Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 381

due to the strong damping of their signals. Inside the region where the
acoustic damping is of minor importance the magnitude of the electromagnetic
perturbations is found to decrease with distance as r 1 , while far from this
region the mean level of the noise falls off more rapidly with distance, that is as
r 3 . According to our estimations the magnitude of the electromagnetic noise
amounts to the value which is greater than or of the order of 1 pT=Hz 1=2 .
(2) In the case of tension cracks the effect treated here can arise independently of
the distribution of the crack plane orientation since the mean effective magnetic
moment of the cracks is always directed in opposition to the geomagnetic field.
In order to obtain the same effect, as the shear crack ensemble is considered, a
certain order of the crack orientation is necessary. For example, the predominant
directions for the shear crack growth can concentrate around the axis of
maximal shear stresses.

10.1.4 Theory of ULF Electromagnetic Noise

Due to Electrokinetic Effect

Perhaps Terada ( 1931 ) was the first that hypothesized a possible connection between
the EQ preparation process and electrokinetic phenomena in the ground. It is in favor
of this hypothesis that during Matsushiro EQ swarm (Japan, in August 1965) the
simultaneous perturbations of the Earth's electromagnetic field and local changes
in subterranean water level have been occasionally observed. The fluid lifting
resulted from diffusion of the subterranean water through the porous rock can be
accompanied by the generation of electrokinetic currents followed by perturbations
of the Earth's electromagnetic field (Mizutani and Ishido 1976 ; Mizutani et al.
1976 ). Many researches have considered the electrokinetic effect as a promising
candidate for explaining the electromagnetic anomalies possibly associated with
EQs (e.g., Fitterman 1978 ; Miyakoshi 1986 ; Dobrovolsky et al. 1989 ; Gershenzon
et al. 1993 ; Surkov 2000a ; Molchanov and Hayakawa 2008 ).
In Chap. 8 we have shown that the electrokinetic phenomenon developed in a
homogeneous conductive ground cannot produce the magnetic field in the atmo-
sphere. The anomalies of both electric and magnetic fields can be generated only
due to any kind of underground inhomogeneities where the rock conductivity and
streaming potential coefficient undergo strong changes. As one example, consider
the earlier model of the media (Fitterman 1978 , 1979a , b , 1981 ; Dobrovolsky et al.
1989 ) as schematically shown in Fig. 10.6 . In the region x<0the conductivity
1 and streaming potential coefficients C 1 are assumed to be constant and in the
next region x>0they are characterized by 2 and C 2 constant as well. The EQ
focus is modeled as an ellipsoidal inclusion shown in Fig. 10.6 with filled area.
The changes in tectonic stress may result in the enhanced/lowered pore pressure in
the focal region. Since a typical time scale of the underground fluid filtration process
is supposed to be much greater than that of EQ preparation, we treat the quasi-steady
regime of the fluid filtration.