Introduction - Extreme Events: A Physical Reconstruction and Risk Assessment - page 1

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Introduction
The problem with natural hazard risk assessments
There is a problem with many natural hazard risk assessments. They do
not incorporate long-term and/or prehistoric records of extreme events; other-
wise known as natural hazards when they affect humans physically, psycholog-
ically, socially or economically. Short historical records are frequently assumed
to be a true reflection of the long-term behaviour of a hazard. Historical records
may be appropriate, in this regard, where they extend for at least several cen-
turies or even a millennium such as in China. However, in many countries, like
theUnited States, United Kingdom and Australia, the historical record is often
not much longer than 100 years. Many assessors of risks from natural hazards
see these short records as appropriate for determining the natural variability
of a hazard. From this they extrapolate to determine the magnitude of less fre-
quent, higher magnitude events and construct probability distributions of the
occurrence of a hazard at various return intervals. Inherent in this process is the
assumption that natural hazards occur randomly over a variety of time scales
and that the mean and variance of the hazard do not change. This may be true
in certain circumstances, especially shorter time periods, but is often not the
case for longer intervals. When we rely upon short historical records we run
thereal risk of not capturing the natural variability of the hazard. Here lies
thecruxofthe problem -- when we do not understand the true nature of the
hazard in question we cannot hope to make realistic assessments of community
vulnerability and exposure and we increase the chance of making an unreliable
estimate of the risk of that hazard. Our ability to increase community safety and
reduce economic loss is dependent upon our understanding of the behaviour of
the hazard. Short historical records rarely display sufficient information for us
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