Geoscience Reference
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Fig. 7.5 A physical
mechanism of geomagnetic
field perturbations due to the
seismic wave propagation in a
conducting rock. 1 —seismic
wave front r l
1
2
C l t,
2 —diffusion front r
D
p m
r d
confining the region where
electromagnetic perturbations
occur, 3 —electric current
lines, 4 —magnetic field
variation lines, 5 —radius r
at which the seismic wave
comes up the diffusion front
D
3
4
5
the GMP and currents propagate ahead of the acoustic wave front. The diffusion
velocity by Eq. ( 7.18 ) falls off with time and distance whereas the acoustic velocity
keeps constant, and so at certain moment the acoustic wave comes up the diffusion
front. Equating the radii r l and r d in Eq. ( 7.15 ) and ( 7.17 ), we can estimate this
moment
4 m
C l D
4
0 C l
t
:
(7.19)
Taking into account that the radii of the acoustic and diffusion fronts are equal at
the distance r C l t
yields
4
0 C l :
r
(7.20)
In the region r<r , referred to the diffusion zone (Surkov 1989a , b , 2000a ),
the diffusion-type propagation of the GMP and terrestrial electric currents is thus
predominant. While the region r>r is called the seismic zone.
The diffusion and seismic zones are sketched in Fig. 7.5 . Below we will show
that the effective magnetic moment M of electric currents is directed oppositely to
the vector B 0 of the Earth's magnetic field.
The ground conductivity usually varies from 10 2 S/m for the upper layer of
sedimentary rocks (1-2 km of the depth) to 10 3 S/m for the basement rocks
(lower boundary is about 10 km depth). The longitudinal seismic wave velocity
changes within 4.5-6.2 km/s (granite, marble, basalt, limestone) (Babichev et al.
1991 ). Using these parameters we obtain an estimate for the diffusion zone size:
r 50-700 km and the time scale t 8-160 s.
 
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