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.

On Sunday, the Alpine Fault experienced its largest quake since 2001. It prompted a GeoNet warning stating: the Alpine Fault had "the potential for larger events and we would like to make sure that you are prepared for a large earthquake at all times".



This piece examines what will happen when the larger quake hits. It was first published in 2017.



The sound of an earthquake will be deafening in the steep, forested valleys of the West Coast.

Birds will be rattled from the trees; The gentle pitter-patter of rivers silenced by the noise of rock being torn from the mountains, cascading into the valleys, stripping trees from forests.

Dogs howl moments before their owners feel that mysterious jolt, what's called a compression wave, that signals the arrival of a disaster.

Arthur's Pass, the main route through the Alps, would be cut off for six months.

Along the western edge of the Southern Alps, a great, long line spanning hundreds of kilometres, thousands of landslides will trigger instantaneously.

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The shaking near the epicentre, not far from Haast, will be unlike any felt in modern New Zealand history.

PETR HLAVACEK/SUPPLIED Milford Sound will be ruined by an earthquake on the Alpine Fault.

Everything not tied to the ground will move. Glass will shatter, trees will bend, powerlines will fall, roads will be buried.

The earthquake will unfurl across the South Island like a zip, dragging north-east. Two minutes after it began, it will hit Christchurch. It will feel like the Kaikoura quake, a deep, rolling bellow, lasting a couple of minutes.

A minute later, Westport will shake. Another minute, Blenheim. Then Wellington. There may be reports of shaking as far as Sydney.

DEBBIE JAMIESON A likely scenario authorities are preparing for is the isolation of entire communities, particularly Queenstown.

An Alpine Fault earthquake will be felt by every South Islander and likely many North Islanders, too. Numerous experts say it will be the largest natural disaster of modern times in New Zealand.

It will take all of five minutes. Then the hard part begins.

The major issue with fault lines is that they're hidden.

Dschwen/Creative Commons An Alpine Fault rupture may be centred near Haast. Pictured is the bridge over the Haast River.

Like veins beneath a person's skin, they can be unearthed if you know what to look for, but it often comes too late.

When the first Polynesian explorers circumnavigated the South Island, coming across the spectacular, snow-capped Southern Alps, they didn't know what it represented; one tectonic plate grinding against another for millennia, pushing the great mountains upwards.

Neither did the first European settlers, who built communities on the West Coast, unaware of the tectonic churning deep into the earth, a great dragon beneath.

John Kirk-Anderson An Alpine Fault rupture may take two minutes to be felt in Christchurch.

It wasn't until the mid-20th century when the Alpine Fault was discovered at all. By then, it was too late.

Two geologists were traversing remote Westland in search of mica, a material used in electrical components; they took the opportunity to do geological mapping of the region, which had not been done before.

With the trained eyes of surveyors, they saw the way the mountain faces dropped in a steep, uniform line, the way rock had eroded to form a valley like depression, and knew what that represented.

DAVID HALLETT/FAIRFAX NZ Helicopters will be crucial to the disaster management effort.

Lives had already been built on the fault; the main road, at points, ran parallel. The town of Franz Josef is on top of it.

All of the West Coast's population centres were built dangerously close to the fault, before anyone knew it was there.

In the decades since, understanding of the fault has grown immensely.

DAVID HALLETT/FAIRFAX NZ Tourists get off bus at the boat harbour Milford Sound.

Last week, research based on a more complete earthquake record revised the return period of a quake to 291 years. The last was in 1717, exactly 300 years ago. Every year that passes pushes us further into the wrong end of that equation.

A succession of earthquakes has the country on high alert. They are now turning to the Alpine Fault, the one everyone knows presents the biggest threat.

Over the last few months, emergency management officials throughout the South Island have been preparing for what to expect, and how to coordinate a response.

It's a programme called Project AF8, and proposes an 8.2 earthquake triggering near Haast.

Because the Richter scale is logarithmic, the size of an 8.2 is deceptive.

The energy released would be four times that of the Kaikoura earthquake, and 700 times that of the 2011 Christchurch earthquake.

In New Zealand, we use the Modified Mercali Intensity scale. It grades earthquakes on how they feel.

An MM1, for example, produces no shaking. An MM5, rated as "strong," produces mild shaking and some damage.

Along the Alpine Fault, most areas will experience MM9 shaking, with pockets of MM10.

That's described as "very destructive;" weak buildings will fall down and many more will be damaged. Key infrastructure will be destroyed, and people will die.

SOUTH ISLAND COMMUNITIES WILL BE CUT-OFF

In the moments after the shaking stops, when that feeling of post-quake paralysis has subsided, it will be dark.

Every region will be alone.

Much of the South Island is expected to lose power immediately. Some communities, particularly on the West Coast and in western Otago, won't have power for weeks.

Depending on the damage to the island's hydroelectric schemes, the electricity problem could extend further. They supply around 50 per cent of the country's power, putting the whole country under strain.

They are built to a high standard, unlikely to be damaged beyond repair. But for a while, at least, they'll be shut off.

For the first responders, communication will be the first problem. The quake will be a South Island-wide event, with an unprecedented area of impact.

The 2011 Christchurch earthquake, for example, was the result of a 16km fault rupture. Despite catastrophic damage in the city, its area of impact was about 50km.

