Lviv Centre of Institute for Space Research, 5 A Naukova St., Lviv, 79060, Ukraine

Transcription

1 Lviv Centre of Institute for Space Research, 5 A Naukova St., Lviv, 79060, Ukraine vakor@isr.lviv.ua

2 Introduction Earthquakes (EQs) prediction is still an unsolved problem, despite the long-term intensive research in this area. Particularly urgent is the problem of short-term predictions (about 1 to 7 days), allowing, if not to avoid, at least, to minimize human and economic losses. The ultra low frequency (ULF) lithospheric magnetic activity before EQ in the frequency range below 1 Hz is recently considered as one of the most informative signatures of an EQ preparation process. Its physical mechanisms are studied and the relation with EQ confirmed. The detection of such an activity meets some serious difficulties first of all, useful signals extraction at the background of much more intensive Pc3-Pc5 ionospheric/magnetospheric pulsations and man-made interference S/N ratio much below unity. Next task is signal source localization at so low frequencies. Important problem: in spite that already many times it was stated and proved that one-point data processing at S/N<<1 can not give reliable results, it is still continued, inventing new processing methods, what discredits the idea and strongly lowers the social confidence in the possibility of EQ prediction. The new ULF precursors detection using spatial filtration was developed. This method is based on synchronous data processing from two or more spaced magnetometers with the use of magnetic field polarization ellipse technique. Its efficiency was confirmed in the three cases from three. The goal of the present report is to attract still once the attention to the method and further confirm its promising efficiency.

3 Theoretical Considerations (REMINDER)

4 ULF band ( Hz) EQ-related signals radiation mechanisms The ULF magnetic field decreases in lithosphere to a little degree - it can be detected at a distance up to km. Mechanical deformations in lithoshpere near hypocentral area of future EQ lead to pressure increase and arising of pre-eq magnetic activity in ULF band due to following main factors: (1) movement of conductive fluids in the Earth s magnetic field (inductive effect), holes displacement in the rocks; (2) displacements of boundaries between high and low conductive crustal areas; (3) electrokinetic effect; (4) piezoelectric or piezomagnetic effects.

5 Problems: the level of anomalous ULF magnetic field before EQ is small enough what needs high sensitive instrumentation and complicated methods for detection of such signals at normally noisy background; Ionospheric/manmade and lithosperic sources discrimination at S/N<1; localization of magnetic precursor source or, at least, determination of an azimuthal direction to it; very often these problems are aggravated by short time of precursor existence (less than 5 minutes).

6 Physical Rationale To extract information about the lithospheric source from the directional dependence of the ULF magnetic field components, the following data processing steps were carried out under some simplifying assumptions: In narrow ULF frequency band the magnetic field components can be represented as harmonic (periodic) signals. At a given frequency, 3 orthogonal components of magnetic field with specific phases allow estimation of the polarization ellipse (PE) parameters. Alternating (induction) currents generated by seismo-em effects can be visualized as small-scale superposition on large-scale telluric current system induced by global induction. These perturbations may be seen as closed loops at distances several times the dimension of source and can be considered to be equivalent to that produced by elementary magnetic dipole placed in the source region. Magnetic moment of such a magnetic dipole source always lies in the PE plane formed by its field components at the measurement points.

7 Magnetic dipole source and components of EM field

8 Formation of M line at two PE planes intersection M line contains the magnetic dipole moment M, which is aligned along it and can be calculated from PE parameters

9 Configuration of M area

10 M lines selection criteria PE major axes ratio is more or equal to cube of the inverse distance ratio between measuring points and centre of partitioning block: B 1 /B 2 =(r 2 /r 1 ) 3, where r 1, r 2 are the distances between magnetic dipole and observation points 1 and 2; B 1, B 2 are the field magnitudes in observation points 1 and 2. For ionospheric sources, which are much more far from both measuring sites in comparison with EQ hypocentres, this ratio is close to unity; for lithospheric source much closer more than ~ 2. The clustering of M lines in the given area allows its selection as M area where ULF signals are generated.

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12 Analysis of the local lithospheric magnetic activity connected with Panzhihua M W 6.0 earthquake (August, 30, 2008, China) Magnetometers position and major seismic events during August 2008 Decomposition of Panzhihua seismoactive area on elementary blocks (potential M area)

13 Time distribution of M block number in depth range 0 30 km during July October, 2008 (elementary block size km, depth zm = 1 29 km, step 2 km) It is clearly seen the increased crustal magnetic activity August,29, one day before EQ, localized just at the EQ depth and only along the line passing the EQ area.

