Geoscience Reference
In-Depth Information
In the nearby zone, the orientation of energy emission was related to the large
extension of the tsunami source. The long and narrow region (stretched out in
the meridional direction), in which deformations of the ocean bottom, caused by
the earthquake of December 26, 2004, were concentrated, formed waves of large
amplitude in the perpendicular, i.e. longitudinal, direction. The waves propagating
in the meridional direction were of essentially smaller amplitude. This effect not
only manifests in simulations, but also follows from analysis of records of mare-
ographs and expedition data. Thus, for example, on the opposite coast of the Indian
Ocean, 5,000 km from the source, on the Somalia coast (East Africa) run-up heights
from 5 up to 9 m have been observed [Synolakis et al. (2005)]. From numerical cal-
culations of the distribution of maximum amplitudes (Fig. 5.10) it is seen that one
of the 'wave rays' ends precisely on this coast.
The main factor, determining the orientation of energy propagation in the re-
mote zone, is now the topography of the bottom of the World Ocean (Fig. 5.11 in
the coloured inset). Analysis of the Indonesian tsunami of December 26, 2004, re-
veals the important role of mid-oceanic ridges in channeling the tsunami energy.
From comparison of Figs. 5.10 and 5.11 it is readily seen that the south-west Indian
Ridge, as well as the Mid-Atlantic Ridge served as waveguides for propagating
the tsunami towards the Atlantic. The Pacific Antarctic Ridge and south-east Indian
Ridges, and, also, the East-Pacific Rise contributed to the penetration of waves into
the Pacific Ocean. It is interesting that ridges cope well with the role of waveguides
until their curvature does not exceed a critical value. Thus, for example, the sharp
bend in the Mid-Atlantic Ridge at the parallel of 40
◦
S contributed to the waveguide
losing beams. As a result, waves of noticeable amplitude were observed at the At-
lantic coast of South America. The numerical model pointed correctly to the signif-
icant amplitude (
1 m) in Rio de Janeiro. Regretfully, no other measurements were
carried out at any points of the Atlantic coast of South America.
In the sourthern direction from the source, the waves propagated along the Ninety-
East Ridge. In accordance with calculations they could have had a significant height
on the Antarctic coast. However, it was practically impossible to check this fact, owing
∼
Fig. 5.11 Bottom topography of World Ocean. Mid-oceanic ridges that essentially influenced
propagation of the Indonesian tsunami of December 26, 2004 (see also Plate 7 in the Colour Plate
Section on page 315)
