Tsunami Information Sheet
Tsunami waves are different from normal waves
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 movement of water all the way to the sea floor.
BEWARE: you cannot surf a tsunami!
A tsunami can really move!
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.
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.
What is a tsunami?
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.
What causes a tsunami?
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.
The natural warning signs of tsunami
In Australia you will most likely receive an official tsunami warning. However, before a
tsunami arrives you may, but not always, experience some natural warning signs.
You may:
• FEEL the earth shake. If you are near the ocean and you feel the ground shake, a strong
earthquake may have occurred and possibly caused a tsunami. However, you may not feel
the earth shake if the earthquake occurred a long way away.
• SEE the ocean drop. Before a tsunami arrives, the ocean level may (but not always) drop
dramatically before returning as a wall of water. If you notice that the water is disappearing,
tell your family and friends and prepare to move to higher ground.
• HEAR an unusual roaring sound. If you hear a loud roaring sound from the ocean (a bit
like an aeroplane ora train), tell your family and friends. This sound may be heard before a
tsunami arrives.
English schoolgirl saves 100 lives
In December 2004, 10-year-old English schoolgirl, Tilly Smith, was enjoying a family holiday
on the coast of Thailand. On the morning of 26 December, her family were enjoying a walk on
the beach when Tilly noticed the ocean drop dramatically. It then returned rapidly covered in
froth, which she described as looking like the top of a beer. As the ocean moved quickly
towards the beach, Tilly knew a tsunami was approaching having learned about tsunami in a
geography lesson a couple of weeks earlier. She raised the alarm and the beach was cleared,
saving 100 lives.
Will tsunami occur in Australia?
While the risk from tsunami is lower in Australia than other parts of the world, tsunami do
have an effect on our coastline. Even with relatively small tsunami, there could be strong
waves, rips and currents and unusual sea levels that can be dangerous. On 17 July 2006,
campers at Steep Point on the Western Australian coast were lucky to escape when a
tsunami hit their campsite. It was strong enough to pick up their 4WD vehicle and move it 10
metres up the beach. They were alerted to the tsunami by a loud roar from the ocean, which
is one of the natural warning signs of tsunami. Above is a picture of the damage to their
campsite.
Will we get an official warning?
The Joint Australian Tsunami Warning Centre runs 24 hours a day to identify any tsunami
threat to Australia. They use sea surface buoys and undersea sensors to measure
earthquake activity and the likelihood of a tsunami affecting Australia. In Australia, warnings
will be issued through the media, but you should also listen to emergency workers, lifeguards
and surf lifesavers.
Remember:
• If a tsunami warning is issued you should follow
instructions and move inland or to higher ground.
• Always listen to lifeguards, Surf Lifesavers and
emergency workers.
• If a tsunami warning is issued DO NOT go back to the
water until you are told it is safe.
IF YOU HEAR A TSUNAMI WARNING MOVE
INLAND OR TO HIGHER GROUND. DO NOT
STAY TO WATCH. DO NOT GO BACK TO THE
BEACH UNTIL YOU ARE TOLD IT IS SAFE.