Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 384

the moment that far away from the EQ focus the last term on the right-hand side of
Eq. ( 10.37 ) can be neglected, we conclude that the electric potential decreases with
the distance x from the fault plane as x 2 . Consequently, the electric field strength
falls off with distance as x 3 . Notice that the quasi-static elastic deformations obey
the similar law but they fall off inversely cubed distance from the EQ hypocenter.
One more approach is based on the assumption that the Earth's crust may
contain the unstable inhomogeneities such as groundwater reservoirs and local
failure zones filled by fluid (Bernard 1992 ). Some of these inhomogeneities are
close to an instability threshold and thus they are very sensitive to small changes
in rock deformation. On the other hand the deformation threshold has to exceed

the threshold for tidal deformation 10 7 . The EQ focus as a source of seismic
activity may trigger the rock fracture in the vicinity of the unstable homogeneity
followed by sporadic fluid fluxes from the groundwater reservoirs into fresh cracks
and pores originated from the rock failure. This may result in the generation of
local electrokinetic effects far away from the epicenter of a forthcoming EQ. In this
picture the effect can be observed by chance if the sensors are located near such an
unstable inhomogeneity. It was hypothesized that this or similar effect can explain
several of a series of successful EQ predictions in Greece on the basis of the so-
called seismoelectric signals (e.g., see Varotsos and Alexopoulos 1984a , b ; 1986 ;
Varotsos et al. 1996 ; Varotsos 2005 ).
Fenoglio et al. ( 1994 , 1995 ) have suggested that the sealed high pore pressure
compartments can occur in the zones of intense shear deformation and around
the fault. The compartment size is supposed to vary from 100 m to 1 km. The
bridging of pore space around the compartment can be due to the deposition
of silica, fracturing and other processes. Under the rock shift or subduction the
sealings between the compartments can be crushed that gives rise to rapid changes
of pore pressure followed by the fluid stream along the cracks and underground
channels. The volume and velocity of the underground fluid can vary significantly
because of the generation of fresh pores, cracks, and channels due to the rock
decompaction or dilatation effect in the zone of intense shear deformation (Scholz
1990 ; Nikolaevskiy 1996 ).
In the model by Fenoglio et al. ( 1994 , 1995 ), the sealed compartment with
volume V has a form of narrow rectangular parallelepiped similar to a plane crack
as shown in Fig. 10.7 . The sealing at the end of compartments breaks down due to
a weak seismic event, which makes for the fluid filtration through the surface S
towards the low pressure region. Suppose that the fluid has advanced a distance
of l thereby producing the electrokinetic current in the volume V . Taking into
account Eq. ( 8.7 ) for the electrokinetic current density j ek , the electrokinetic current
moment is estimated as

d D j ek V D C jr P j V; (10.38)

where jr P j is absolute value of the pore pressure gradient. Substituting Eq. ( 10.38 )
for d into Eq. ( 7.3 ) gives a rough estimate of magnetic variation resulted from the
electrokinetic effect