The Big One: What will an Alpine Fault quake feel like?

One hundred and fifty seconds.

That's how long it could take to transform the South Island.

The big quake strikes somewhere deep in the Fiordland wilderness, erupting with a magnitude 7.9 mainshock.

The ripples roll north, roaring beneath the West Coast at a blistering pace of 4km per second.

At just under a minute into the sequence, Haast has been hammered.

At the opposite end of the island, people walking about the streets of Christchurch haven't felt a thing.

That's until the seismic waves punch their way into the Canterbury basin's soft sediment, and the severe shaking starts.

One by one, towns along the 410km-long rupture zone of the Alpine Fault meet this fearsome freight train head-on. By the time it's over, just shy of two minutes later, pandemonium has been unleashed.

Upwards of 10,000 people cut off in an instant. Highways blocked in more than 120 places.

Perhaps a billion cubic metres of material, dumped from the Southern Alps into roads, rivers, and native habitats. A black-out across virtually the entire West Coast south of Greymouth.

Death and injury. Tourists stranded everywhere.

"In the Christchurch earthquakes, it was all on us – but everyone else was coming to the rescue," the University of Canterbury's Professor Brendon Bradley told a public lecture in 2016, after showing a simulation of an Alpine Fault event in action.

"In this particular case, the whole South Island will feel the impacts - and we may not be the first ones that get help."

This is The Big One.

A quake overdue?

If you've been hearing a lot about the Alpine Fault lately, it's for good reason.

A wave of fresh scientific findings has changed what we know about the fault, culminating in a multi-agency effort to prepare the communities in the firing line.

Hundreds of people have turned out to hear a series of public meetings held across the South Island over recent months.

The message: the Alpine Fault is set to go, probably within our lifetimes, and we need to be ready for it.

We're often warned of two goliath natural hazards in New Zealand.

One is the Hikurangi Subduction Zone, a sprawling offshore plate boundary east of the North Island which has the potential to trigger a tsunami-making, mega-thrust earthquake.

The other is the 600km-long Alpine Fault, marked by the mountain range it's pushed up with every violent rupture: the Southern Alps.

Over about 12 million years, that's amounted to an incredible 20km of uplift. Only the much faster pace of erosion has kept the alps' highest point below the 4000m mark.

This rapid, dramatic process has also hauled up faulted rock from deep below the surface, giving scientists critical insights into the constant geological scrum taking place under New Zealand.

Some of the biggest findings have come in the past decade, sprouting from two major studies.

Until now, scientists knew of four large quakes over 1000 years; today, they can point to 27 over 8000 years.

Further, they've also learned that the southern section of the fault has ruptured repeatedly with similar characteristics in size and in time intervals between events.

These intervals have ranged between 100 and 500 years – a trend that's enabled scientists to roughly work out the probability of a quake over any given future time period.

Given that the Alpine Fault last rumbled to life in about 1717AD, with an 8.1 quake that shunted its southern side eight metres further south in a matter of seconds, the median estimate for the next one happened to be back in 2010.

But since time periods always varied, we couldn't truly say the fault was "overdue" a big show – nor could we say whether the Canterbury quakes, or the 7.8 Kaikoura Earthquake, had eased or added pressure.

GNS Science's Dr Kelvin Berryman, whose recent work with Dr Ursula Cochran pulled a long record of Alpine Fault quakes from South Westland's Hokuri Creek, expected there had been no influence.

But there might have been some transfer of stress from the 7.8 Dusky Sound quake that shook Fiordland in 2009, being that it was both big and close to the southern end of the fault.

In any case, another major rupture was inevitable.

"Currently the estimate for a rupture on the southern section of the fault is about 30 per cent in the next 50 years," Berryman said.

"But when we factor in other information from the central and northern sections of the fault, the overall probability of a large earthquake somewhere on the Alpine Fault is quite a bit more."

And by large, he really did mean "large".

"The rupture will produce one of the biggest earthquakes since European settlement of New Zealand, and it will have a major impact on the lives of many people," he said.

"In between earthquakes, the Alpine Fault is locked. All these things mean that the Alpine Fault is a globally significant geological structure."

Another scientist whose work has greatly widened our understanding, Victoria University's Dr Jamie Howarth, pointed out that the same sections of the fault which produced the most recent rupture had also triggered ones with shorter lengths – and hence lower magnitudes.

