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At the time t>t
and the distance r>r
other tendency begins to prevail.
At this stage the GMPs are generated in the vicinity of the seismic wave front and
propagate together with the wave front at the velocity C
l
. In such a case the wave
front, where the mass velocity reaches a peak, plays a role of the effective moving
source, which “radiates” the GMP in all directions. The region r>r
is referred
to as seismic/acoustic zone. We shall consider these zones separately because the
attenuation factor and other properties of the GMP in these two zones are different.
7.2.4
Electromagnetic Forerunner of Seismic Wave
Considering Eq. (
7.12
), we note that the term
r
.
V
B
0
/, playing a role of the
GMP source, is nonzero only behind the seismic wave front, i.e., in the region r<r
l
covered by seismic wave. Nevertheless, due to the diffusion the GMP and electric
currents penetrate into the region ahead of seismic wave front where the medium
is at rest. Certainly, the radial field diffusion in the direct and reverse directions is
developed nonsymmetrically first because of the geometrical factor and second due
to the motion of diffusion source, i.e., the acoustic wave. This fact is analogous, in
part, to the known Doppler effect arisen from the motion of the radiation source.
We remind that the characteristic wavelength in the direction of the radiation source
motion is shortened and vice versa it becomes longer in the reverse directions. A
similar effect can take place in our case in spite of the fact that the diffusion and
radiation are quite different processes.
The accumulation of the GMP ahead of acoustic wave results in the formation
of the so-called electromagnetic forerunner of the acoustic/seismic wave that
propagates in front of seismic wave (Surkov
1989a
). Here we can only estimate
the characteristic size of the electromagnetic forerunner. If the decrease in the
amplitude due to the divergence of acoustic energy in the spherical wave is ignored,
the quasistationary stage occurs so that the characteristic length and duration of the
forerunner keep approximately constant. In such a case the forerunner speed C
l
has to be on the same order of magnitude as that of the diffusion front. Equating
the diffusion velocity (
7.18
)toC
l
gives the characteristic duration, t
m
m
=C
l
,
and size,
m
m
=C
l
13-180 km, of the electromagnetic forerunner. It is not a
surprise that
m
is on the same order of magnitude as the diffusion zone size r
and
t
m
t
.
It follows from above estimate that the electromagnetic forerunner can propagate
not far as several seconds ahead of seismic waves. In the case of a solitary compres-
sion impulse, the forerunner signal increases in time up to the moment of the seismic
wave arrival, and then the signal changes the sign (Surkov
1997a
,
b
). It appears
that the electromagnetic forerunner can be detected only at short distances from
the earthquake epicenter or explosion point. For example, the theory predicts that
at the distances about tens kilometers from the earthquake epicenter the forerunner
amplitude decreases to the value from several pT to 1 nT for magnetic component
and from several nV/m to 1 V/m for electrical component, correspondingly.
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