Wellington's tsunami risk

NIGHTMARE SCENARIO: A computer-generated image showing the potential impact of a 10-metre wave on downtown Wellington.
NIGHTMARE SCENARIO: A computer-generated image showing the potential impact of a 10-metre wave on downtown Wellington.

A tsunami generated by an earthquake in Cook Strait could sweep kilometres inland within three or four minutes, putting the lives of up to 170,000 Wellington residents at risk.

There would be no time for any warnings and people need to think now about how they can get to higher ground immediately after a strong quake, says Wellington Region Emergency Management officer Rian van Schalkwyk.

"The magnitude of that quake and tsunami in Japan is the sort of thing we can expect here. The moment there is a big local quake that is so strong that you can't stand upright, you need to get to higher ground or up in a building as quickly as possible. There is no time to hop in a car to drive away – just go up as quickly as possible.

NIGHTMARE SCENARIO: A computer-generated image showing the potential impact of a 10-metre wave on downtown Wellington.
NIGHTMARE SCENARIO: A computer-generated image showing the potential impact of a 10-metre wave on downtown Wellington.

"In Japan the quake was 200 kilometres offshore, but for us it could be 15km to 20km ..."

A tsunami caused by a quake that close "could be 15 metres high and it will come into the harbour and destroy everything in its path", he said.

The biggest threats were faults through Cook Strait and the Wairarapa coast and the possibility of huge landslides in the Cook Strait trench.

Mr van Schalkwyk said the tsunami triggered by the 8.2 magnitude quake on the Wairarapa Fault in 1855 was up to five metres high when it swept over Lyall Bay and Kilbirnie into Evans Bay.

A 10-metre wave struck Palliser Bay on the south Wairarapa coast. The tsunami also flooded Porirua Harbour, and hit Titahi Bay and the Kapiti Coast.

Mr van Schalkwyk said Greater Wellington regional council had drawn up maps showing evacuation zones for the most vulnerable areas. In the red, orange and yellow zones people need to be prepared to evacuate if there is a tsunami. The red zone is mainly for beach areas that could be hit by a one-metre tsunami generated thousands of kilometres away and during which people are advised to stay off the beaches.

The orange zone is based on a tsunami similar to that which happened in 1855, and the yellow zone is based on how far the devastation might extend if a 10-metre wave struck.

A tsunami of that size would threaten Wellington's commercial heart – the waterfront, the central business district and all the flat land as far south as the Basin Reserve, the suburbs of Island Bay, Lyall Bay, Kilbirnie, Miramar, Seatoun, Eastbourne, much of Lower Hutt, Titahi Bay, Porirua, Plimmerton, Paekakariki, Paraparaumu, Waikanae and Otaki.

Mr van Schalkwyk said these zones were based on best estimates but might need revising after events in Japan. "You don't really know exactly how far it will go. If you look at Japan, they indicated safe zones, but I heard their tsunami passed that by two kilometres.

"If something like that happens, it will be a big beast" and there was no doubt many people could die.

The death toll from the 1855 quake was uncertain, but between five and nine. At the time, Wellington's population was small – about 3000 – and few people lived on the coast.

Earth scientist Rodney Grapes said the 1855 quake – the most violent in New Zealand since European settlement – lasted for 50 seconds. It ripped a 156km fault along the eastern side of the Rimutaka Range from north Wairarapa and into Cook Strait.

The two sides of the fault shunted past each other by 12 metres and the western side of the lower North Island was tilted and thrust upwards. Much of Wellington was thrown up by three metres.

Ten minutes later a tsunami roared in. For eight hours, beaches and roads around Wellington Harbour were repeatedly inundated as water sloshed around it as if it were a giant basin, with tides falling and rising two to three metres every 20 to 25 minutes.

Tsunami researcher James Goff said the 1855 wave was as big as that which struck Japan.

Professor Goff, director of the Australian Tsunami Research Centre in Sydney, said he found debris left from the tsunami about a kilometre inland from Lake Ferry, more than three kilometres inland in Abel Tasman National Park and in a lagoon on Kapiti Island.

MIND THE LINE, IT MIGHT SAVE YOUR LIFE

Blue lines on roads in Island Bay mark the furthest point that a worst-case tsunami has been calculated to reach.

Since the lines were painted in February, after consultation with GNS Science, almost every coastal suburb has expressed an interest in having them.

"If there was a big earthquake in Wellington, and you live on the coast and have seen that line on the street, then hopefully you grab your wife and kids and go to behind where that line is," Wellington emergency management office senior adviser Dan Neely said.

KEEPING SAFE AND SOUND

This is the current tsunami advice from Wellington City Council:

If you feel a strong earthquake or notice the sea receding, immediately make for higher ground – at least 35 metres above sea level is advised. If you cannot go higher, go inland at least 1.5 kilometres. You may have only a few minutes.

Do not return for at least an hour, or until told by authorities.

Do not wait for a tsunami warning.

Avoid rivers and inlets, which magnify tsunami waves.

Warnings for long-distance tsunamis come from the Pacific Tsunami Warning Center in Hawaii. They will be passed on using local radio, a loudhailer or warning siren.

Turn on the radio, follow all instructions, and take essential items if you are told to evacuate.

Do not go to the beach to watch the waves.

If on a boat, stay on the water. Do not return to port, where the water levels and currents will be unpredictable.

Source: Wellington City Council

» SEE THE CIVIL DEFENCE WEBSITE HERE TO LEARN HOW TO BE PREPARED

The Dominion Post