There had been some events over magnitude 7.5 over the past few thousand years, and others over 8.0.

"The results are important because they suggest that the Alpine Fault has a greater diversity of earthquake behaviour than previously thought," Howarth said.

More work was underway.

One new $300,000 Marsden Fund project, led by GNS seismologist Emily Warren-Smith, aimed to find out why some places along the West Coast seemed to have often acted as barriers during ruptures.

Berryman believed further radiocarbon dating of organic material taken from fault trenches would also shed more light, as would a denser network of instruments on the fault.

"There are some instruments out there, but not at the same density as along the Hikurangi subduction margin," he said.

"We know that there is tremor-like activity at depth below the Alpine Fault, but it isn't well recorded or understood."

The big shake

Just what a big Alpine Fault event would actually feel like is another pivotal question scientists have been trying to answer.

Traditionally, they would have looked at the shaking levels from earthquakes of a similar type and size from around the world, and then applied those average values to the fault.

The problem was, however, was they didn't think this historic data was actually relevant, because of unique local factors.

Instead, they've turned to sophisticated physics-based computer simulations.

Berryman said the physical laws around seismic wave propagation was well known – we just had to build in those factors like rock and crust properties, the current state of stress, and the direction of the rupture.

"This technique is still in development, but by choosing a suitable range of parameters, we can work out a realistic range of likely future ground shaking."

Berryman noted that the 7.9 scenario described at the start of this article was just one of a range of possibilities.

But, generally, recent research indicated there would likely be between seven to nine metres, and one to two metres, of horizontal and vertical movement respectively in a rupture.

It was known from large quakes in similar mountainous environments globally that strong shaking would kick off what Howarth described as a "hazards cascade" of landslides and flooding.

"The limited data we have suggests this cascade could persist for years to decades after the next Alpine Fault earthquake."

University of Canterbury geologist Dr Tom Robinson expected there wouldn't be widespread destruction of people's homes on the West Coast, even though that's where the worst shaking would be felt.

This was because the majority of those homes were wood-framed, making them more flexible even amid the rocking and rolling of a massive rupture.

"In the 2016 Kaikoura earthquake, we saw many wooden framed buildings close to, and even on top of, the fault rupture suffer damage but remain standing," Robinson said.

By contrast, the unreinforced masonry of all of those aged commercial buildings that line the main streets of small-town New Zealand wouldn't fare well at all.

"On top of that, in the larger urban areas like Greymouth and Westport, there are also a variety of reinforced concrete structures that are typically large, commercial buildings," he said.

"Many of these were built in the 1970s before building codes and these older structures could also suffer quite severe damage."

More modern multi-storey reinforced concrete buildings should have been built to code, meaning that while they may be damaged, they still shouldn't collapse.

"However, as Christchurch showed, the collapse of even one large building can have devastating results."

An impact that far fewer West Coast locals might have pondered was to the region's vulnerable transmission network.

Damage to pylons would be expected through Arthur's Pass and close to the rangefront due to landslides and rockfall, resulting in widespread loss of power across virtually the entire coast, south of Greymouth.

"This would be particularly bad for dairy farmers, as their capacity to milk cows and then store that milk would be severely affected," Robinson said.

"Depending on the level of damage to the transmission network, it could take several days before power is restored to places like Greymouth and Hokitika.

"However, further south, it may be several weeks or even months before mains power can be restored."

Cut off

It would also take months – at the least – to clear away countless landslides from vital transport routes.

Robinson's research has indicated that roads would likely be blocked at State Highway 6 just south of Franz Josef, 20km northeast of Haast and 10km east of Haast, and at State Highway 94 just south of Milford Sound and State Highway 73 just north of Arthurs Pass.

If these sections were cut off, reaching the West Coast would only be possible via State Highway 7 - and with access only to 50km south of Hokitika.

"Safely clearing landslide debris from these routes is likely to prove extremely challenging – each one on their own could be as difficult and costly as SH1 has proven," he said.

"The effect of this will be that anything south of Franz Josef on the West Coast will be inaccessible by road, possibly for several months at the very least.

"While this region is already fairly remote and has some experience of being cut-off, most recently due to the loss of the Waiho Bridge, an Alpine Fault event will be on an entirely different scale."

While the rail tunnel between Otira and Arthur's Pass was likely to survive, landslides and rockfall at the entrances would present a major blockage.

On the West Coast, the strong shaking was likely to bend rails, as was seen at Darfield and Kaikoura.

The region's mining industry, particularly, relied upon the rail network to transport products to Lyttleton for national and international distribution.

Continuously providing supplies into the small but spread-out local population for at least several months would be a logistical nightmare.

"Another point to consider," Robinson added, "is the timing of the earthquake".

"If it were to occur during peak tourist season, there could be on the order of 5000 tourists stranded as well, adding further strain on local capacities until they can be safely evacuated."

His study pointed out that 1.3 million travellers visited the West Coast each year, with enough capacity for 4000 visitors each night in the popular Franz Josef area alone.

The Milford Sound area alone attracted 650,000 visitors each year - equating to 1700 each day - and in the event of a quake, hundreds could be spread out in the region and on foot.

He suggested the number of tourists requiring evacuation by sea and air would be five times that of the Kaikoura event.

"In my opinion, the transport networks provide the greatest challenge in the wake of an Alpine Fault earthquake, and the 2016 Kaikoura earthquake should act as a major warning for what we should expect," Robinson said.

"Learning from that event will undoubtedly be of huge benefit, and shows that we need to be thinking now about how we regain access to large areas of the West Coast, as well as how we go about rebuilding these networks in the longer-term."

The NZ Transport Agency's system manager, Pete Connors, said Kaikoura had indeed served up some important lessons.

Work on restoring Kaikoura's transport network – specifically the rail link, the harbour and the inland Waiau road - was still ongoing two and a half years later.

"This work includes making the inland road from Peketa south of Kaikoura to Waiau and Culverden back to Waipara much more resilient and able to take all traffic in an emergency," Connors said.

There was also a significant programme to make the Lewis Pass-SH7- St Arnaud route between Picton and Christchurch much safer for freight and larger volumes of traffic as an alternate route to SH1.

"Every seismic event will have more effects upon some areas than others - and the Transport Agency will respond based on what actually occurs and where the damage is."

Getting ready

West Coast Regional Council's chief executive, Mike Meehan, said coasters weren't oblivious to the risk.

"They are aware of the hazards they face, and in general, they're quite a resilient community."

That said, the region recognised that serious preparations were needed.

One previous proposal even explored shifting the entire township of Franz Josef to Lake Mapourika about 10km away, but found that the $300 million cost of it would be too much for the tourist hub's meagre 444 rateable properties.

A regional business plan had been put together to address some of the issues, but support for that work was recently turned down by the Government's Provincial Growth Fund, Meehan said.

"Since then, we've been looking at other ways to advance what's required."

More widely, scientists, authorities and emergency planners were working together under a sweeping, nationwide effort called Project AF8.

The project has developed scenarios so South Island communities can plan how they might respond to impacts such as road closures, power losses, dislocation, fatalities and injuries.

Based on the first seven days of emergency response, AF8's planning covered such things as how to shelter tens of thousands of stranded people – and how to provide immediate medical care.

Civil Defence director Sarah Stuart-Black was confident this leg-work would pay off.

"If an Alpine Fault earthquake were to happen today, we now have more comprehensive planning arrangements in place at all levels of society to help us respond more effectively as a nation."

The project's science leader, Dr Caroline Orchiston, recently wrapped up a three-week tour of 11 communities and nine schools.

"We found that there is widespread interest in the AF8 science and preparedness information throughout the South Island, particularly regarding the secondary hazards and impacts associated with an AF8 earthquake," she said.

In Murchison, which was rocked by a 7.3 quake in 1929, about 10 per cent of the town turned out for the public talk.

"In nearly every location the question of 'how long should we be prepared to look after ourselves?' came up, particularly on the West Coast," Orchiston said.

"Our key message is that, while we can't predict earthquakes - we can prepare for them."

The Alpine Fault

• Running about 600km up the spine of the South Island, the Alpine Fault is the on-land boundary of the Pacific and Australian tectonic plates.

• The fault has ruptured four times in the past 900 years - 1717AD, 1620 AD, 1450 AD, and 1100 AD - each time producing an earthquake of about magnitude 8.

• The fault has a high probability - estimated at 30 per cent - of rupturing in the next 50 years. The rupture will produce one of the biggest earthquakes since European settlement of New Zealand.

• Researchers have estimated a major event could leave at least 10,000 people stranded, and block roads and highways in 120 places.