Hikurangi megathrust quake could be more destructive than Japan's with 30m waves

A rupture of New Zealand's largest faultline could cause damage and destruction on a scale comparable to the Japan quake of 2011 with tsunami waves as high as 30 metres, a geologist has warned.

On March 11 that year, a magnitude 9 quake hit off the eastern coast of Honshu, Japan's largest island. The devastation caused by the six minutes of shaking and resulting tsunami killed almost 16,000 people and caused $346 billion in economic losses - the costliest natural disaster in recorded history.

That quake happened where the Pacific plate is subducting beneath the smaller Okhotsk plate. Just off New Zealand's east coast, the Pacific plate is subducting beneath the Indo-Australian plate at a boundary called the Hikurangi subduction zone.

GNS Science
New Zealand's largest fault, the Hikurangi subduction zone Photo credit: GNS Science

A Hikurangi megathrust quake the same strength as the one that hit Japan would release more than 11,000 times the energy of the 2011 Christchurch quake. Even the huge 2016 Kaikoura quake would pale in comparison - a magnitude 9 quake being 63 times stronger than a magnitude 7.8.

Civil Defence Emergency Management (CDEM) groups from across the North Island are presently working together to develop an emergency response plan, assuming a quake of a similar strength to the one that hit Japan will eventually strike.

"The scenario we are using to support the development of this response plan is a very realistic example of what we could face in our lifetime, or that of our children and grandchildren," said project lead Natasha Goldring of Hawke's Bay Regional Council.

Martha Savage, professor of geophysics at Victoria University of Wellington's School of Geography, Environment and Earth Sciences, told RadioLIVE on Friday the longer the wait, the bigger the quake.

"The one in Japan... it had been almost 1000 years between ruptures. That's one of the reasons why it was surprising."

The previous time the Japan fault ruptured was 869, causing a tsunami that reached four kilometres inland. Evidence for previous quakes on the fault places them about 1000 years apart.

New Zealand's recorded history is much shorter, so the frequency at which the Hikurangi zone ruptures is not well understood. But scientists believe it's capable of causing a magnitude 9 'megathrust' quake, like the one that hit Japan.

And because of its location and our prior lack of research, the result could be a lot worse.

"Because the subduction zone is closer to us than it is in Japan, we would have large tsunamis and we wouldn't have as much warning - they could come in as early as six minutes," said Prof Savage.

"That would be very devastating to people near the coast - you could have up to 15, 20, 30m waves if you're right on the coast."

New Zealand's warning systems also pale in comparison to what they have in Japan. Tokyo residents for example had about 80 seconds' warning before the shaking reached its peak, and a tsunami warning was issued in just three minutes - well ahead of the waves' arrival on the coast 15 minutes later.

"We could do more if we had enough money, although we are constrained by the geometry," said Prof Savage. "In Japan for instance, they are putting in a network of seismometers on the seafloor. They're cabled together, and if we had enough money we might be able to do that and give ourselves maybe a couple more minutes' warning."

The world's major tectonic plates.
The world's major tectonic plates. Photo credit: File

Thirty sensors were placed on the seabed at Hikurangi in October.

"I think they should spend some money on more instruments on the seafloor," said Prof Savage. "At the moment we put them out for a couple of years at a time... it would be nice. But the Government has other priorities too."

If - or when - it hits, she says the North Island stands to bear the brunt of the damage. The Civil Defence planning reflects this, with initial response plans to focus on Gisborne, Bay of Plenty, Hawke's Bay, Manawatu-Whanganui and Wellington.