shaped tunnel, situated in a steep part of the Southern slope of the Carpathians. Other three observatories are under construction, two of them inside Vrancea seismogenic zone and another outside it. At the MLR Observatory, a triaxial Bartington fluxgate magnetometer and a high resolution acquisition system are used to measure and record the geomagnetic field fluctuations. The data are transmited by internet at the National Institute for Earth Physics of Bucharest. The working data are represented by the geomagnetic data as recorded at MLR observatory (Fig. 1). In order to discriminate local and global phenomena, the MLR geomagnetic data are compared with data provided by the international INTERMAGNET organization (www.intermagnet.org), from 2 stations situated outside the epicentral region (Fig. 1 and Table 1): Surlari (SUA) from Romania and Tyhany (THY) from Hungary, mostly for the period October 2004.
3 The geomagnetic method on precursory phenomena associated with 2004... 251 Table 1 The location of the magnetic observatories used for data comparison Magnetic Observatory Country Latitude Longitude Altitude (m) Code MLR Romania 45.49N 25.95E 1360 SUA Romania 44.68N 26.12E 84 THY Hungary 43.10N 17.54E 187 We have used the magnetic components: Hx, Hy the horizontal components along the north-south direction and east-west direction, respectively, Hz the vertical component of the geomagnetic field, and the total field vector amplitude, measured in nanotesla. The magnetic activity Kp indices were computed for each station using the program Kasm provided by INTERMAGNET. Fig. 1. MLR, SUA and THY magnetic observatories location and the 27 October 2004 earthquake epicentre. VRI and PLO, the next locations of other two magnetic stations (that will be operable before the end of 2008) are situated in the vicinity of the epicentre. 3. MAGNETIC FIELD OBSERVATIONS Anomalous changes of the geomagnetic field can occur before and during seismic events. As the lithosphere deforms, rock properties may change in response to changes in stress (piezomagnetism) or changes in the distribution (and
252 I.A. Moldovan et al. 4 composition) of fluids in the crust. The reported expected changes are in the range of a few nanotesla. The problem of the identification of the seismomagnetic effects in the geomagnetic time series is complicated by the presence of disturbances, mainly due to irregular transient time variations generated in the terrestrial ionosphere and magnetosphere and depending also on the geological structure of the area. In order to reduce such noise we have used two methods. The first one was proposed by Hayakawa et al. (1996) and mentioned and applied by Gladychev et al., 2002 (Kamchatka). This method was also applied (on daily basis) in Romania, in the previous work (Enescu et al., 2001, 2002 and 2004). The results of the data processing have been represented in diagrams of the magnetic impedance Bz(t)/BH(t), where Bz is the vertical component of the geomagnetic flux density and BH its horizontal component. The method (on a-posteriori investigation basis) showed that there were 83% cases in which intermediate depth earthquakes with Mw 4.0 presented some kind of precursory anomalies of the geomagnetic field. However, for the studied time interval no significant earthquakes occurred, except the event of October 27, 2004, for which the method failed to reveal a precursory anomaly in a reasonable time-window before the seism. It were reported two anomalies, situated one in July and the other in September, but it s difficult to connect the events (anomalies and earthquake) when they are separated by such a long time interval (Fig. 2). Bz/BH Eq Bz/By 1.89 1.885 1.88 Bz/BH 1.875 1.87 1.865 400 350 300 250 200 150 100 50 Bz/By 1.86 0 01/06/04 01/07/04 31/07/04 30/08/04 29/09/04 29/10/04 Fig. 2. Bz/B H and Bz/By ratios variation at MLR magnetic observatory, during 5 month in 2004 (between June and October). This includes the earthquake produced on 27th of October 2004. The second method consists in comparing the geomagnetic field recorded by several stations located as well as in the area of investigation as out of it. The comparison facilitates the detection of anomalous signals which are different and therefore can be the probable earthquake precursors.
