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to the absence of mareograph stations. Two stations (one Japanese, 'Syowa', and
the other French, 'Dumont d'Urville'), located approximately 2,000 km to the West
and East, respectively, from the point of incidence of the main 'beam', registered
moderate wave heights (of amplitude 60-70 cm).
In most cases of tsunami records, obtained in the eastern and central regions of
the Indian Ocean (Fig. 5.9), only the first several waves exhibited the maximum
amplitude. Further, the amplitude approximately exponentially decreased in time.
The duration of anomalously large level oscillations amounted to 12 h. Numerical
simulation shows that such a character of level oscillations at the coast corresponds
to those cases, when waves of maximum amplitude, being focused by an extended
source, travelled directly from the source to the observation point. The pronounced
orientation of wave emission, seen well in Fig. 5.10, once more confirms the fact
that one of the most important factors determining tsunami propagation in the near
zone is the shape of the source.
Tsunami records, obtained in the western part of the Indian Ocean and in other
oceans, reveal a significant duration of tsunami 'sounding', while level oscillations
of maximum amplitude were observed with an essential delay after the onset of
the first wave. This is due to enhancement of the role of waves reflected from
the coasts and from irregularities of the ocean bottom, and also to propagation
along natural waveguides—submarine ridges. The relatively slow, but energy-saving
waveguide propagation provides for a late onset of the largest waves. Numerical
simulation has shown that a wave perturbation often consists of two (or more)
clearly distinguishable packets. One of them has a relatively small amplitude and
propagates straightforwardly with a high velocity, 'taking advantage of' deep ar-
eas of the ocean. The second packet has a greater amplitude, but propagates slower
along underwater ridges (elevations).
It is interesting that the tsunami penetrated the Pacific Ocean via two routes:
directly from the Indian Ocean and through the southern part of the Atlantic Ocean,
bypassing the Drake Passage between South America and Antarctica. Numerical
simulation reveals the waves that arrived in the Pacific Ocean from the West from
the Indian Ocean, and those that came from the East through the Atlantic, to have
commensurable amplitudes. For all the Pacific coast, with the exception of southern
Chile, the onset of waves arriving from the East occurs later.
To conclude the section we note that no destructions related to the tsunami of
December 26, 2004 were reported outside the Indian Ocean. But experience in ob-
servations and simulations of the global propagation of tsunamis shows that the pen-
etration of waves into all oceans is possible, in principle. Such a danger can be
withstood, if a global system of tsunami warning is created.
5.3 Tsunami Run-up on the Coast
Of all problems relevant to tsunami dynamics, the description of wave transforma-
tion in the coastal belt, together with flooding of the coastal zone or uncovering of
the ocean bottom, represents one of the most difficult tasks. This is, first of all, due
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