Alpine Fault is better understood – but that won't stop the Big One

Friday, 14 June 2019

Dr Caroline Orchiston explains the Alpine Fault and the damage the earthquake will wreak. (Video first published in May 2018)

This is it: Franz Josef. The place which will be split apart by titanic forces when the Alpine Fault can't take the pressure anymore.

That could be today, it could still be 100 years hence – although the probabilities are against that, given scientists have found the fault has ruptured 27 times during the past approximately 8000 years.

The last great magnitude 8-plus quake it generated was in 1717, give or take a year or two. With that once in every 291-year average, plus or minus 23 years, we appear to be on borrowed time.

GNS Science principal scientist Dr Kelvin Berryman doubts last Sunday's quake was on the Alpine Fault and says there are several small faults just west of it.

No other community has such a vested interest in this slumbering monster, which does a bad job of hiding directly below the tourist mecca's main street, its petrol station, its police station and motels. Once you've seen the hump, it's difficult to unsee it.

**READ MORE:

Whataroa's Alpine Fault Tours takes people to the well-known exposure of the fault at the nearby Gaunt Creek.

* Big Bay quake on Sunday largest along Alpine Fault since 2001

* Videos show devastating impact across South Island if Alpine Fault ruptures

There is now a 27 per cent chance of an Alpine Fault earthquake occurring in the next 50 years.

* Questions over future of West Coast's SH6 if Alpine Fault quake hits

* New study says Alpine Fault quake interval shorter than thought: GNS Science

The Alpine Fault scarp crosses the main street of Franz Josef directly under the petrol station.

* Scientists examine Alpine Fault for signs of stress following earthquake**

The only other places straddling the fault almost to the same extent are St Arnaud, the Gloriavale community at Haupiri and Fox Glacier village just down State Highway 6, though there are plenty of settlements and towns within five to 10 kilometres of its path.

Alpine Fault Tour's Elisabeth Frankish explains how the fault will rupture in the expected magnitude 8+ earthquake.

Franz Josef residents living with the fault now have a greater appreciation of what is in store than they did only 10 years ago, thanks to the efforts of scientists and civil defence workers, and some incredible, high-profile, big science in the form of the Deep Fault Drilling Project at Gaunt Creek about 20km away. As a result, they are becoming better prepared.

But the fact remains that, even after dozens of studies, some with international researchers, costing millions of dollars, nothing has changed - nobody can predict when the Alpine Fault is going to rip asunder.

A wet winter's evening in Franz Josef village, taken from the top of the Alpine Fault scarp.

We know far better the characteristics of the fault, and its likely behaviour when it does rupture. But how close it really is to the end of its seismic cycle is still as unknown as it was when all this research began several decades ago.

The massive Alpine Fault is due for another big earthquake and scientists have been drawing up a scenario of what the devastation would look like. (First published May 2018)

ALPINE FAULT ROADSHOW

The Project AF8 roadshow is in town.

It's a rainy Tuesday evening in June with an early dusk made even earlier by the thick, wet, looming West Coast bush.

Franz Josef is quiet. No thumping helicopters, just one tourist coach parked on a side road, a few visitors puddle-jumping along the street hunkered under umbrellas, restaurant owners looking hopeful. A few wisps of chimney smoke hang low in the air.

Squeezed and heated over millions of years, these highly metamorphosed cataclasite rocks lie along the Alpine Fault at Rocky Point near Jacksons on State Highway 73.

It may be quiet but it's not uneventful. Amid the pelting rain, there's a steady stream of people making their way to the Alpine Theatre for a presentation by AF8's science lead, Dr Caroline Orchiston, of the University of Otago, and Dr Rob Langridge, a senior earthquake scientist at GNS Science.

The Alpine Fault Magnitude 8 collaboration is a three-year partnership of the South Island's six emergency management groups and features scientists from universities and Crown research institutes as well as emergency services personnel, iwi members and health-authority workers.

Big Bay in northern Southland with Lake McKerrow on the right. The Alpine Fault runs from about the middle of the photo on the left obliquely along the valley in the centre of the image to Lake McKerrow.

Funded to the tune of about $650,000 by the Ministry of Civil Defence and Emergency Management, with similar contributions from the scientific community, it fills a gap in terms of the effects the impending earthquake will have on people living in communities such as Franz Josef and on South Island infrastructure.

Tonight it's Franz Josef's turn. Close to 80 people and their wet-weather gear fill the theatre, less than 100m from the lump in the road that looks like a whale lurking just below the surface.

GNS Science earthquake geologist Dr Robert Langridge studying layers in a trench across the Alpine Fault at Calf Paddock near Springs Junction. (File photo)

Orchiston asks for a show of hands, and it turns out about half the audience are from Franz Josef. About a third say they are in the tourism business.

There are a lot of serious faces as the grim scenario is explained. But Orchiston also reminds the group there are good sides to the Alpine Fault.

The Alpine Fault crossing Calf Paddock at Marble Hill, near Springs Junction. The Pacific Plate is upthrust on the left, with the Australian Plate on the right of the scarp.

Without it, there would be no uplift forming the Southern Alps, no rain forest and unique fauna on the western side of the divide, no hot foehn nor'wester blasting down the Canterbury Plains. It has shaped the South Island in so many ways, she says.

Langridge – who has spent years digging and exploring trenches across the Alpine Fault at various sites along the West Coast – takes up the cudgels. He doesn't want to confine himself to the fault and instead talks about 'multi-hazard cascades', using the November 2016 Kaikōura earthquake as the example.

The two trenches dug across the Alpine Fault at Calf Paddocks near Springs Junction where geologists have studied previous earthquakes. (File photo)

That magnitude 7.8 quake set off many landslides, blocking rivers, forcing them to change course and alter valley landscapes. Other slips formed post-quake lakes behind rock dams, which posed a major hazard when they failed and released tonnes of water downhill.

You only need to look at the slips on the range front around Whataroa and Harihari from storms earlier this year to get a sense of what an Alpine Fault quake will do to unstable slopes and rivers, he says.

The Deep Fault Drilling Project site at Gaunt Creek near Whataroa, directly on the Alpine Fault. Scientists and engineers drilled into the fault several years ago and are still analysing what they found.

Franz Josef residents now have a good appreciation of the fault in their midst so there is little point going over old ground, Langridge tells them. But it is worth remembering the devastation from the Kaikōura quake would pale in comparison with what is expected in the Alpine Fault quake.

'Surface rupture will cause significant damage to buildings. The surface rupture in Kaikōura was around 180km of fault – in this Alpine Fault event, we're talking about at least twice that length.'

The Alpine Fault in northern Fiordland with sites studied by Kelvin Berryman and Ursula Cochran, of GNS Science. Their findings showed the fault has ruptured 27 times in about the last 8000 years.

There is now a 27 per cent chance of that earthquake occurring in the next 50 years. Langridge shows an image of a stopwatch with '1717' – for the most recent quake – at the 0-second mark at the top. The watch's second hand is now within 10 seconds of the same mark.

'It could well be that the event is further on in time. But the point of this image is to tell you we are very late on in the seismic cycle of the Alpine Fault.'

A member of the audience asks about aftershocks. Langridge says if the quake is of magnitude 8.0 or higher, 'it would have to have some magnitude 7 aftershocks'. There is also a chance the redistribution of stress may cause other faults around the central South Island to generate quakes of comparable magnitude to the largest aftershocks.

An hour and a half later, the meeting winds up. The following night, in Hokitika, 130 people pack the Westland RSA rooms to take part in the roadshow.

Such community engagement from the scientists is a far cry from just a decade ago.

In March 2009, more than 60 of the world's best quake scientists were in Franz Josef to plan the fault-drilling project. It's fair to say such engagement was not on their agenda. Residents were unsuccessful in their efforts to invite the scientists to an informal meeting to share their thoughts on the Alpine Fault.

EXCITEMENT AMONG SCIENTISTS

Sunday morning's magnitude 5.5 earthquake close to Big Bay in northern Fiordland generated a frisson of excitement among scientists and quake watchers, particularly after early reports from GeoNet that it appeared to have been centred on the Alpine Fault.

Analysis this week has cast doubt on that, with the consensus view it was probably caused by movement on a fault alongside and very close to the Alpine Fault. Whatever the truth is, the very shallow 3.24am quake was the largest sizeable quake along the Alpine Fault since a magnitude 5.8 shake on December 8, 2001, further northeast near Lake Ellery, closer to Haast.

The Alpine Fault is the boundary between the Pacific crustal plate and the Australian plate. From space it looks like a straight line but in reality it is made up of a number of slightly curvy connecting strands, some of which are classified 'active' and others as historic.

The fault runs northeast from the northern side of the entrance to Milford Sound, along the western side of the Southern Alps for about 800 kilometres before morphing into the Marlborough fault system. Researchers have found each major earthquake results in an average of about 2m of uplift and up to 8m of horizontal slip along around 400km of the fault.

The Big Bay quake occurred where the Alpine Fault is changing its form as it gets closer to where it runs offshore and into the Puysegur subduction zone.

University of Otago earthquake scientist Professor Mark Stirling says the quake 'may have been' on the fault.

'However, it would be too much of a stretch to assert that the recent earthquake is an Alpine Fault foreshock. It occurred in an area that already has relatively high seismicity, and there have been other quakes with similar magnitudes in this general area in the recent past.

'Unfortunately, the GeoNet station coverage is also very poor in this area, so there could be significant location errors with respect to the quake's location. The depth is 5km, which is one of the three 'restricted depths' used by GeoNet (5, 12 and 33km) … when depth cannot be reliably constrained.

'The current seismicity of the Alpine Fault is very low, but our aftershock modelling suggests that huge amounts of earthquakes will occur on the fault in the weeks, months and more after a major earthquake. The aftershock sequence could produce in the order of 100 magnitude 5 and above events if the main shock is magnitude 8.

'However, we don't know if the earthquake will be magnitude 8, as the last earthquake was prehistoric. The magnitude 8 and 400km rupture are popular estimates, but by no means are they estimated on the basis of any observational data.'

Victoria University of Wellington earthquake scientist Professor Rupert Sutherland says the quake did not have slip or vertical movements 'consistent with surface observations of the Alpine Fault near there'.

'But we can't rule out that it was on the Alpine Fault. It was certainly very close, but that is not unusual – there are a lot of smaller active faults in that region.

'Any event so close to the fault is worthy of closer consideration. The main problem is that the network is sparse around there and it is very hard to do any further analysis.'

GNS Science principal scientist Dr Kelvin Berryman has been to the Big Bay and Lake McKerrow area several times on foot and by helicopter to study the fault. He doubts Sunday's quake was on the Alpine Fault and says there are several small faults just west of it.

Despite thick beech forest and swamp 'well worth steering clear of', there are walking tracks in the area.

'It's not impossible to get through. It's GPS that has changed things from when we started working there 20 or 30 years ago, when you didn't really know where you were when you got into the bush.'

Compared with the expected magnitude 8-plus quake, the magnitude 5.5 was 'in the noise' and less likely to signal something than perhaps a magnitude 6.5 might have, he says.

THINK OF THE CHILDREN

In nearby Whataroa, Elisabeth Frankish is opening up the Alpine Fault Tours shop. Inside there are rock samples, various maps and photographs, possum-fur souvenirs and a banana-box model of the structure of the Alpine Fault and how it will rupture.

For $60 for adults, and $25 for children, you can go on a two-hour trip to the fault at Gaunt Creek, close to the drilling site. In summer there can be 10 or more tours a week; at this time of the year, maybe one or two.

During the past five years, visitors have been taken to what is probably the best-known exposure of the fault, where mylonite rocks from the Pacific Plate are being thrust up over gravels on the Australian Plate. The business owners, Vickie and Gray Eatwell, realised the potential for the venture when giving permission to scientists to cross their land en route to the Gaunt Creek site.

Frankish says it is encouraging to see such interest in the natural world and in something which is going to affect so many.

Has she thought of how she will react when the Alpine Fault unzips?

'I'll think of my children first, and pray.'

Asked what will it mean for the business, she says it will have a significant impact like for anything else on the Coast. But months down the track, it will be interesting to see how the earthquake has changed the look of the fault at Gaunt Creek.

This burgeoning interest in the inevitable quake is good news for those communicating its dangers.

After the Franz Josef meeting, Langridge and Orchiston tell Stuff the growing understanding of the fault and the coming quake means they do not have to focus so much on the geology. Instead, residents can think more about preparedness and other possible scenarios.

'We can take the geology more as a given,' Langridge says. 'So I could talk about other things of interest too, like the multi-hazard cascades.'

Have scientists discussed what clues might tip them off to an impending major Alpine Fault quake? How about if Sunday's Big Bay quake had been followed by more quakes of similar or larger magnitude up and down that section of the fault? Would that be a possible sign?

'You only know something is a foreshock once it is followed by something larger, but you can't tell that at the time,' Orchiston says.

'We can't predict earthquakes.'