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phase with respect to seismic signals. A peak of the electromagnetic signal should be
expected in the vicinity of the seismic front where the mass velocity of the medium
amounts to the maximum value.
2
ı
B
LJ
LJ
that means that the diffusion of
the electromagnetic perturbations dominates over the effect of “frozen-in” magnetic
field lines. Let
a
and k
a
D
2=
a
be the characteristic size/wavelength and
acoustic wave number of the seismic wave, respectively. Using these parameters,
the above inequality reads as
Consider first the limit when
j
@
t
ı
B
j
m
LJ
LJ
r
!
j
ı
B
j
m
j
ı
B
j
k
a
:
(7.21)
Substituting the frequency of the acoustic wave !
D
k
a
C
l
into Eq. (
7.21
), we obtain
f
C
l
=.2
m
/
D
0
C
l
=.2/;
(7.22)
where the frequency f is related to the cyclic frequency ! via f
D
!=.2/.
Taking numerical values C
l
D
5 km/s and
D
10
3
-10
2
S/m we find that
inequality (
7.22
) keeps as f
(5-50) mHz.
In the same manner we can estimate the source function; that is, the last term on
the right-hand side of Eq. (
7.12
) by the order of magnitude
jr
.
V
B
0
/
j
V
0
B
0
k
a
;
(7.23)
where V
0
denotes the magnitude of the mass velocity. So, neglecting the term on
the left-hand side of Eq. (
7.12
) and equating the terms on the right-hand side of this
equation gives
m
j
ı
B
j
k
a
V
0
B
0
k
a
:
(7.24)
The value
j
ı
B
j
in Eq. (
7.24
) can be considered as the order-of-magnitude estimation
of the perturbation of the Earth's magnetic field caused by seismic wave propaga-
tion. So we finally come to
V
0
a
B
0
2
m
V
0
C
l
B
0
2f
m
:
ıB
max
(7.25)
Thus, such parameters as the mass velocity, seismic frequency, and ground con-
ductivity may greatly affect the magnitude of magnetic perturbations. For example,
taking a typical seismic frequency f
D
1 Hz, V
0
D
1-10 cm/s, B
0
D
5
10
5
T
and the abovementioned values of the parameters we obtain that ıB
max
3-30 pT.
The above estimate for magnetic field variations is valid not only for the earth-
fixed, that is, for the motionless reference frame but also for the reference frame
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