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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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