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capable of separating and distinguishing these waves at teleseismic distances due
to the different properties of the longitudinal, transverse, and surface waves such as
polarizations and different speed of propagation.
Here we start with qualitative estimations and analysis of Eq. (
7.12
). For sim-
plicity consider only a longitudinal seismic wave propagating in the homogeneous
elastic wave with constant velocity C
l
. Far away from the seismic source the shape
of the longitudinal wave front is close to spherical one. This approximation is valid
for a distance, which is much greater than a typical size of the explosion chamber or
earthquake focal zone. In this notation we can consider a point seismic source which
radiates the longitudinal seismic wave followed by the GMPs. If the seismic event
happens at the origin of coordinate system at the moment t
D
0, then the radius, r
l
,
of the spherical wave front increases with time t as
r
l
C
l
t:
(7.15)
As we have noted above, the GMPs and terrestrial electric currents can propagate
in the ground due to diffusion in the conductive medium. To estimate the character-
istic velocity and the size of the region covered by diffusion process, we compare the
left-hand side of Eq. (
7.12
) and the first term on the right-hand side of this equation
by the order of amplitude
ıB
t
m
ıB
:
(7.16)
r
d
Here r
d
is the characteristic distance at which the diffusion front spreads for the
characteristic time t. It follows from Eq. (
7.16
) that r
d
.
m
t/
1=2
. A more accurate
analysis produces the similar relationship which differs from above estimate by a
factor of 2 (Surkov
1989a
,
b
,
2000a
), i.e.,
r
d
2.
m
t/
1=2
:
(7.17)
The diffusion front serves as the effective boundary surface between the inner
region covered by the diffusion process and the rest medium surrounding the
perturbed region. In the diffusion region the GMP reaches a peak value at least at
the initial stage of the process. It should be noted that there is not a distinct surface
of separation between these two regions. However the boundary is abrupt enough
so that outside the diffusion zone the GMP decreases exponentially with distance,
say, as exp
r
2
=r
d
or anything else. The effective velocity of the diffusion front
propagation is
m
t
1=2
dr
d
dt
:
(7.18)
At the initial moments, while as t
!
0, the velocity of the GMP and electric current
diffusion is much greater than the seismic/acoustic velocity C
l
. This means that
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