Physics of Tsunamis by Boris W. Levin & Mikhail A. Nosov

Author:Boris W. Levin & Mikhail A. Nosov
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Figure 4.4 presents the dependences of amplitudes of gravitational waves and of “fast” surface oscillations upon the distance from the source center r. The data correspond to the duration of the ocean bottom displacement, , which does not violate the general nature of the conclusions, since the form obtained for the solution of (4.46) and (4.47) reveals that the parameter does not affect the decrease of amplitude with distance. In all cases, the amplitude varies weakly immediately above the source zone. Outside the source zone the amplitude of gravitational waves decreases approximately like (corresponding to the known asymptotic estimates, see Pelinovsky 1996), while the amplitude of oscillations drops like . Here, displacements of the ocean bottom with and without residual deformation lead practically to the same oscillation amplitude, while the amplitudes of gravitational waves differ noticeably. Figure 4.4 permits to conclude that “fast” surface oscillations are 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 represented 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 . 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 Hz, and precisely they are termed the T-phase.

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