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Tsunami Hazard in New Zealand

New Zealand has had tsunami in the past so people need to know what to do.

In New Zealand there here have been about 10 tsunami higher than 5 metres since 1840. Some were caused by distant earthquakes, but most by seafloor earthquakes not far off the coast.

A nearby coastal seafloor earthquake is the only warning people may get before a tsunami arrives. Such a tsunami can arrive within minutes - before there is time to issue a warning.

Very large distant earthquakes give more warning time. Some tsunamis are turbulent, foaming walls of water filled with debris and sand that crash ashore and sweep inland. Others are just rapidly rising or falling water levels over minutes to an hour. Both are dangerous. Both can travel over land at speeds faster than a person can run.

Risk of tsunami

The risk of tsunami in the Pacific Ocean is higher than for other oceans because of the Pacific Plate boundary or ‘Ring of Fire’. This zone has frequent earthquakes due to movement of the tectonic plates. New Zealand is therefore at risk of tsunami.

Sources of tsunami

Tsunami can come from local sources or far off sources. If a tsunami is generated close to shore then there may be very little warning. Tsunami generated by local earthquakes can arrive at the nearest coast before scientists can calculate the location of the earthquake and issue a warning and activate tsunami sirens. A strong earthquake may be the only warning of a tsunami.

If an earthquake lasts for more than a minute or is strong enough that it is difficult to stand then you need to move to higher ground. If you are at the beach and notice the sea suddenly recedes you also need to move to higher ground.

For tsunamis caused by distant earthquakes such as in South America there will be warnings. Warnings from the Pacific Tsunami Warning Centre PTWC, will be issued by the Ministry of Civil Defence and Emergency Management if needed. Tsunamis from South America, Alaska and Japan take more than 12 hours to reach New Zealand.

What to do if there is a tsunami

Locally generated tsunamis

  • If you are near a beach and feel a strong earthquake or notice the sea receding unusually, move to higher ground immediately (you may have only a few minutes).
  • Take yourself and others to higher ground - at least 35m above sea level is best.
  • If you cannot go higher, go inland - at least 1.5km.
  • Do not return for at least one hour or until advised by authorities.
  • Do not wait for a tsunami warning before moving to higher ground.
  • Avoid rivers and inlets because they magnify tsunami waves.

Long distance tsunamis

Authorities will give warnings about tsunamis if they are coming from far away. These warnings come from the PTWC in Hawaii to MCDEM in New Zealand. Tsunami warnings will be passed to you on your local radio, via a siren, or loud hailer.

  • Turn your radio on and follow instructions.
  • Take essential items with you if you are told to evacuate.
  • Do not go to the beach to watch the waves come in.
  • If you are on a boat, stay on the water. Do not return to port. Tsunamis can cause rapid changes in water levels and unpredictable currents in ports and harbours.

 

Many people live close to the coast and need to know what to do if there is a tsunami. What warning will people get that a tsunami is coming? Image: LEARNZ. 

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New Zealand is at risk of local tsunami and tsunami from a long way away. This map shows far off sources of tsunami for New Zealand. Can you find out where the last tsunami that hit New Zealand came from? Image: GNS Science.

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Long distance tsunamis can be large events but there will be several hours warning. This map shows how long it could take for waves to reach New Zealand. What is your emergency plan if there is a tsunami near you? Image: Christchurch City Council. 

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The main sources of locally generated tsunamis are from earthquakes in the Kermadec Trench, Hikurangi Trench or Puysegur Trench. Why do you think these areas are of greater risk than some other closer off-shore faults? Image: GNS Science.

Design and complete a poster to tell people what to do if there is a tsunami.