Earthquakes in Kermadec Trench, South America most likely to pose tsunami risk to Auckland

Tuesday, 2 April 2019

The last major tsunami in Auckland, in 1960, was caused by an earthquake in Chile (file photo).

Earthquakes in the Kermadec Trench and South America pose the biggest threat of generating major tsunamis as high as five metres in Auckland in the next year, GNS Science says.

Senior tsunami scientist Dr William Power said tsunamis in Auckland were always a possibility, but earthquakes from those places could have a massive effect if a tsunami followed, depending on strength.

The Kermadec Trench runs between East Cape and Tonga and is part of the Pacific Ring of Fire. It is an area of frequent quake activity and occasional large quakes, Power said.

GNS modelling suggested those earthquakes could generate tsunamis with a maximum height of 3m at the time of hitting the coast and had a 1 in 300 (0.33 per cent) chance of happening in the next year.

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The Kermadec Trench runs between East Cape and Tonga, and is a region of frequent quake activity.

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Tsunamis can potentially be generated almost anywhere along the Pacific Ring of Fire and can potentially reach New Zealand.

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A tsunami with a maximum height of 5m at the coast had a probability of about 1 in 2000 (0.050 per cent) per year, he said.

A tsunami siren test will sound at noon on Sunday in the Rodney and Waitākere areas.

Those tsunamis could run-up higher onshore in some places.

'The probabilities are likely to be lower than these within large harbours such as Waitematā and on the west coast,' Power said.

'The probabilities are considerably higher on Great Barrier Island, and somewhat higher on the coasts north of Auckland city.'

The last major tsunami to hit Auckland was in 1960. It was caused by an earthquake in Chile.

Though the waves were only about a metre high in the Auckland Harbour, the strong currents caused ferries to lose control both at the Queen's Wharf basin and at Devonport.

People are being advised to be aware of tsunami risks and how to respond to any threat.

But any large earthquake or tsunamis generated from around the Pacific Ring of Fire could potentially reach New Zealand, Power said.

'The size of impact on Auckland depends on many factors, and on the whole more distant earthquakes need to be larger to have the same effect as closer ones.

'A magnitude 9 earthquake in South America could have similar tsunami impact to a magnitude 7 earthquake close to Auckland.

'Shallower earthquakes tend to cause larger tsunamis than deeper ones, all other things being equal.'

On the whole, Auckland city faced a lower threat from tsunami than many other parts of New Zealand.

SIREN TO SOUND

On April 7, the region's twice-yearly tsunami siren test will sound at noon in the Rodney and Waitākere areas.

Coinciding with the end of daylight saving, the test sequence will be three sets of tones, each signalling a specific action people are advised to take in a real emergency.

Auckland Emergency Management acting director Sarah Sinclair said the test was important to make sure the sirens worked and that Aucklanders knew how they sounded like.

'There's no cause for alarm during the testing and the public isn't required to do anything,' she said.

KNOW THE SIREN TONES

The siren patterns and the way they will sound in an emergency are:

Alert (dash – dash – dot – dot) sounds for 15 minutes and indicates the threat of a tsunami. Evacuate beaches, prepare for full evacuation and monitor the radio or television for more information.

Evacuate (dot – dot – dot) continuous bursts for 15 minutes indicates immediate evacuation to the nearest safe high ground. Avoid using personal transport unless essential.

All clear (continuous tone) sounds for 5 minutes indicate threat has passed. Return to premises if they have not been affected and follow directions of the emergency services if your area has been affected.

WHAT TO DO IF THERE IS A TSUNAMI

Civil Defence advises anyone on the coast to leave immediately if there is a long and strong earthquake.

If you are at the coast and feel a strong earthquake that makes it hard to stand up, or a weak rolling earthquake that lasts a minute or more, see sudden rise or fall in sea level, or hear loud and unusual noises from the sea, then move immediately to the nearest high ground, or as far inland as you can. Walk or bike if possible. Do not wait for official warnings.

For more information, visit civildefence.govt.nz.