Engineering Geology

Chapter 8
record of tsunamis is required in any risk assessment. This involves a study of the seismicity
of a region to establish the potential threat from earthquakes of local origin. In addition,
tsunamis generated by distant earthquakes must be evaluated. The data gathered may highlight
spatial differences in the distribution of the destructiveness of tsunamis that may form
the basis for zonation of the hazard. If the historic record provides sufficient data, then it may
be possible to establish the frequency of recurrence of tsunami events, together with the area
that would be inundated by a 50, 100 or even 500 year tsunami event. On the other hand, if
sufficient information is not available, then tsunami modelling may be resorted to using computer
models. These provide reasonably accurate predictions of potential tsunami inundation
that can be used in the management of a tsunami hazard. Such models permit the extent of
damage to be estimated and the limits for evacuation to be established. The ultimate aim is
to produce maps that indicate the degree of tsunami risk that, in turn, aids the planning
process, thereby allowing high risk areas to be avoided or used for low intensity development.
Models also facilitate the design of defense works.
Various instruments are used to detect and monitor the passage of tsunamis. These include
sensitive seismographs that can record waves with long period oscillations, pressure recorders
placed on the sea floor in shallow water, and buoys anchored to the sea floor and used to measure
changes in the level of the sea surface. The locations of places along a coast affected by
tsunamis can be hazard mapped that, for example, show the predicted heights of tsunami at a
certain location for given return intervals (e.g. 25, 50 or 100 years). Homes and other buildings
can be removed to higher ground and new construction prohibited in the areas of highest risk.
Resettlement of coastal communities and prohibition of development in high risk areas has
occurred at Hilo, Hawaii. However, the resettlement of all coastal populations away from possible
danger zones is not a feasible economic proposition. Hence, there are occasions when
evacuation is necessary. This depends on estimating just how destructive any tsunami will be
when it arrives on a particular coast. Furthermore, evacuation requires that the warning system
be effective and that there be an adequate transport system to convey the public to safe areas.
Breakwaters, coastal embankments and groves of trees tend to weaken a tsunami wave,
reducing its height and the width of the inundation zone. Sea walls may offer protection
against some tsunamis. Buildings that need to be located at the coast can be constructed
with reinforced concrete frames and elevated on reinforced concrete piles with open spaces
at ground level (e.g. for car parks). Consequently, the tsunami may flow through the ground
floor without adversely affecting the building. Buildings are usually orientated at right angles
to the direction of approach of the waves, that is, perpendicular to the shore. It is, however,
more or less impossible to protect a coastline fully from the destructive effects of tsunamis.
Ninety per cent of destructive tsunamis occur within the Pacific Ocean, averaging more than
two each year. The Pacific Tsunami Warning System, PTWS, is a communications network
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