Emergency Dispatch Vol 4 2014

14
SURVIVING A TSUNAMI
Thai Police Patrol Boats prepare for impact. / Thailand 2004
A tsunami is a series of fast, low and long ocean
waves, that move out from a central area, due to
a sudden disturbance of a large body of water.
In the past, tsunami have been known as tidal
waves but this is not correct as tsunami are not
caused by tides.
Tsunami (pronounced soo-nah-mee) is a
Japanese word meaning ‘harbour wave’. It
came from an old story about some Japanese
fishermen who, when out in the deep ocean, did
not notice a tsunami travelling underneath them.
When they arrived back at port they found the
harbour
destroyed.
In the deep ocean a tsunami can travel up to
950km/hr, the speed of an aeroplane and may
be less than one metre high. This is why the
Japanese fishermen did not notice the tsunami
in the deep ocean. Tsunami wavelengths are
extremely long with hundreds of kilometres
between wave crests in the deep ocean.
movement of water allthe way to the sea floor.
The most common cause of tsunami is large,
undersea earthquakes that occur along the
boundaries of the Earth’s tectonic plates. As
the plates move, stress slowly builds up over
hundreds, or even thousands, of years along
geological structures known as faults.
During an earthquake the stress along these
faults can be released within a matter of seconds.
This causes a sudden movement of the sea floor
which disturbs the ocean above. The affected
ocean then spreads out from the location of the
earthquake as a tsunami.
Although undersea earthquakes are the most
common cause of tsunami, they can also be
caused by volcanic eruptions, landslides and
even asteroid impact in or near the ocean.
As tsunami approach the coastline they slow
down but do not lose energy. The back of the
wave catches up with the front, causing the
wave to grow in height – up to several metres. It
is not so much this movement of water but the
energy moving through it that makes tsunami so
dangerous.
Tsunami are different from normal waves you see
at the beach. With a normal wind swell wave, it is
only the water on top of the ocean that is moving.
In contrast, the passage of tsunami involves the