5 The geomagnetic method on precursory phenomena associated with 2004... 253 The latest approach is using the local geomagnetic data recorded at Muntele Rosu Observatory during the last 10 years and the remote geomagnetic data from the observatories situated outside the epicentral zone. The method has conducted to almost perfect matches between recordings from Vrancea zone and those from neighboring observatories, excepting the days prior to earthquakes, as is the case of October 2004 seism (Figs. 3 5). Fig. 3. Magnetic field components variation at MLR observatory, situated at 70km from the epicenter of the earthquake produced in October 2004. The bar marks the moment of the earthquake. The literature from the last decade has reported anomalies not larger than 10 nt for earthquakes with Mw>6.0 and epicentral distances not larger than 130
254 I.A. Moldovan et al. 6 km. Because Vrancea presents intermediate-depth earthquakes, with depths below 60 km and the Muntele Rosu Observatory is located at the border of the seimogenic area, such that the hypocentral distances are at the limit of those 130 km (and even beyond it) we didn t expect to record anomalies larger than 10nT. Fig. 4. Magnetic field components variation at SUA observatory, situated at 130km from the epicenter of the earthquake produced in October 2004. The bar marks the moment of the earthquake. Despite of this, starting with 10 of October 2004, the eastern component of the local geomagnetic field, recorded at this observatory, left the general pattern (recorded by other observatories) and started to decrease on its own (Fig. 6).
7 The geomagnetic method on precursory phenomena associated with 2004... 255 Fig. 5. Magnetic field components variation at THY observatory, situated at 1000km from the epicenter of the earthquake produced in October 2004. The bar marks the moment of the earthquake. The issue is that after a relative steep decaying it reached a low peak of about 40nT, which is far over the expected values of an anomaly from a hypocentral distance of about 122 km. After this, the value of the eastern component starts to increase slowly following a determined slope, toward a normal, mean value. By coincidence (?) the earthquake occured when the value of this component restored to its mean value. It has to be observed that two days after the anomaly starts (on 12 of October) the k indexes show higher values which denote solar storms, and these are easily visible on the recordings. But this is still after.variation of Kp index at 3 geomagnetic observatories (SUA, THY, MLR) during October 2004 shows that most of the time it was a calm month over large areas from the Carpatho-Pannonian area.
256 I.A. Moldovan et al. 8 100 60 Bx(SUA)=22.600 nt Bx(MLR)=23.200 nt Bx(THY)=21.450 nt 20 27.10.2004 h=98.6km Mw=6.0 Bx (nt) -20-60 -100 1.10.2004 Time (days) 100 By(SUA)= 1670 nt By(MLR)= 220 nt By(THY)= 1100 nt 60 20 By (nt) -20-60 -100 1.10.2004 Time (days) 27.10.2004 h=98.6km Mw=6.0 100 60 Bz(SUA)=42.300 nt Bz(MLR)=43.390 nt Bz(THY)=42.860 nt 20 Bz (nt) -20-60 -100 1.10.2004 Time (days) 27.10.2004 h=98.6km Mw=6.0 Fig. 6. Comparison of the magnetic field variation at MLR, THY and SUA observatories on x, y and z components. We have tried to imagine what could be the determining factor of this anomaly, recorded only at MLR. The result is that, at present, we still don t have a
9 The geomagnetic method on precursory phenomena associated with 2004... 257 plausible answer. We have studied again all the 10 years of geomagnetic recordings looking for some similar anomaly to relate to. But there haven t been any. For this reason, we decided to analyze not only the geomagnetic records from the MLR, SUA and THY observatories as in the previous figures, but the differences between these records (Figs. 7 and 8). To emphasize these differences we have also computed the angle between the geomagnetic vectors from MLR, SUA and THY (Fig. 9). In every figure the annomaly is still present and seems to be related only with the 27th October 2004 earthquake. Fig. 7. Variation of the differences between the magnetic records from MLR and SUA. The differences are made on the three components and the total amplitude magnetic field. The bar marks the moment of the earthquake produced in October 2004. The anomaly is present only on the y axis.
