8 Dec 2016

When the Kekerengu Fault ruptured

From Our Changing World, 9:06 pm on 8 December 2016

Scientists began detailed research and analysis of the Kekerengu Fault, that runs from North Canterbury through Marlborough and out to sea, earlier this year. They said they knew it posed a significant risk to the northeastern South Island and also to Wellington if linking faults in Cook Strait ruptured at the same time.

What they didn’t count on was it actually happening.

There is now evidence to show nine faults ruptured in the fatal 7.8 magnitude Kaikōura earthquake, and the Kekerengu fault was the major player.

GNS senior scientist Russ Van Dissen, a north Californian who came to New Zealand as a Fulbright scholar in the late 1980s to work on active faulting, is very familiar with the Marlborough landscape.

RNZ News joined him on a recent field trip to the hills behind Ward, where the Kekerengu fault rupture is most evident.

And the coast rose out of the sea

As we drove south out of Blenheim, past damaged wineries and creased hillsides, he pointed out signs of previous large quakes. The first glimpse of the sea revealed a whole new coastline created by the November quake. More than 60 kilometres of coastline south of Kaikōura to from Cape Campbell that juts off Marlborough has been shoved up, exposing new rock platforms.

Mr Van Dissen described them as much like a "two-tiered wedding cake", where the top layer was once lapped by the sea, but now the whole bottom layer and base was exposed as well.

He said the coastline at this point rode on the back of the Needles Fault, the offshore extension of the Kekerengu fault, which was one of seven that ruptured to create the magnitude 7.8 quake.

Mr Van Dissen, who married a Kiwi after he arrived in New Zealand, has been immersed in the Kaikōura landscape for the past three decades. He said the scale of change in the coastline is not the only breathtaking thing about the quake - also breath-taking is that it happened in a flash.

"These fault ruptures happened at high speeds - faster than a speeding bullet, like Superman-type speed. If you'd been standing here you probably would have been knocked on the ground and when you stood up you would have seen a new landscape."

He said farmers in the area spoke of the noise that went on for hours, of sea water draining away from the uplifted rocky platform.

Driving the damaged road

Contractors with clipboards manage the road cordon at Ward. They let only official vehicles through. Unstable land and the huge amount of road works underway to repair State Highway One make it impassable.

As we drove through a large dip in the road, surrounded by a crew of road workers busy fixing it, Mr Van Dissen explained we were crossing the Kekerengu Fault. It ruptured for 36 kilometres on land and almost as far again out to sea.

It was here that we met GNS earthquake geologist Dr Nicola Litchfield and other members of the research team. She said the road had been "stepped" down and across by several layers, shifting the middle line by a couple of metres at one point.

A broken farm

The most dramatic scenes are across the farmland of Winterhome Station, whose owners have allowed the scientists access to carry out emergency studies. Along the rupture itself, it was as if a giant mole has raced underground and over hills at the speed of sound, tossing up great sods of earth.

Mr Van Dissen described the effects on a hillside as being more like the claws of a giant, prehistoric monster having scraped through it.

"The earthquake initiated at 10-20 kilometres of depth but the rupture has been big enough to break all the way up to the ground surface."

Two geologists standing in a trench

Russ van Dissen and Jamie Howarth are dwarfed by a rupture in the Kekerengu Fault. Photo: RNZ / Tracy Neal

Building on previous research

This earthquake confirmed some ideas that he and others were already developing, based on recent field work that they have been carrying out at this very spot.

"Why we chose to study here is that it's an old pond site. We were hoping to intercept peat layers we could date with radiocarbon dating."

Mr Van Dissen said after they cut a slice through the faultline, cleaned the walls down and made a real detailed map of the different layers, the trench revealed faulted peats, unfaulted peats, and deposits they could date, and when they pieced the story together they identified three big ruptures in the last 1200 years."

And now we've just had the fourth.

Adrian Benson is a geophysics technician at Victoria University, and has been helping to survey topographic information around the trench site. Last week he went back for a look, and said the difference to the land was startling, and greater than people might imagine.

Further on, a chasm three metres deep and almost as wide revealed the truth around the scale of movement.  Mr Van Dissen said the size of the fissures on this stretch of the fault were unusual.

"It's not often you see a fault that's ruptured by 10 metres.

"Fissures in the ground are common - the size of these is uncommon because this displacement is way bigger than common."

Lessons for the Alpine Fault

GNS earthquake geologist Dr Jamie Howarth said there were distinct signposts within these cracks in the earth, which told them about past quakes.

He said they helped him with his research into his particular field of expertise - the Alpine Fault. The boundary between the Pacific and Australian plates runs almost the entire length of the South Island. It has a habit of rupturing about every 300 years, and the last time was 1717.

But the recent quake has thrown some of their theories.

"This particular rupture has let the cat out of the bag in some respects, because it's ruptured a series of faults that we wouldn't have thought would rupture together. That has significant implications for how we go about modelling seismic hazard in this country. That's something we're going to have to grapple with in the months to years ahead," Dr Howarth said.

In terms of New Zealand's other major subduction threat - the Hikurangi fault, which is New Zealand's largest source of earthquake hazard - the science was still too young to really know, he said.

The Ministry of Business, Innovation and Development has recently announced funding for a research programme into the faultline for the purpose of gaining a greater understanding of the risk, Dr Howarth said.

And then there were nine faults

Scientists say they now have evidence that nine faults ruptured in last month's magnitude 7.8 Kaikōura earthquake.

Earthquake geologist Rob Langridge confirmed that the known tally has increased from seven faults to nine, after he and another scientist flew further over the area around Kaikōura on the 6th December.

Dr Langridge says the quake now stands as being of a similar complexity to the 1855 Wairarapa earthquake, in which multiple onshore and offshore faults ruptured, and which caused a tsunami.

He says the simultaneous rupture of multiple faults is also similar to the 1906 San Francisco quake which travelled over hundreds of kilometres of the San Andreas fault.

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