Geoscience Reference
In-Depth Information
1.3 Tsunami Magnitude and Intensity
Estimation of the degree of tsunami danger for one or another coast (long-term
tsunami forecast) is primarily based on the statistical analysis of events, that oc-
curred in the past. Tsunamis evidently vary in strength within wide limits: from
weak waves, that can be registered only with the aid of instruments, up to terri-
ble catastrophic events devastating the coast along hundreds of kilometers. How
can one estimate the strength of a tsunami? The point is that without the introduc-
tion of some quantitative characteristic of this strength it is not only impossible to
perform any statistical analysis, but also to speak of estimating the degree of dan-
ger. The determination of such a quantitative characteristic is quite a non-trivial
problem, the ultimate resolution of which has not yet been achieved. Similar diffi-
culties are encountered by seismologists determining the strength of an earthquake.
On the one hand, an earthquake is characterized by objective physical parameters
showing the energy emitted by the source, or the released seismic moment. These
parameters are measured quantitatively, and the scale of earthquake magnitudes is
made to correspond to them. On the other hand, there exists a descriptive scale of
earthquake intensities, which is related to the so-called macroseismic data, based on
the results of in situ studies. Clearly, in practice, it is precisely the intensity scale that
is important, but contrary to the magnitude scale it is not rigorous, from a physical
standpoint.
Going back to tsunamis, we note that this phenomenon is also characterized, on
the one hand, by objective and quantitatively measurable parameters (energy, am-
plitude, period, etc.), and on the other hand—by subjective descriptions, reflecting
the scale and degree of the destructions caused by the wave or the character of
its manifestations on the coast. Like in the case of earthquakes, for estimation of
the tsunami danger precisely these subjective descriptions are more important than
abstract physical parameters. The inhabitants of coastal regions are not interested
in the energy of the approaching wave in joules, but they are interested in whether
the wave is dangerous to their lives, what damage may be done and how it can be
avoided. And, until further modelling is realized of the entire process starting from
the actual formation of a wave up to its run-up onto the shore, such a situation will
remain intact.
The first attempt at classification of tsunamis was made by Sieberg, who intro-
duced a six-point scale of tsunami intensities by analogy with the scale of earth-
quake intensities [Sieberg (1927)]. This scale was not related to the measurement
of physical parameters (wave heights, run-up lengths, etc.); it was based on the de-
scription of macroscopic effects, revealing the degree of destruction. Subsequently,
the Sieberg scale was somewhat modified [Ambraseys (1962)].
The Sieberg-Ambraseys tsunami intensity scale
1. Very light. Waves can only be registered by special tide gauges (mareographs).
2. Light. Waves noticed by those living along the shore. On very flat shores waves
are generally noticed.
