Geoscience Reference
In-Depth Information
Fig. 2.20 Waves formed by a piston-like displacement of large amplitude. Calculations are
performed within the framework of long-wave theory: solid line—linear theory, dotted line—
nonlinear theory. The numbers, indicating the curves, show the ratio of the basin bottom defor-
mation amplitude and the basin depth
In his experiments Hammack applied the traditional method of wave registra-
tion on water, making use of a parallel-wire wavegraph. For this reason he had to
excite waves of higher amplitudes. From Fig. 2.19 it is seen that the points, cor-
responding to positive (empty circles) and negative (full circles) displacements of
the basin bottom, are stratified, i.e. lie respectively below and above the theoretical
dependence obtained within the framework of linear theory. Note that the 'stratifi-
cation' effect of experimental points is observed only in the case of large dimen-
sions of the generation area (for instance,
b
/
h
= 12
,
20 or 6,10). This is readily
explained by the large relative dimensions of the generation area (the quantity
b
/
h
)
being achieved by the choice of a small depth of water in the canal,
h
,owingto
which the relative amplitude of the displacement became comparable to the depth.
Large relative amplitudes were accompanied by manifestation of non-linear effects
[Kostitsyna et al. (1992)]. Attention must also be drawn to the fact that in the case of
prolonged displacements the 'stratification' effect became noticeably smaller, which
was evidently related to the drop in the relative wave amplitude. Figure 2.20 clearly
demonstrates the manifestation of non-linearity in the case of tsunami generation by
piston-like displacements of large amplitude.
2.3.3 Running and Piston-like Displacements
The idea of the deformation of a basin bottom being a process taking place simul-
taneously throughout the entire active region is, naturally, far from reality, although
it does serve as an illustrative model of tsunami generation. Actually, deformation
of a basin bottom is a consequence of the fault at the earthquake source propa-
gating along a certain plane. In the case of strong earthquakes the fault plane may
