Tsunami 35 metres high the worst case scenario

Dr Graham Leonard, a natural hazard scientist from GNS Science, is an expert in hazard mapping and warning preparedness for volcanoes and tsunami.
Dr Graham Leonard, a natural hazard scientist from GNS Science, is an expert in hazard mapping and warning preparedness for volcanoes and tsunami.

The worst case tsunami scenario for New Zealand would be 35-metre-high wave hitting the North Island's east coast, according to experts.

Stuff today hosted a live chat with tsunami experts Graham Leonard, William Power, both from GNS Science, and Fred Mecoy from the Wellington Emergency Management Office, ahead of the one-year anniversary of the Japan quake and tsunami on March 11 last year.

The disaster killed more than 15,800 people and destroyed entire towns.

The experts said such events were very ''infrequent'' but parts of the North Island could resemble the worst impacted areas of Japan. Waves reached up to 5km over land and 10km up rivers in Japan last year.

Leonard emphasised such a disaster would be the "worst case scenario".

"Parts of the upper South Island, such as Marlborough, could also be strongly affected," he said.

The North Island's east coast was more "commonly affected" by tsunamis, while in the South Island different sources impacted each of the coasts. 

People living on a hill by the North Island's east coast would have to be 35 metres above sea level in the worst case scenario, Leonard said. That height dropped as you went inland.

Leonard said if a tsunami were to hit Wellington, people would be "generally safe" above the fourth floor of a building.

Aucklanders would probably be safer on a lower floor as a tsunami would be smaller.

He said whether staying inside a building, that was high enough, was the safest option during a tsunami was an area of active research in New Zealand.

''Initial findings suggest reinforced concrete buildings hold up well as long as they survive the quake and are taller than the arriving tsunami,'' Leonard said.

The concern was for people to stay out of the water as large objects could be drifting, causing injury.

"In Japan there was not a high survival rate for those caught in a wave," Leonard said.

A Stuff reader expressed concern about New Zealand's warning systems, saying they were not anywhere near adequate when compared to Japan.

But Mecoy said New Zealand was better prepared for an event like Japan's since the 2004 Indian Ocean tsunami.

As to whether New Zealand could do more, he said: "absolutely".

The country had a national warning system for the emergency management sector, with each local authority then forwarding the message using their own methods.

The quake itself should be taken as the main warning sign, Mecoy said.

"It will be huge - so difficult to stand up, and/or last for a minute or more.

"For distance source tsunami, from elsewhere, we use mobile sirens on vehicles, helicopters and emergency text alerts, which people can sign up for."

Leonard said tsunamis were triggered depending on whether the seabed was uplifted or subsided during an earthquake.

"That is normally caused by faultlines offshore."

He described a "substantial tsunami" as "anything that might cause damage", measuring from half to a metre in size, if the waves arrived on high tide.

"But they could get up to tens of metres. Larger ones are less common than smaller ones."

Power said in the open ocean a tsunami wave could go as fast as a jet plane, but as the wave approach the coast, it typically moved a few tens of kilometres per hour.

Several tsunamis had hit the New Zealand coast in the last 200 years, coming from South America in 1868, 1877 and 1960.

Some were generated locally - in 1855 in Wellington and Wairarapa, and north of Gisborne in 1947, Power said.

The largest tsunamis in recorded history were about 200 years ago, when a few reached 10 metres high on land. Prehistoric geology suggested they had reached heights of 35 metres.

When asked whether they had nightmares about tsunamis and quakes, Mecoy said: "Only that people won't heed warnings."

Power didn't have nightmares, but he did think about it quite a lot, while Leonard said ''the images of the destruction [in Japan] do affect me a bit".

Dr William Power is a Tsunami Scientist from GNS Science whose expertise is in tsunami modelling and tsunami hazard.
Dr William Power is a Tsunami Scientist from GNS Science whose expertise is in tsunami modelling and tsunami hazard.
Fred Mecoy is the manager of emergency preparedness at Wellington Emergency Management Office (WEMO)
Fred Mecoy is the manager of emergency preparedness at Wellington Emergency Management Office (WEMO)
RED ALERT: Wellington tsunami zones. The red zone is the highest risk area, but orange and yellow zones would also need to evacuate.
RED ALERT: Wellington tsunami zones. The red zone is the highest risk area, but orange and yellow zones would also need to evacuate.

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