Understanding the Alpine Fault

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Understanding the impact of an earthquake along the Alpine Fault is the focus of a series of public science talks in the South Island.

Science lead for Project AF8 and deputy director of the Centre for Sustainability Caroline Orchiston will speak at a session in Wanaka next Thursday.

The Alpine Fault runs for about 600km along the west of the Southern Alps.

Fault line . . . A graphic shows an indicative line of the Alpine Fault in relation to Wanaka and Queenstown. PHOTO: PROJECTAF8.CO.NZ

“What we know about the Alpine Fault is that it tends to produce big earthquakes, roughly every 300 years,” Dr Orchiston said.

The last big earthquake was about 1717AD, and over the last 8000 years there had been about 27 earthquakes.

“The evidence suggests that the Alpine Fault has this habit of producing big earthquakes as a reasonably regular occurrence.”

Although it was not possible to predict when and where an earthquake might hit, a hazard scenario had been developed that described what a magnitude 8 Alpine Fault event with a rupture length of more than 400km might look like.

“In this case we estimate or suggest that the fault would actually break the surface right up from Milford Sound all the way up to around about Greymouth. That is the scenario we are working to.”

Using this model as an example enabled local communities and councils to understand what the impact and consequences of an earthquake might be on built and social environments, she said.

But an earthquake might happen in “many different types of ways”.

“When you look at a satellite map of the South Island you can see the fault as what appears to be a pretty straight line, but of course on the ground it is not quite as simple as that.”

The fault “moved around” and it was not clear where an earthquake might break the surface.

“So we can’t say exactly where but we know that, for example, it goes through Franz Joseph township, and we know that it is cutting right underneath a lot of infrastructure and the highway in various places along the West Coast.”

Wanaka was about 75km from the Alpine Fault, and Alexandra was about 135km from the fault “as the crow flies”.

“Obviously this is a big long fault, and an earthquake has to start somewhere on that fault.”

Where the quake initiated and where the seismic energy moved across the fault would have a big impact on the way it would affect the region, Dr Orchiston said.

Showing the effects of an earthquake was more than just scaring people, she said.

“I get asked that quite a bit, about the scaring factor.”

The aim instead was that by understanding challenges there was the ability to be better informed, and make choices that could minimise disruption after an earthquake.

“The preparations we’ve made in advance are going to make a big difference to the way our lives are going to play out in the aftermath.

“If we build better bridges and have some retrofitting in our buildings then the outcomes will be better.”

The Science Beneath our Feet: a public science talk – will take place from 7pm on Thursday March 28 at Lake Wanaka Centre.