Mechanics of Fluids

480 Flow with a free surface
10.14 TSUNAMIS
Cyclical or wave motion in the seas is established in three broadly different
ways. Gravitational forces have a fundamental influence on all three, but
they are distinguished by the fact that certain features of the motion differ
by orders of magnitude. First, there are the tides. Tidal motion is the regular
raising and lowering of sea level, with a periodic time of approximately 12 h
to 12.5 h, caused principally by the gravitational pull of the moon, with
the sun and planets also making smaller contributions. Second, there are
surface waves. These are mainly generated by the movement of the winds
over the seas, although the motion of ships and other surface disturbances
also contribute to this form of wave motion. Aspects of the motion of surface
waves were covered extensively in Section 10.13. Third, there are tsunamis.
The tides and wind-generated waves are with us all the time, but tsunamis
only occur infrequently. Tsunamis will be discussed here, where, in order
to focus attention on their main characteristics, we shall neglect tidal action
and wind-generated wave motion.
The term tsunami is a Japanese word which means harbour wave. The
term tidal wave was formerly used to describe the phenomenon, but this
expression has been abandoned because it incorrectly attributes the waves
to the tides. A tsunami is a natural phenomenon, consisting of a series of
waves generated at sea as a result of a triggering event. Once these waves
have been set in motion they are very persistent, eventually arriving at a
coastline, and it is there that the full effect of a tsunami is manifested. No
two tsunamis are the same. A small tsunami, resulting for example from
an earth tremor, is scarcely perceptible and is only detectable with sensitive
measuring equipment. However, at the opposite extreme, a tsunami can
cause an enormous amount of damage to property, with associated human
casualties. This was vividly demonstrated by the South East Asian tsunami
of 26 December 2004, resulting from an earthquake of magnitude 9.0 on
the Richter scale, when some 300 000 lives were lost, mainly in Indonesia,
Sri Lanka and India.
There are four main ways in which tsunamis are initiated. Earthquakes are
the most frequent cause. Then, in descending order of occurrence, tsunamis
can also be generated by a major landslide, the eruption of a volcano or the
descent into the sea of an object, such as a meteorite or asteroid, from space.
Each tsunami is unique, depending in part on the details of the triggering
event, the distance from the origin to the neighbouring coastlines, and the
morphology of the coastline. We shall examine the broad characteristics of
a tsunami, in turn considering events at the location where it originates, the
movement of the waves over the deep ocean, and finally the arrival of the
waves in shallow water at the coastline.
We start our discussion of the origins of a tsunami by considering an
earthquake, which results from the relative movement of two tectonic plates
along a fault line, one plate passing beneath the other in a process known
as subduction. This movement causes the sudden release of an enormous
amount of strain energy, resulting in a substantial deformation of the sea bed,
some parts elevating, others subsiding. As a consequence, a large volume of