Devastating megathrust tsunami striking NZ east coast a once every 7000 year prospect, says geophysicist

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Kiwis alarmed by the prospect of a devastating tsunami hitting our east coast with little warning should rest somewhat assured, because the stats say it's occurred barely once a millennium. 

Wellington geophysicist William Power has tempered fears a megathrust earthquake will imminently strike the Hikurangi subduction zone - a fault line running from Marlborough past the east coast in the Pacific Ocean.

It comes as University of California researcher Steven Ward this week released a computer simulation of 12-metre tsunami waves immersing the length of New Zealand's east coast if a magnitude nine quake were to hit Hikurangi.

While Power says it could possibly happen, a megathrust quake the size Ward recreates has only occurred once over the past 7000 years.

"Our best guide to the likelihood of future events comes from what we know about past ones," Power says.

"Over the past 7000 years there appear to have been eight to 10 large Hikurangi earthquakes capable of causing tsunamis, and only the largest of these appears to be possibly comparable to the one in Steven Ward’s model.

"We do not know when the next one will happen, it could be in the next ten years or it might not be for several hundred years."

However, even in the case a megathrust earthquake were to hit the Hikurangi subduction zone, Power says New Zealand's tsunami evacuation zones are extensively scoped out across the coastline, and cordoned off conservatively for the worst case scenario.

"The tsunami evacuation zones are designed to be sufficient to encompass even very rare tsunamis, including some events that we do not even know for sure if they can happen but choose to be cautious about the possibility when we cannot rule them out," Power says.

"Hikurangi tsunamis like the one modelled by Steven Ward can indeed reach some parts of the east coast in under ten minutes, and this is why people should evacuate immediately following a 'long or strong' earthquake and not wait for an official warning."

Power, however added that tsunamis travel more slowly over shallow water, and that for many people along the New Zealand coastline, there would be more than 10 minutes before a wave arrived - but people should still respond promptly.

"The really key piece of preparation is to make sure that everyone knows the ‘long or strong then get gone’ message, and where their local evacuation zones are. Because of the short time before a tsunami can reach our coasts if people evacuate immediately after an earthquake this can save more lives than anything else."

Power said while nowhere along the New Zealand coast is without some level of tsunami threat, most at-risk areas are the east coast of the North Island, and around Banks Peninsula and Fiordland in the South Island.

Auckland City's tsunami evacuation zone map. Red covers the beach and adjacent low-lying areas most likely to be hit by a tsunami. Orange would be evacuated if there was a medium to large tsunami threat. Yellow covers the largest area that would need to be evacuated in the event of a maximum-impact tsunami

Source: Supplied

A directory of tsunami evacuation zone maps for different parts of the country can be found on the Civil Defence website.

The maps are designed to help people evacuate in the event of a nearby earthquake, and also to help evacuate in response to official warnings of a tsunami approaching from further away.

Power says the key piece of preparation for Kiwis is to abide by the "long or strong then get gone" message, where after feeling a powerful quake people get to higher ground and not wait for official warnings.

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