Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 392

perturbations is believed to be indicative of the presence of seismo-electromagnetic
signal (Hayakawa et al. 1996 ). According to this criterion, one may expect that the
value of B z =B h on the ground surface is on the order of, or even more than unity,
as the underground source is operative. In order to discriminate the global and local
variations, (1) the data obtained at one ground-based station is compared with data
of other remote stations; and (2) the temporal evolutions of B z =B h and of averaged
power spectra are compared with the trend of K p index of global geomagnetic
activity. This technique has been successfully applied to two shallow EQs (the
hypocenter depth was 20 km in both cases) happened at Kagoshima, Japan on March
23, 1997 (M s D 6:5) and on May 16, 1997 (M s D 6:3) (Hattori et al. 2002b ,
2004 ). The observation at ground-based station 60 km far from the EQ epicenters
has shown (1) the increase in polarization ratio B z =B h before the seismic events,
and (2) a distinct correlation between the changes of field polarization and of local
seismic activity.
According to the simplest theory, the ULF underground sources can be split into
two types: current element and magnetic dipole. As is seen from Eqs. ( 7.1 )-( 7.5 )
and Figs. 7.1 - 7.3 , in all cases the ratio B z =B h is a function of the angle between
the vector of dipole moment and the position vector. This implies that the above
technique is sensitive to the epicentral distance since the value B z =B h can vary with
distance (through ) especially as the ground-based station is situated near the EQ
epicenter.
Schekotov et al. ( 2007 , 2008 ) have developed the polarization technique by
analyzing the components of polarization matrix. A possibility to locate an EQ
epicenter has been demonstrated on the basis of the data collected at Karimshino
station (52:83 ı N, 158:13 ı E, Kamchatka, Russia) for seven or more years.
However, the validity of this technique to find direction of the underground ULF
source has not been discussed.
One more polarization technique for locating the source of pre-EQ electromag-
netic activity has been proposed by Dudkin et al. ( 2010 ) and Dudkin and Korepanov
( 2012 ). The theoretical basis of this technique is based on the assumption that this
source is equivalent to an effective magnetic dipole which has axially symmetrical
magnetic field (that is, only two components, B r and B ). It should be noted that this
assumption is valid if only the vector of magnetic dipole moment is perpendicular
to the boundary between the earth and the atmosphere, otherwise there must be all
the components of the magnetic field including azimuthal one.
However, in the model by Dudkin et al. ( 2010 , 2012), the magnetic field
polarization ellipse and the vector of magnetic moment are assumed to be in the
same plane. Since the polarization ellipses are measured at two different ground-
based stations S 1 and S 2 , the intersection line of the polarization ellipse planes
contains the magnetic dipole vector M , as shown in Fig. 10.8 . Thus the measurement
of the polarization ellipses parameters makes it possible to calculate a position
of M which is assumed to be located at the EQ focus. A basic limitation of this
method is that it can be applied only for magnetic-dipole sources (poloidal-type
magnetic field) whereas the actual underground source may be an electric-dipole
source (toroidal-type magnetic field).