Risk and Avoidance - Tsunami - page 190

10

Risk and Avoidance

10.1 Introduction

dismissed as hyperbole by a primitive culture. Such atti-
tudes are natïve, doctrinaire, and ignore a body of evidence
that has validity.
Much of the world's coastline has never experienced a
large tsunami in its historical record, be it European or
otherwise. This is clearly illustrated in Fig. 10.2 , which
shows that portion of the world's coastline subject to his-
torical observation in 1500 and 1750. The boundaries in
Fig. 10.2 are liberally defined. Outside of eastern Asia, they
reflect the presence of Europeans rather than any other
society. In Chap. 9 , the period around 1500 was identified
as a possible time when a major asteroid struck the south-
west Pacific. No one, who could put pen to paper, was there
to observe any such event. A second event may have
occurred in the Australian region in the early part of the
eighteenth century. Again, the event would have gone
unrecorded except by the odd Dutch merchant ship or ship
of adventure.
There are various methods for assessing the vulnerability
of coastal populations to the threat of tsunami. One of the
simplest is to map population densities. Such population
density maps are readily available over the Internet and
indicate that the most vulnerable regions of the world are
the coastlines of China, India, Indonesia, Japan, the Phil-
ippines, the eastern United States, the Ivory Coast of Africa,
and Europe. This approach ignores the economic impact of
tsunami. Without belittling the death tolls due to tsunami
that have occurred along isolated coastlines such as the
Aitape coast of Papua New Guinea or the Burin Peninsula
of Newfoundland, tsunami will have their greatest impact
along densely populated coasts of developed countries or
where large cities are located. For example, the next
earthquake to strike Tokyo would have a worldwide eco-
nomic impact. Here, any associated tsunami would destroy
the shipping infrastructure so vital to that city's economy. It
is possible to evaluate similar vulnerable coastlines in two
ways. First, densely populated, economically developed
coastlines can be detected by the amount of light they emit
at night. The United States Air Force operates the Defense

There are four problems in assessing the risk of modern
tsunami to any coastline. All four suffer from popular
misconceptions. The first problem involves the construction
of probability of exceedence curves for the occurrence of
tsunami based upon historical records. Such an approach is
flawed logically and scientifically. It runs into a problem at
the extreme end because many coastlines are devoid of
credibly documented events. Second, it is often assumed
that a coastline is immune from the threat of large tsunami
if it has not recorded any. Any study attempting to show
otherwise is assumed fundamentally wrong. This concept
treats the occurrence of tsunami as being some stochastic or
random process mainly generated by earthquakes. Because
the sea is flat, so is the seabed. Any idea of submarine
landslides is discarded. So too is any consideration of vol-
canoes as a cause of tsunami unless the smoke can be seen
on the horizon. The idea that asteroids could cause tsunami
is considered erroneous because no such phenomenon has
been observed in the European-based historical record. The
laws for tsunami cannot be understood just by documenting
tsunami that have occurred in the historical record. They
must be set within the context of such events over hundreds
of millions of years. Large catastrophic tsunami tend to
occur because of the same processes that produce small
ordinary tsunami. Conversely, if catastrophic phenomena
such as asteroids have generated large tsunami, then they
can be implicated in some of the smaller, more frequent
events. Third, tsunami events of all sizes tend to be clus-
tered in time. This includes the now recognized phenome-
non that earthquakes close to subduction zones do not
necessarily close a seismic gap (Lorito et al. 2011 ); but will
recur in the same area until they do. Clustering of events
also incorporates the notion of coherent catastrophism
involving asteroid impacts (Asher et al. 1994 ). Fourth,
legends about tsunami, for whatever reason, are dismissed
as myths, and, if any legend in a tsunamigenic region
describes a tsunami as bigger than the historic record, it is