Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 111

4.0
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Time UTC (hours)

Figure 5.1. Example of a tsunami trace from a tide gauge record, Port Villa,
Vanuatu, November 1999. Note the number of waves in the tsunami wave train.

steep topographic gradient of the shore relative to the wavelength of a tsunami
allows the wave to surge onto the land at tremendous speed. For example, the
Aitape tsunami in Papua New Guinea on the 17th July 1998 travelled across
thesand barrier at Sissano Lagoon at approximately 80 km h − 1 (the wave was
15mhigh as it travelled across the beach). The destructive capabilities of such
ahigh-velocity wave across dry land are highlighted when it is remembered
that moving water in such situations has 1000 times the force of air travelling
at the same velocity. Though strictly not an earthquake-generated tsunami, as
it appears to have been generated by a submarine landslide which itself was
caused by an earthquake, the size and velocity of the waves during this event
were catastrophic to the local people living on the sand barrier (the official
death toll was 2200 people but it was quite possibly much higher) (Kawata et al .,
1999).
Tsunamis have also travelled vast distances across oceans to impact shores
far removed from the generating earthquake. The most famous of these 'telet-
sunamis' is the Boxing Day (26th December) tsunami of 2004 that impacted a
number of countries in the south Asia region and also east Africa (Somalia and
Kenya). A magnitude 9.3 earthquake occurred just to the west of Banda Aceh off
thenorthern tip of Sumatra. Within minutes tsunamis 10--15 m high impacted
thewest coast of northern Sumatra resulting in the deaths of over 100 000 peo-
ple. The tsunami wave train radiated northwards into the Bay of Bengal, south
to Western Australia and Antarctica, west towards India, Sri Lanka, the Maldives
and East Africa and east towards Thailand and Malaysia. The eastward travel-
ling waves first approached the coast of Thailand as a trough causing the water