Geoscience Reference
In-Depth Information
and ambiguous. Besides the earthquake magnitude, a tsunami intensity may depend
on many parameters: the hypocentre depth, shape and orientation of the fault area,
duration of processes at the earthquake source, ocean depth, etc. It is seen that suc-
cess in the investigation of tsunami generation is related not only to resolution of
the hydrodynamic part of the problem, but also to progress in resolving such a diffi-
cult problem as description of the earthquake source. It must be noted that the large
spread is also due to the tsunami intensity not being a rigorously defined physical
quantity like, energy. At any rate, a certain positive correlation within the depen-
dence under consideration can be identified: earthquakes of higher magnitudes are
generally accompanied by tsunamis of higher intensities. The dependence presented
is a good illustration of the magnitude criterion applied in the tsunami warning sys-
tem. It is seen that the formation of practically all significant tsunamis (
I
>
2) was
due to earthquakes of magnitudes
M
w
>
7.
The process of tsunami generation has been studied relatively weakly, which
largely due to the fact that no measurements have hitherto been performed at tsunami
sources. Indeed, all the information on processes proceeding at a tsunami source
has been obtained by remote measurements done with mareographs (coastal and
deep-water devices), hydroacoustic systems or seismogrpahs. Evidence provided
by witnesses of underwater earthquakes is quite scarce, and it naturally concerns
phenomena that took place at the ocean surface. In principle, it became possible
to investigate a tsunami formation at its source in 1996, when a set of measuring
devices comprising several sensors of bottom pressure (JAMSTEC, Japan Agency
for Marine-Earth Science and Technology) was established on the continental slope
close to the Japanese islands. The Tokachi-Oki earthquake of 2003 was the first
strong seismic event with its epicentre located in the immediate vicinity of the JAM-
STEC sensors. In Sect. 3.1.6 data will be analysed on variations of the bottom pres-
sure, registered by the JAMSTEC sensors at the source of the 2003 Tokachi-Oki
earthquake.
In simulating a tsunami of seismic origin a convenient method is usually ap-
plied that permits not to deal with the description of the generation process in
a straightforward manner. The 'roundabout manoeuvre' consists in the following.
An earthquake is considered to suddenly cause residual deformations of the ocean
bottom (actually the duration of the process at the source may amount to 100 s and
more). The residual deformations of the bottom are deduced from the parameters
of the earthquake source. The calculation technique will be presented in detail in
Sect. 2.1.3. Then, the assumption is made that the displacement of the bottom is
simultaneously accompanied by formation at the surface of the ocean of a perturba-
tion, the shape of which is fully similar to the residual deformations of the bottom.
The perturbation of the water surface (the initial elevation), thus obtained, is then
applied as the initial condition in resolving the problem of tsunami propagation.
It is interesting that the possibility to transfer sea-floor perturbations up to the sur-
face is based on the actual structure of the equations for shallow water requiring
the sole condition that the sea-floor deformation process be rapid. If, contrariwise,
one applies, for instance, potential theory, then, even if the process is instantaneous,
the perturbation of the liquid's surface and the residual deformation will differ from
each other.
