Tsunamis are long waves
generated by submarine earthquakes. Tsunami is Japanese for "harbour
wave". They are
often called tidal waves; but this is a misnomer, since tsunamis have nothing
to
do with tides.
Before 2004 the
strongest tsunami in known history was produced by the eruption of the
Krakatau of the Sunda
Island group in 1883. It reached a wave height of 35 m and claimed
36,830 lives. Four
tsunamis with heights in excess of 30 m have been documented in the Pacific
Ocean since 684 A.D. A
strong tsunami in the Atlantic Ocean was observed in 1755 after an
earthquake near Lisbon
(Portugal).
In the vicinity of the
epicentre of an earthquake, tsunamis can result in extreme wave heights.
Once they reach the
open ocean and travel through deep water tsunamis have extremely small
amplitudes but travel
fast, in 4000 m water depth at about 700 km/h.
(This speed can be estimated
by using the wave speed equation given above:
We have g = 9.8 m/s,
h = 4000 m, so (9.8 x 4000)1/2 = 200 ms-1= 700 km/h.) On
approaching a coast they build up wave height again through shoaling. The
period of tsunamis is in the range 10-60 minutes.
Tsunamis were used to
estimate the depth of the ocean in 1856, when direct depth measurements
were virtually
impossible, by observing their phase speed. The result for the North Pacific
was
4200 - 4500 m, which
was a considerable improvement on the previous estimate of 18,000 m.
The most destructive
tsunami known occurred on 26 December 2004. It was generated by an
earthquake in the
vicinity of the Andaman Islands and northern Sumatra and caused death and
destruction in
countries around the Indian Ocean. The death toll is estimated at between
265,000
and 320,000, although a
final accurate figure may never be known.
Because of the
destructive force of tsunamis, a tsunami warning system has been set up. It
uses
seismographic
observations of earthquakes and calculates arrival times around the coastlines
of
the oceanic basin.
Another possibility is the monitoring of compression waves linked with
volcanic eruptions;
they travel at the speed of sound (1500 m s-1) in the SOFAR channel. No
warning
system is available for areas in the vicinity of the epicentre.
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