Geoscience Reference
In-Depth Information
intensity one may consider the flooded area, instead of the wave height. This char-
acteristic may turn out to be a successful and promising alternative to the wave
heights on the coast. A clear advantage of the flooded area consists not only in that
it can be conveniently measured by remote means (from satellites, airplanes, etc.),
but also in that this characteristic automatically reflects the scale of the catastrophe
that took place.
Abe and Hatori proposed to modify the magnitude scale so as to take into account
the weakening of waves, as the distance from the source increases [Abe (1979),
(1981), (1985), (1989); Hatori (1986)],
M
t
=
a
log
h
+
b
log
∆
+
D
,
where
h
is the maximum wave amplitude on the coast measured from the foot up to
the crest in meters,
is the distance from the earthquake epicentre to the point of
measurement in kilometers,
a
,
b
and
D
are constants. Such a definition resembles
the definition of magnitude in seismology.
An essentially different approach to the definition of tsunami magnitude was put
forward in [Murty and Loomis (1980)]. Here, the calculation of magnitude is based
on estimation of the tsunami's potential energy
E
(in ergs),
∆
−
19)
.
ML = 2 (log
E
The definition of magnitude based on the wave energy is, naturally, the most ad-
equate definition, from a physical point of view. However, it is not always possible
to calculate the wave energy. At any rate, at the present-day stage calculations can
be based on the potential energy of the initial elevation of the water surface, con-
sidering it to be identical to the residual displacements of the sea-floor. These dis-
placements are calculated from the earthquake parameters by the Okada formulas
[Okada (1985)].
It must be noted that the Imamura-Iida magnitude or the Soloviev-Imamura in-
tensity gives an idea of the wave height on the coast and, consequently, permit to
judge the scale of destructions. But, although the Murty-Loomis tsunami magnitude
ML is a physically correct quantity, it cannot be unambiguously related to the man-
ifestation of a tsunami on the coast.
Recently, a new detailed 12-point descriptive tsunami intensity scale was pro-
posed in [Papadopoulos, Imamura (2001)]. Its elaboration was based on the more
than 100-years-long experience, accumulated by seismologists in drawing up earth-
quake intensity scales. This scale is not related to any quantitative physical parame-
ters (wave amplitudes, energy and so on), it is organized in accordance with the fol-
lowing three features:
(A) Its influence upon people
(B) Its impact on natural and artificial objects, including boats of different sizes
(C) The damage caused to buildings
Therefore, a tsunami of large amplitude that hits a weakly inhabitated coast may
be assigned a low intensity in accordance with the Papadopoulos-Imamura scale.