14 Number of detected lithospheric (M lines) and ionospheric signals and Kp index values during July October 2008

15 Depth distribution of magnetically active crustal blocks in the monitored area one day before EQ the depth corresponds to EQ USGS catalogue data

16 Spatial distribution of crustal magnetically active blocks and Faults map Spatial distribution of crustal magnetically active blocks in the depth range 6 8 km on August 29. Red dots relates to the EQ epicentres on August Faults map of Sichuan seismoactive region and detailed view of the monitored area. The red line represents the maximum of M lines azimuth angle distribution in crust on 29 August The area of pre EQ magnetic activity maximum is marked with a green star and the EQ MW 6.0 epicentre is marked by red star. Populated places are denoted by blue circles.

17 More details in: Dudkin, F., Korepanov, V., Magnetic Field Polarization Ellipse: A New Approach for Detection of Pre Earthquake Lithospheric Activity, Chapter in book The Frontier of Earthquake Prediction Studies., M. Hayakawa ed., Nippon Senmon Tosho Shuppan Co Ltd., Tokyo, 800 p., ISBN , pp , 2012.

18 Looking for two closely spaced points for the synchronous analysis of ULF magnetic data

19 URL site: URL ftp: ftp://ftp.nmh.ac.uk/wdc/obsdata/

20 N MS1 MS2 Dist, km lat lon alt, km Opened Closed HRS DATA MIN 1 Abisko (ABK) Kiruna (KIR) Dourbes (DOU) Manhay (MAB) Gingin (GNG) Gnangara (GNA) Great Wall (GTW) Livingston Island (LIV) Istanbul-Kandilli (ISK) Iznik (IZN) Lovo (LOV) Uppsala (Fiby) (UPS) Nagycenk (NCK) Wien Kobenzl (WIK) Pelabuhan Ratu (PLR) Tangerang (TNG) Qiongzhong (QGZ) Sanya (SAN)

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22 Japan, Kanto region Kanto region Magnetic stations parameters [KNZ] Kanozan [KAK] Kakioka Latitude, deg Longitude, deg Elevation, m Digital Sampling, s 1 1 Resolution, nt About 450 EQs with magnitude > 5 occurred during years

23 Cumulative lithospheric magnetic activity in Kanto region in XY YZ XZ

24 Conclusions (1) For achievement of significant progress in EQ prediction it is necessary to organize the ULF magnetic field continuous observation by network of magnetometric stations in zones of increased seismic activity. Up to now there are any network of the magnetometric stations in seismo active areas. The proposed method of an increased magnetic activity localization for lithospheric sources needs 3 component magnetometers placed at least in two (better 3) observation points at distance about 50 70km. Technical requirement to the magnetometers: frequency range: Hz (better 10 4 Hz) in dependence on source depth resolution: min nt, better 1 nt Online access to the magnetometers data is a must.

25 Conclusions (2) The direction finding method for magnetic dipole like sources was developed. In majority of cases it gives a possibility for sources separation of lithospheric and ionospheric origin. The proposed method is based on clear physical considerations and can be efficiently realized with a network of EQ warning systems consisting of several (minimum 2) sensitive 3 component magnetometers operating in ULF frequency band. The method was approved on magnetic field data in ULF range obtained from sensitive magnetometers located in: 1) two spaced ULF stations in Koyna Warna seismoactive area, India; 2) three spaced measuring sites in seismo active region of Sichuan province, China; 3) two spaced measuring sites in Kanto region, Japan. It was found that the EQ EM precursors amplitudes appear mostly in the frequency band Hz 1 30 days before the EQ and are rather low (~ 1 80 pt) and completely masked by much stronger ULF signals from ionospheric sources For reliable determination of slow crustal nucleation processes preceding EQ (which form pre EQ ULF lithospheric magnetic activity, i.e. magnetic precursors) it is necessary to cover the monitored area with magnetometer density not less than one magnetometer per 2000 sq. km (the distance between measuring sites less than km).

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27 Spatial distribution of crustal magnetically active blocks N Date Time Lat Lon Depth, km Mw :25: :37: :44: :15: