No one is safe from increased earthquake risk

Wellington is due a major earthquake along its main fault line (Getty)
Wellington is due a major earthquake along its main fault line (Getty)

There was a time when earthquakes weren't a major concern for the New Zealand public.

Before the deadly Christchurch shake in 2011, the country hadn't faced a major urban earthquake since the destruction of Napier in 1931.

Entire generations of Kiwis grew up largely untroubled by the Earth's violent tremors - it simply wasn't something they had to think about.

But that all changed on September 4, 2010 when a powerful magnitude 7.1 quake struck near Darfield on the Canterbury plains. It aggressively rocked Christchurch and the surrounding towns causing plenty of damage but no loss of life.

"We were very lucky" became a popular catch cry.

But five months later that luck ran out as Christchurch itself was ripped apart by a shallow 6.3 quake and 185 people lost their lives.

Earthquakes and the results of their destructive power were suddenly seared into the Kiwi public consciousness. We became earthquake experts overnight.

"Wellington could be next" many people suggested.

Since a magnitude 7.8 struck near Dusky Sound in Fiordland in 2009 there have been nine other earthquakes of major significance in New Zealand. That's more than one 'big one' a year.

No one is safe from increased earthquake risk

Seismologists are researching that very question.

Dr John Ristau from GNS Science says:  "Between 1848 and 1942 there were regular earthquakes in New Zealand with magnitudes greater than seven, the biggest being the 8.2 quake that struck the Wairarapa in 1855.

"But since the mid-20th century things seemed to quiet down, there weren't really that many major earthquakes, and the ones that did happen tended to be centred more in Fiordland and it was easy for people to ignore them," says Dr Ristau.

"It does appear since the July 15 2009 Dusky Sound 7.8 earthquake in Fiordland we've had a series of major earthquakes.

"The big question is whether that is just an aberration, or whether that period from the 19th to the 20th century where there were lots of earthquakes is normal for New Zealand, and we're just getting back to that?

"Or, is this current series of big earthquakes just an overall historical bump and will things go back to normal? That, we don't know yet." says Dr Ristau.

Dr Ristau says statistical seismologists will examine what the Kaikoura earthquake has done to the surrounding fault lines.

"They'll examine the stresses and what the probability is of those fault lines having major earthquakes, and see how aftershock patterns are spreading out around the country.

"Ultimately all this research will be able to provide is probability, no absolutes." He added.

The Dusky Sound earthquake in Fiordland in 2009 was New Zealand's biggest since the devastating 7.8 quake that levelled Napier in 1931.

The remote location of the Dusky Sound quake, combined with its low frequency energy output, meant it caused minimal damage to buildings and infrastructure. Seismologists at the time claimed New Zealand had "dodged a bullet".

Dr Ristau says there could be a link between the Fiordland quake and the series of big earthquakes that have followed in the South Island.

"One possible interpretation is that the Dusky Sound earthquake changed the stress patterns around the South Island and it's starting to trigger more of these larger earthquakes, but that's very hypothetical, we can't say for certain.

The South Island's main fault line, the 600km long Alpine Fault, hasn't ruptured since 1717.

Dr Ristau says there is a high probability of a magnitude 8.0 earthquake or greater on the Alpine fault within the next 50 years.

"Based on historical records, the minimum time for a major earthquake on the Alpine Fault is around 100 years and the maximum time about 300 years. Next year it will be 300 years since the last rupture, so we're definitely in that time window."

The New Zealand capital was always viewed as the city most likely to be hit with the 'big one' until Christchurch claimed that deadly honour in 2011.

Dr Ristau says: "The most noteworthy fault for Wellington is the Wellington fault line which runs right through the city centre and up the Hutt Valley.

"It's really a matter of time until there is a major earthquake along the Wellington fault. As to when that really happens we don't know.

"It's been several hundred years since there was a last major earthquake, and we're a little more uncertain as to what the average time between the earthquakes on it is, but it seems that we're right in the middle of a cycle with the Wellington fault so there's little less risk compared with the Alpine fault and having an imminent earthquake on it.

"However, you can never discount the possibility that it will happen." He added.

Dr Ristau says a similar quake to the shallow magnitude 6.3 that levelled parts of Christchurch could do even more damage in Wellington.

"As Christchurch showed, you don't need a really big earthquake to cause massive amounts of damage. If we have, say, just a magnitude 6 occur right dead centre underneath Wellington or just outside Wellington, it will cause major amounts of damage and probably casualties."

There is arguably an even greater threat to Wellington and the rest of the North Island however; the subduction zone that runs beneath the entire island.

Dr Ristau says: "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.

"The Pacific plate is pushing beneath the Australian plate, that's what we call a subduction zone.

"Currently that whole thing is locked just south of the North Island all the way past the north end of the North Island.

"These subduction zone earthquakes are the largest type of earthquake that you can get." Dr Ristau added.

Recent subduction earthquakes include the devastating 2011 quake and tsunami in Japan, and the Boxing Day quake and tsunami in South East Asia in 2004.

Dr Ristau believes a subduction quake, where the entire zone ruptures, would produce an enormous earthquake in the North Island of at least magnitude 8.5 or greater.

No city, including Auckland, would be spared.