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Fig. 7.6
Normalized vertical
component of the GMPs
resulted from the longitudinal
seismic wave propagation.
The numerical calculations
are made for a distance
500 km from the seismic
source (Surkov
1989b
). The
ground conductivity is taken
as
10
4
.
dB
z
/
B
0
2
1
1 S/m. The
arrow
shows the moment of seismic
wave arrival at the
ground-recording station
D
0
t,
s
0,1
0,2
-1
-2
The typical time-dependence of the co-seismic signal calculated by Surkov
(
1989b
) for the seismic zone is shown in Fig.
7.6
. Detonation on the ground surface
was served as a source of seismic waves. The calculations were performed for a
distance 500 m from a point of the detonation. The arrow corresponds to the moment
of seismic wave arrival at the ground-recording station. The magnetic forerunner is
seen in the initial part of the signal before the arrow.
Notice that, in practice, the signal caused by the detonation of high explosive
charges on the ground surface can be much more complicated due to the effect of
electric charges and currents appearing inside dust clouds and explosive products
(e.g., see Soloviev and Surkov
2000
; Soloviev et al.
2002
). Aerial shock wave
propagation is accompanied by the perturbation of heavy ions and charged aerosol
densities in the atmospheric surface layer which in turn results in local perturbations
of the Earth' electric field (Soloviev and Surkov
1994
). One more effect is due to an
impact of the aerial shock wave (SW) on instruments.
7.2.5
Estimates of Typical Amplitude and Frequencies
of Co-seismic Signals
As is seen from Fig.
7.6
, the signal is not a monotonic one behind the front of
seismic wave. This part of the electromagnetic signal is similar, in character, to a
pattern of vibrations caused by seismic wave. It is usually the case that the frequency
and the characteristic size/wavelength of the electromagnetic vibrations are close to
those observed in seismic waves, although the electromagnetic signals are shifted in
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