Tsunamis

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 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.

How fast can they 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 the 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 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.

What are the natural warning signs of a 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 or a train), tell your family and friends.  This sound may be heard before a tsunami arrives.

What is the risk to the Northern Territory from tsunami?

Historically, tsunamis are rare long the NT coastline.  The NT has a low to moderate risk from tsunami because it is protected by shallow waters and a large tidal variation.  The most likely impact is considered to be unusual tidal and current variations, though some coastal and island communities face some inundation risk, particularly if they are low lying.

Will we get an official warning?

The Joint Australian Tsunami Warning Centre (JATWC) 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.

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.

 

Where can I find more information on tsunamis in Australia?

More information is available through: