Today's quake East Cape's most dramatic in years

2:51 pm on 2 September 2016

It's alarming, but today's large magnitude 7.1 tremor off the East Cape - and its impact - could be considered run-of-the-mill in the history of the area.

Scientists have turned their focus to the Hikurangi margin, off the east coast of the North Island, and further beyond into the Kermadec to Tonga trench, in recent years due to the risk of large quakes generating tsunamis.

Tolaga Bay

Photo: RNZ / Claire Eastham-Farrelly

GNS senior geophysicist Dr William Power said today's quake was the most dramatic in recent years.

However, in 1947 a collision between the Pacific and Australian plates generated a quake which resulted in a wave reaching 10m in height north of Gisborne.

Fortunately, no-one was killed.

While the Hikurangi Margin and Kermadec to Tonga trench were often dealt with as separate areas, they were really one long continuous fault, albeit with changing characteristics, Dr Power said.

There had been larger events, thought to be up to about magnitude 8, in the Kermadec-Tonga trench in 1917, 1976 and 1986.

In 2008 Dr Power co-authored a report which painted a stark picture: "A tsunami caused by an earthquake on the Hikurangi plate interface is thought to be a plausible candidate for the most destructive tsunami New Zealand is likely to encounter over a 1000-year timeframe - capable of severe damage to urban areas on the east coast."

The conclusion? "Self-evacuation to over 35m in the event of a large earthquake is an appropriate precautionary response for those present close to the coast."

That remained the worst case scenario advice, which GNS had to consider for civil defence purposes, Dr Power said.

Since the study was released, a number of evacuation maps had been created to guide people on what to do in their specific area, as it varied.

"It is totally appropriate for us to worry about large events but hope they never happen," he said.

Evidence suggested a very large quake created a tsunami which hit Great Barrier Island and Northland about 1450AD, he said.

However, that was the subject of much scientific debate.

If the pattern suggested was followed, the quake would have been in the vicinity of magnitude 9.

It was important that scientists continued to try and model the impacts of such big events to help planning in case of disaster, Dr Power said.

Today's quake could increase the risk of larger events due to the stresses along the fault changing.

The Pacific plate was being thrust beneath the Australian plate in a process known as subduction, at a rate of about 5cm a year, Dr Power said.

The most important question was whether they were "getting stuck together", he said.

"If they are gently sliding past each other the risk from earthquakes is thought to be less."

GPS measurements at the Kermadec end of the fault suggested the plates were coupled together around Raoul Island.

NIWA principal scientist Dr Geoffroy Lamarche said research he worked on indicated that tremors greater than magnitude 8.0 strike in the Hikurangi Margin on average every 820 years, while quakes measuring magnitude 7.5 or above strike every 230 years.

This was discovered by looking at the number of submarine mudslides caused in the past, and dating the sediment in the mudslides precisely.

The exact relationship between the quakes, mudslides and tsunami was still being studied, Dr Lamarche said.

The worst case scenario, which no one could yet prove was possible - but nor could they prove it was not - was a "plate interface earthquake" that stretched 550km from the Wairarapa to the East Cape, he said.

The 2011 Japanese quake and tsunami which measured magnitude 9 had changed some of the thinking in relation to the potential for these.

An upcoming study, involving scientists from the US, would beginning drilling into the Hikurangi plate interface next year.

That would be a "once in a life-time opportunity" to discover the risks of a plate interface earthquake, Dr Lamarche said.

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