Geoscience Reference
In-Depth Information
part of the basin there exist several (or one) propagating modes of period
T
, while
in the shallow-water part no mode exists for such a period. Assuming in expression
(3.35)
n
= 1, one can readily find the critical period for the given depth
H
:
T
c
=
4
H
c
.
(3.36)
The frequency corresponding to the critical period is called the cut-off frequency.
Modes of periods inferior to
T
c
do not propagate in the considered waveguide. For-
mula (3.36) also permits to calculate the critical depth
H
c
for a given period of
elastic waves,
T
. An elastic wave will not penetrate the region, where the depth is
smaller than the critical depth,
H
<
H
c
=
cT
/
4. A displacement of the ocean bottom
with residual deformation of duration
τ
forms elastic waves of period
T
∼
τ
, which
are capable of penetrating down to depths
H
c
∼
/
4. The examples of calculations,
presented in Figs. 3.9, 3.10 and 3.12 correspond to
c
τ
= 10 s, i.e. the critical depth
amounts to
H
c
= 3
.
75 km. From the figures it is seen that manifestations of com-
pressibility of the water column correspond to those cases, when the depths exceed
the critical value
H
c
.
Note one more interesting effect, related to the shape of the ocean bottom relief.
We intend the possibility for the lowest mode of elastic oscillations to be captured by
regions of local depressions of the ocean bottom (deep-water trenches or hollows).
Indeed, if the lowest mode originates in the region of a local maximum depth,
H
max
,
then it exhibits the period
T
max
= 4
H
max
/
c
. This mode cannot leave the region,
where it originated, since to do so it would have to propagate up the slope.
In conclusion of this section, we note that in tsunami catalogues [Soloviev
et al. (1997), Soloviev, Go (1974), (1975)] cases are repeatedly mentioned,
when tsunami waves throw out onto the coast deep-water fish (unknown species,
'sea monsters'). Moreover, cases have been described, when deep-water fish rised
up to the surface before an earthquake. We shall present two quotations from
the catalogue of tsunamis in the Mediterranean sea.
1783, February, 5,
12
h
τ
30
min. Calabrian Arc.
38
◦
25
N
,
15
◦
50
E.
Catastrophic Calabrian earthquake, which initiated a long period of seismic ac-
tivity in the south-west of Italy continued for several years.
Unusual events at sea are described, which can be considered short-time
precursors of the earthquake. At the beginning of February close to Messina and at
other sites deep-water fish Chichirella started appearing in large numbers, although
it usually does not leave the seabed and digs into the seabed silt.
1887, February (March), 23,
6
h
20
min. Ligurian sea, Italy, France.
43
◦
42
N
,
08
◦
03
E.
Strong earthquake that occupied an area of 570,000 km
2
Deep-water fish or fish
rarely seen in winter were found thrown out onto the beaches of Nizza, San-Remo,
Savona.
Such a behaviour of the marine inhabitants is readily explained by the distribution
of the dynamic pressure amplitude. In attempts at avoiding the influence of uncom-
fortable changes in pressure, caused by the underwater earthquake, the fish goes to
those regions, where the variations in pressure are minimal, i.e. to shallow-water
regions or to the surface.
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