Fault unlocks new geological data

6:03 am on 1 December 2016

The freshly ruptured Kēkerengū Fault has unearthed a few more secrets about New Zealand's tectonic past, and will help map out a plan for what the country can expect in future, scientists say.

The ruptured faultline can be clearly seen on this Ward farm.

The ruptured faultline can be clearly seen on this Ward farm. Photo: RNZ / Tracy Neal

The faultline begins in north Canterbury and goes through south Marlborough.

GNS senior scientist Russ Van Dissen, an expert on the Kēkerengū fault, describes it as dramatic.

"It looks like a big, huge ploughshare has just gone screaming across the landscape from as far as we can see in one direction to as far as we can see in the other direction.

The fault was one of six to have ruptured at 12.02am on November 14. Their combined jolt created a magnitude 7.8 quake - one of the largest ever recorded in New Zealand.

View RNZ's full coverage of the earthquakes here

Adrian Benson stands beside a fence that was once straight.

Adrian Benson stands beside a fence that was once straight. Photo: RNZ / Tracy Neal

Mr Van Dissen said the huge rupture confirmed a theory they had been working on in the months leading up to the quake.

"When we pieced the story together and actually got the results back we identified three big ruptures in the last 1200 years.

"So in the last 1000-ish years, this fault has ruptured three times - and now it's the fourth time."

GNS scientists Jamie Howarth and Russ Van Dissen discuss the forces that have re-shaped the land.

GNS scientists Jamie Howarth and Russ Van Dissen discuss the forces that have re-shaped the land. Photo: RNZ / Tracy Neal

Mr Van Dissen said there were only a few faults in the country that such a short recurrence interval.

"Four hundred years sounds like a long time to humans but as far as the faultlines go, it's one of the most active in New Zealand."

The colossal forces shifted the landscape distances between 9m and 11m in a few seconds.

Nicola Litchfield of GNS explained the damage to a road at the fault line.

"The road has a number of steps in it, and if you follow one of the lines - either down the side or down the middle you can see it's stepped over several times - and that's where the Kēkerengū fault ruptured the road in the earthquake."

GNS scientists Russ Van Dissen and Nicola Litchfield show how State Highway 1 at Ward has been shunted sideways in the quake - the centre line is misaligned.

GNS scientists Russ Van Dissen and Nicola Litchfield show how State Highway 1 at Ward has been shunted sideways in the quake - the centre line is misaligned. Photo: RNZ / Tracy Neal

The scale of the damage becomes apparent while walking on a nearby farm. Mr Van Dissen said it was unusual how big the fissures are. Some are more than 3m deep and almost as wide.

"And that's because it's not often you see a fault that ruptures by up to 10 metres. Fissures in the ground are common but the size of these are uncommon because this displacement is way bigger than common," he said.

Russ Van Dissen and Jamie Howarth take samples from a giant chasm created by the quake on a Ward farm.

Russ Van Dissen and Jamie Howarth take samples from a giant chasm created by the quake on a Ward farm. Photo: RNZ / Tracy Neal

Jamie Howarth of GNS has a special interest in the Alpine Fault, and said he wondered what the Kēkerengū fault might tell him.

"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 in an earthquake of this magnitude.

An old stone church in Ward which remains largely intact after the quake.

An old stone church in Ward which remains largely intact after the quake. Photo: RNZ / Tracy Neal

"That has implications for how we go about modelling seismic hazard in this country."

Mr Howarth said traces of organic materials buried in the layers of dirt were fairly accurate indicators of a timescale between ruptures.

Adrian Benson inspects a land slump.

Adrian Benson inspects a land slump. Photo: RNZ / Tracy Neal

"We can do some statistics to get the condition of probability of having a rupture of a given size on the Alpine Fault in a given period of time from now. Those numbers sit between 30 and 50 percent chance of having a great earthquake on the Alpine Fault in the next 50 years," Mr Howarth said.

However, the risk of that happening was no different to before the quake, he said.

The scientists said the thing we can control, is being prepared.