Geoscience Reference
In-Depth Information
3. Rather strong. Waves generally noticed. Flooding of gently sloping coasts.
Light sailing vessels carried away on shore. Slight damage to light structures
situated near the coasts. In estuaries reversal of the river flow some distance
upstream.
4. Strong. Significant flooding of the shore. Buildings, embankments, dikes and
cultivated ground near coast damaged. Small and average vessels carried either
inland or out to sea. Coasts littered with debris.
5. Very strong. General significant flooding of the shore. Quay-walls and solid
structures near the sea damaged. Light structures destroyed. Severe scouring of
cultivated land. Littering of the coast with floating items, fish and sea animals
thrown up on the shore. With the exception of big ships all other type of vessels
carried inland or out to sea. Bores formed in estuaries of rivers. Harbour works
damaged. People drowned. Wave accompanied by strong roar.
6. Disastrous. Partial or complete destruction of manmade structures for some dis-
tance from the shore. Strong flooding of coasts. Big ships severely damaged.
Trees uprooted or broken. Many casualties.
Numerous attempts were made in Japan to introduce a quantitative characteristic of
the tsunami strength. Imamura introduced, and Iida further improved, the concept
of tsunami magnitude [Imamura (1942), (1949); Iida (1956), (1970)]. A proposal
was made to estimate the magnitude by the formula
m
= log
2
H
max
,
where
H
max
is the maximum wave height in metres, observed on the shore or mea-
sured by a mareograph. In practice, the Imamura-Iida scale is a six-point scale (from
−
1upto4).
In attempts at improving the Imamura-Iida scale S. L. Soloviev introduced
the following tsunami intensity:
I
=
1
2
+ log
2
H
,
where
H
is the average tsunami height on the coast closest to the source. At present
such a definition of the tsunami intensity is widespread, and the corresponding scale
is conventionally termed the 'Soloviev-Imamura tsunami intensity scale'.
Note that the Imamura-Iida definition of magnitude is, generally speaking, un-
ambiguous. It only requires knowledge of the maximum wave amplitude. The
Solviev-Imamura definition of intensity is not mathematically rigorous and, con-
sequently, provides for much 'freedom' in calculating the average height of tsunami
waves. At any rate, both scales are not very sensitive to small errors in the deter-
mination of wave heights, since it is the logarithms of these quantities that count.
It is also important to note that in the case of numerous historical events and, more
so, of prehistoric events (paleotsunamis) the only available information comprises
estimates of wave heights at a single point or at several points along the coast. Thus,
both scales are quite convenient and will still be applied in practice for a long time.
Anyhow, as a base characteristic to be measured in calculating the magnitude or
