Mega-tsunami: Would Auckland survive?

Mega-tsunami: Would Auckland survive?

The recent Kaikoura earthquake sent powerful shockwaves not only through the Earth's surface, but also the organisation tasked with informing Kiwis that a major tsunami is imminent.

Civil Defence has copped a lot of flak since Monday morning's quake.

Following the magnitude 7.8 earthquake, equal in power to the one that destroyed Napier in 1931, Civil Defence initially issued no tsunami warning - but 55 minutes afterwards did so, claiming a tsunami was possible. A 2m-high tsunami had already hit the Kaikoura coast by then.

Some people in Kaikoura and Wellington had already left for higher ground, but others had not.

Now as the Government and Civil Defence scramble to implement a new public alert text-messaging system that could be still 18 months away, Kiwis are wondering when and where the next inevitable 'big one' will strike, and if a mega-tsunami could follow.

That includes almost 1.5 million people living in Auckland.

Tsunami expert Dr NAK Nandasena told Newshub not only does Auckland face a tsunami threat from earthquakes, but also volcanic eruption.

But he believes an undersea earthquake, like the magnitude 9.1 quake that killed almost 20,000 people in Japan in 2011, is the biggest threat.

"Japanese researchers did not expect such a great magnitude earthquake."

Most people were killed when enormous waves up to 10m high struck the coast near Tohoku. These powerful waves were caused by a subduction zone earthquake.

Mega-tsunami: Would Auckland survive?

The 2011 mega-tsunami on Japan's east coast was caused by a magnitude 9.1 subduction earthquake (Getty)

It's worth noting a subduction zone runs under the entire North Island, and a similar rupture here would produce waves similar to those that barrelled into Japan's north-east coast.

"Just off the East Coast of the North Island you have the Pacific tectonic plate and the North Island sitting on the Australian tectonic plate," says GNS Science seismologist Dr John Ristau.

"The Pacific plate is pushing beneath the Australian plate, that's what we call a subduction zone. These subduction zone earthquakes are the largest type of earthquake that you can get."

One of the great urban myths in Auckland is that the two main islands adjacent to the city would protect most sea-side suburbs and the CBD from significant tsunami damage.

Dr Nandasena believes Waiheke and Rangitoto could actually amplify tsunami waves coming into Waitemata Harbour.

"The two islands can block some volume of tsunami water, so some damage would be reduced, but on the contrary, a tsunami can refract around the islands which would result in more energy.

"It could release more energy compared to the situation when there is no islands. And the other factor is reflection from the two islands that also amplifies a bad situation."

So those living in the seaside suburbs of Devonport, Mission Bay, St Heliers and Bucklands Beach would appear to be in the most dangerous areas. This is where a mega-tsunami would hypothetically hit Auckland at its most powerful.

Dr Nandasena says this is hard to predict.

"It's all dependent on certain parameters. Those are the earthquake magnitude, the water-depth and local flow-depth. You also have to take into account the wave velocity at the shoreline and the topography.

"In Auckland we have a very mild slope or a flat ground slope; we would expect an inundation level that could be as long as up to one kilometre.

"Inundation length is the length water can move, but the water depth is the height of the water at ground level, so these two parameters can affect things differently."

Dr Nandasena says Auckland's high-rise office and apartment blocks could save lives.

"If you feel a great tremor and if you're living near by the sea, rush to a higher elevation, a nearby hilltop, or the fifth floor of a building."

Dr Nandasena says there is no clear evidence when a mega-tsunami last hit Auckland, but some Maori oral histories suggest a 6-m high wave did hit the area sometime in the 1300s.

The last major tsunami to hit New Zealand was in 1868 when a magnitude 8.5-9.0 earthquake in Chile sent huge waves across the Pacific. The New Zealand coastline from Great Barrier Island to Bluff was affected, and the waves in some places were up to seven metres high.

The entire Maori village of Tupuangi in the Chatham Islands was washed away.

We are now living in a period of increased earthquake activity in New Zealand.

Before 2010 and the massive 7.1 tremor that rocked Canterbury, Kiwis had largely forgotten about the destructive power of earthquakes.

Let's hope it doesn't take a mega-tsunami hitting New Zealand to remind us of the destructive power of the sea.

Kim Vinnell looked extensively at what could happen to Auckland in the event of a mega-tsunami tonight on Story - watch here