Giant quake risk off east coast of North Island

The subduction zone off the North Island East Coast has been ranked as being at high risk of generating a giant earthquake.

New research rated a 600-kilometre section of the Hikurangi-southern Kermadec subduction segment, starting at the lower North Island and stretching to the northeast, as having the highest risk of producing a giant quake of a greater magnitude than 8.5.

A team led by Associate Professor Wouter Schellart, of Monash University in Melbourne, developed a global map in which individual subduction zone segments were ranked in terms of their predicted capability to generate such a giant. The largest earthquakes happen only at subduction zones.

Each zone segment was ranked according to six parameters. In the highest risk segments all six parameters fell within the ranges seen in previous quakes of greater magnitudes than 8.5.

According to the research, the Hikurangi-southern Kermadec subduction zone segment had the characteristics of the locations of previous giant earthquakes, Schellart said.

"It should be capable of generating a giant earthquake as well."

The researchers looked at all 23 active subduction zones on Earth, making up about 50,000km of subduction zone plate boundary. That was divided into a total of 241 200km-long segments. Of the 222 segments with enough data, 105 were rated as being at the highest risk of having a giant earthquake, including the three segments off the lower North Island.

Schellart said the surface of the subduction zone trench that was the boundary between the Pacific Plate and the Australian Plate was about 100km to 200km off the east coast of the lower North Island.

The Hikurangi subduction zone was part of a larger zone called the Hikurangi-Kermadec-Tonga subduction zone which stretched all the way to Tonga.

"Only the southern part of it, that's the Hikurangi segment, has these particular properties that, at least according to our research, make them capable of generating a giant earthquake," Schellart said.

"The southernmost part of the Hikurangi subduction zone is currently experiencing compression. The crust in the southern part of the North Island is being shortened and compressed, which implies that the subduction zone fault has high stress on it, which can mean that the fault is temporarily locked."

If that part slipped it could create a large earthquake and potentially could result in a cascading effect, causing segments further to the north to slip as well.

"If a large segment of a fault slips that generates a large earthquake," he said.

In a giant earthquake a very large part of a fault slips.

In the Boxing Day 2004 quake off Sumatra, which caused a tsunami that killed more than 200,000 people, about 1300km of the subduction zone fault slipped. In the 2011 Japanese quake, about 400km to 500km slipped.

In the Hikurangi subduction zone "numerous" hundreds of kilometres could slip to the northeast, "and then it would be a giant earthquake", Schellart said.

"I would think that the biggest worry would be, like in Sumatra and in Japan, that if a giant earthquake would happen it would cause a tsunami."

The biggest risk would be between East Cape and Cook Strait, the area immediately bordering the edge of the subduction zone.

In Japan and Sumatra it had taken about 15 minutes for the first tsunami waves to hit the coast, in those cases, the distance from the plate boundary to the land regions was comparable to New Zealand.

He was not an expert in tsunami modelling but thought the tsunami could be "metres" high.

He could not comment on when such an earthquake might happen.

Fairfax Media