Geoscience Reference
In-Depth Information
Sect.
7.2
) propagating along the ground surface just at the same velocity and with the
same periods. For example, the first Rayleigh wave originated from the great Alaska
EQ propagated at the velocity 3.0-3.3 km/s with period 23 s. The maximal vertical
ground displacement was about 4:2cm that results in the atmospheric pressure
excess in the near surface layer about 4:0Pa (Bolt
1964
). Considering the Mach
number M
D
C
R
=C
a
where C
a
is the sound speed in the air, we should note that
its value is about M
9-10. In this case the wave vector of atmospheric acoustic
waves is nearly vertically directed. According to the calculations by Golitsyn and
Klyatskin (
1967
) the angle included between the acoustic wave vector and the
normal must be about 6
ı
. Moreover, the Rayleigh waves can excite only acoustic
waves but not IGW in the atmosphere.
The amplitude of mass velocity of an upgoing acoustic wave increases with
altitude because of an exponential decrease in air density and pressure. At the
E-layer of the ionosphere (90-100 km) the mass velocity of the acoustic wave V
a
can reach a value about tens m/s that may greatly affect the ionospheric plasma.
The relative variations of the ionospheric plasma density is of the order of n=n
V
a
=C
a
, that is about several percent or more. We consider this effect in more detail
in the next section.
Much emphasis has been paid in the past on the studies of EQ precursors in
the ionosphere. Space-borne observations by OGO-6, Intercosmos-19, Intercosmos-
24, Aureol-3 and DEMETER satellites over seismo active regions have shown the
ionospheric perturbations which can be associated with impending EQs. Among
them ULF/ELF/VLF electromagnetic variations and noses occasionally observed
several hours before an EQ (Migulin et al.
1982
; Gokhberg et al.
1983
; Larkina
et al.
1985
; Parrot and Mogilevsky
1989
; Chmyrev et al.
1989
; Parrot
1990
,
2012
;
Serebryakova et al.
1992
; Molchanov et al.
1993
), changes in ion composition
(Boskova et al.
1993
; Pulinets et al.
1994
), small scale plasma inhomogeneities
(Chmyrev et al.
1997
), weak variations of particle precipitations (Voronov et al.
1989
; Galper et al.
1989
; Serebryakova et al.
1992
) and etc. However, analyses
of space-borne data collected by GEOS-2 (Matthews and Leberton
1985
), DE-
2 (Henderson et al.
1993
) and by ISIS-2 (Rodger et al.
1996
) satellites have not
shown any correlation between seismic activity and low-frequency electromagnetic
variations in the ionosphere.
The IGW is frequently considered as a possible mechanism/source for the pre-
seismic phenomena in the ionosphere (e.g., see Molchanov and Hayakawa
2008
;
Sorokin et al.
2003
). However, the acoustic mechanism for pre-EQ perturbations
in the ionosphere is very questionable since the ground surface vibrations due
to seismic wave propagation can hardly be applied to the phenomena under
consideration because of the weak amplitude of the acoustic waves caused by such
vibrations (Hayakawa et al.
2007b
). Indeed, the pressure variations in the near-
surface layer of the atmosphere can be estimated as follows:
P
D
C
a
@
t
u
C
a
u
T
;
(10.45)
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