258 I.A. Moldovan et al. 10 Fig. 8. Variation of the differences between the magnetic records from MLR and SUA, MLR and THY and SUA and THY only on the horizontal y East component. The bar marks the moment of the earthquake produced in October 2004. The first graph also represents the variation of Kp index during the month of October 2004. 4. CONCLUSIONS After this experience we have more questions than answers. One of these questions is: why this slow variation manifests on only one axis (y)? What are the odds for a magnetic perturbance to be perpendicular to the (x z) plane, such that no noticeable influencies are recorded to z and x axes? The only explanation seems to be that the perturbing magnetic vector had a very small inclination angle related to
11 The geomagnetic method on precursory phenomena associated with 2004... 259 Fig. 9. Variation of the angle between the geomagnetic vectors from MLR and SUA and SUA and THY during October 2004. the horizontal. Does this mean that there was a human intervention in the vicinity of the sensor? We don t think so, because there is a restricted area, serviced only by qualified personnel. Moreover, the perturbation doesn t appear suddenly as it would if a magnetic object would be inserted in the immediate neighbouring of the sensor; the perturbation has a clear, definite slope of developing, which almost excludes any artificial cause. Is that local-perturbation real or it s just a failure of the y flux-gate sensor inside the magnetometer? And what are the odds for a defective sensor to manifest its malfunction prior to a significant earthquake, during the preparing stage, and to recover just at the time of it? If we assume that the sensor was in a perfect state of operation, and there really was a local, slow evolving geomagnetic anomaly, within the preparation area, and if we also assume that all observations would lead us to the conclusion that we have to deal with a precursory phenomenon, could we, however, state beyond any reasonable doubt there is an earthquake preparing? And how relevant is this last question, at all? Of course, these might be just rhetorical questions and nothing more, but it s really frustrating the mode in which we are always one step behind the facts; turning this feeling into a motivation should be the clue in finding if chasing the earthquakes through the fields of electromagnetism is much more than a sport.
260 I.A. Moldovan et al. 12 The main conclusion of this work that can be taken into account is that if you want to look for a geomagnetic precursor of a strong earthquake in the Vrancea area, it is necessary to have a sensitive and dense geomagnetic network in the epicentral area. That s why, starting with July 2006, we have made a research consortium who s project is financed by the Romanian Ministry of Research and Education, through the Programme Excellency Research. Using specific instrumentation that will provide information on acoustic (both earth s seismic and atmosphere s infrasonic activities), electric, magnetic and electromagnetic fields, the consortium will verify if there are correlations which could be established between the behavior of these fields and the preparatory stage of strong intermediate-depth earthquakes in Vrancea zone. At the same time, the observations will be improved by differential measuring methods involving simultaneous, time-sychronized, data aquisition from sensors located inside and far from the epicentral zone (Plostina, Vrancioaia, Surlari, Magurele and international observatories of geomagnetic field) and will take place in ELF and sub-elf bands, between DC and 30Hz. This study confirms the main results obtained by other authors, namely that the great majority of Vrancea earthquakes of magnitudes smaller than 6 are not associated with precursory anomalies in the geomagnetic field behaviour. The earthquakes larger than 6, might have associated anomalies, but until now, we could not investigate the possibilty. Acknowledgements. The authors thank to all scientists that will share their experience and could suggest their opinion related to this unusual geomagnetic anomaly. REFERENCES 1. Biagi P.F., and Contadakis, M.E., (editors), Seismo-tectonic electromagnetic effects, precursory phenomena and seismic hazard, Nat. Hazards Earth Syst. Sci., 6, 2006. 2. Contadakis, M.E., Biagi P.F., (editors), Earthquake Precursory Phenoma, Nat. Hazards Earth Syst. Sci., 3, 2003. 3. Enescu D., Enescu B.D., Moldovan I.A., Contribution to the short-term prediction of Vrancea earthquakes. Romanian Journal of Physics, 46, 237 253, 2001. 4. Enescu D., Enescu B.D., Moldovan I.A., Chitaru C., New results obtained through the electromagnetic method for short term prediction of Vrancea (Romania) earthquakes. Romanian Journal of Physics, 47, 9 10, 901 917, 2002. 5. Enescu, D., Moldovan, I.A., Enescu, B.D., Solar activity, geomagnetic perturbations and Vrancea (Romania) earthquake short-term predictability, Romanian Journal of Physics, 49, 1 2, 145 170, 2004. 6. Gladychev, V., Baransky, L., Schekotov, A., Fedorov, E., Pokhotelov, O., Andreevsky, S., Rozhnoi, A., Khabazin, Y., Belyaev, G., Gorbatikov, A., Gordeev, E., Chebrov, V., Sinitsin, V., Lutikov, A., Yunga, S., Kosarev, G., Surkov, V., Molchanov, O., Hayakawa, M., Uyeda,
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