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Fig. 3.4 Dependences of maximum amplitude of 'fast' surface oscillations (thin line) and of maxi-
mum amplitude of gravitational waves (thick line) upon the distance from the source centre. Curves
1, 2 correspond, respectively, to displacements of the bottom with residual deformation (piston-like
displacement) and without residual deformation (membrane-like displacement). Calculations are
performed for τ = 1, R = 10
oscillations are to be considered local effects, the appearance of which should be
noticeable either immediately at the tsunami source or at relatively small distances,
not exceeding several sizes of the source.
The general picture of tsunami excitation in a compressible ocean can be rep-
resented as follows. When a vertical displacement of the ocean bottom occurs
the water column is shifted correspondingly, and under the force of gravity it
gradually starts to spread out, at the same time undergoing elastic oscillations.
Therefore, the tsunami source not only serves as a source of gravitational tsunami
waves, but also of low-frequency acoustic waves, the emission of which is possible
at the characteristic frequencies
ν k = 0 . 25 c (1 + 2 k ) H 1 . Waves of low energy-
carrying modes exhibit lengths that significantly exceed the width of the underwater
acoustic channel and that, consequently, cannot be captured by it. In this case the
entire thickness of the ocean must serve as the waveguide, while the elastic waves
considered will effectively be scattered on irregularities of the ocean bottom and
of the water surface and be absorbed by the elastic ocean bottom. Most likely, it is
precisely for this reason that at large distances from the source only relatively weak
components of the signal are observed at frequencies
ν
> 1 Hz, and precisely they
are termed the T-phase.
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