New study plumbs the depths of the Wairau and Alpine faults

It's no surprise to most of us that New Zealand is perched on top of a collection of fault lines.

Russ Van Dissen in the Kēkerengū Fault line rupture. Source:

But what might be a surprise is how little we understand about some of them, and that we're still finding new ones.

The Wairau Fault, and faults nearby, is the subject of a new study gearing up to try and properly understand it, and what might happen if it ruptures.

The study is being led by GNS earthquake geologist Dr Russ Van Dissen.

“Our hope in this investigation is to shed more light on the aspects of those earthquakes, largely which direction they rupture from,” Van Dissen told Summer Times.

The Wairau Fault links up with the Alpine Fault near the Nelson Lakes it extends for about 140 kilometres, he says.

“It’s important because it points either towards or away from Wellington and that could have a significant impact on the level of ground shaking in the next earthquake.”

His team are observing scratch marks made along the faultline to see if it gives a clue as to how a rupture is likely to behave, he says.

“As one side of a fault moves past another it makes scratch marks, you can imagine like a bear claw ripping across the plain as one side moves past the other.

“And the shape of those scratch marks very well could have clues on which way the rupture moved from, or propagated from.”

The Kēkerengū Fault in the north east of South Island moved during the Kaikōura quake and he says it is displaying scratch marks akin to “lazy rainbows”.

“It’s mainly a sideways slipping fault, the scratch marks look like lazy rainbows. They were metres long and they had a curve to them which looked like a lazy rainbow.

“We’ve done modelling that’s consistent with [the idea] if the rupture’s moving towards you the shape of the rainbow is convex, it looks like a rainbow instead of a U but on the other side of the rupture it should look like a U.”

His research team are digging along the Kēkerengū Fault conducting a kind of archaeology, he says.

“We take out excavators and dig along the fault and like an archaeologist gently remove layers of the earth until we get along that plain where the displacement happened, and we start recording the shape of the scratch marks.

“And through that we hope to build up a better knowledge, testing our ideas, does the shape of these scratch marks really indicate did the rupture come towards you? Meaning the shaking will be a lot stronger, or did it go away from you?”

He hopes the study will have direct applications for building codes – moving beyond human safety to more resilient buildings.

“Meaning having things be operable after the earthquake, more easily repaired, less damaged, less costly to repair.

“Better design parameters and this is what we are aiming for.

“If the Wairau Fault moves, Wellington’s going to be shaken, but we want to go past that and say it will be stronger than expected or weaker than expected for this fault line and that will enable, hopefully, the engineers to design more appropriately to achieve better performance objectives.”

The Kaikōura earthquake revealed a number of faults that were unknown, and geologists are constantly learning and adding to the sum of our knowledge, he says.

“We’re getting a lot better at recognising the dominoes, but what we need to start learning better is how they may fall.”