An Alpine Fault rupture would likely be 400km long, stretching across much of the island. Everyone in the South Island would be within the area of impact.

An earthquake could trigger anywhere on the fault, but experts believe it is most likely to originate at the southern end, near Haast, where the Australian and Pacific plates are most tightly coupled.

Modelling shows that from its starting point, the shaking will radiate outwards, moving north-east.

Its size means those at the top end will get a couple of minutes of warning; for everyone else, the destruction will be immediate.

The damage to everything on a rupture itself is catastrophic, as seen in Kaikoura.

A road near Waiau on the rupture was obliterated; a house on a rupture north of Kaikoura was split in two.

Large, remote communities, such as the Gloriavale Christian community near Haupiri, which is just several hundred metres from the fault, will likely be devastated and completely cut-off.

The extent of the damage further afield will be unknown for weeks. Helicopters will be dispatched to isolated areas, but there will be so many it could take weeks to get to everyone.

Communications across the West Coast and the Queenstown-Lakes district could be down for weeks.

For a long time, people will be left to themselves in isolated areas, completely and utterly alone.

MAJOR RIVERS WILL BE DAMMED

Many of the large problems caused by the earthquake will stem from one singularly massive problem: landslides.

Research in 2013 estimated the amount of material produced in landslides after an Alpine Fault earthquake to be one billion cubic metres, covering an area of 30,000km².

It's impossible to predict where and how severe the landsliding will be. There are too many variables.

At the very least, it's likely thousands will be triggered instantly, crashing soft soil and rock into dozens of steep, narrow valleys, up to 300km from the epicentre.

The immediate problem this presents is one of transport. Three roads lead to the West Coast, and all are exposed to landslides.

The region's roading network was poorly designed to handle failure. When prime minister Sir Keith Holyoake officially opened the road to Haast in 1965, it had to be closed just hours later because heavy rain caused a slip.

That road will be completely destroyed by landslides, experts say, much like the road north of Kaikoura was last year.

Where the road crosses the fault, it could be horizontally displaced by 10 metres. Haast will be the last township to be re-connected to the rest of the world, up to three months later.

Arthur's Pass, the main route through the Alps, would be cut off for six months. For about six weeks the entire West Coast may be isolated, cut off by road and by rail.

The broader ramifications of roading failure are severe.

If it takes two days to repair a downed powerline in normal circumstances, but it's on a road that will take six months to fix, its repair time is six months and two days.

Aftershocks will continue for months. Some may be magnitude 7, destructive in their own right, too dangerous for contractors to approach.

Water and sewage supplies may be cut off near the fault. Many in the affected region will be tourists.

A likely scenario authorities are preparing for is the isolation of entire communities, particularly Queenstown.

There are three ways in and out of the township, all vulnerable to landsliding. Its airport, like many others, will be closed temporarily.

At the height of summer, at any one time, tens of thousands of tourists are in Queenstown. Many will be completely unaware there was an earthquake risk in the first place. Some won't speak English.

They will be trapped in a town without power or means of communication with the outside world.

Unlike locals, they don't have a support system. Local authorities don't have accurate records of how many visitors are in a town at any one time.

Hundreds of tourists were evacuated from Kaikoura by helicopter and by sea, which was a massive co-ordination challenge.

Multiply that number by at least 10, and you have the issue in Queenstown.

These scenarios will play out in individual communities throughout the South Island, each enormous in their own right.

If all goes well, those will be the worst problems to deal with. It's possible they'll be much worse.

THE TSUNAMI RISK

Picture Milford Sound when the earthquake hits.

Hundreds of visitors are scrambling to their feet, dazed, struggling to hear beneath the sonic blast of rocks crashing down from the mountains.

The fault crosses the entrance of the fiord.

Researchers have found evidence of past landslides from earthquakes on the sound's bottom. The tsunamis generated by those events would have ranged from 40cm to 47m.

Past disasters internationally show that tsunamis are difficult to escape, and cause catastrophic damage.

There could be 1000 visitors at Milford Sound on any one day, who will need to find higher ground within minutes. By the time the shaking stops, the wave will be on its way.

It sounds fantastical, but it's an issue authorities are urgently preparing for.

In Norway in 1934, a landslide-triggered tsunami in a fiord similar to Milford Sound killed 41 people.

Modelling has shown tsunamis could similarly generate on Lake Wakatipu and Lake Wanaka, swamping low-lying parts of the nearby townships.

Fatalities are a difficult subject to broach. Scientists are wary of being seen as fear-mongering, and deaths are inherently difficult to predict.

Because much of the West Coast is remote, the catastrophic death tolls from earthquakes overseas simply won't happen. As seen in Kaikoura, most buildings that weren't directly on the fault survived.

In 2013, disaster management officials held a preparation exercise related to a theoretical Alpine Fault earthquake based on years of research and data from experts.

It was not a prediction: simply an event deemed to be realistic for the purposes of the exercise.

It was called Te Ripahapa, and for its scenario, it put the death toll at 455 dead and more than 7000 injured.

Most of those theoretical deaths came from building collapse, most in the Grey District. It attributed another 162 deaths to tsunamis at Milford Sound and Lake Brunner.

The actual number could be either much lower, or higher. It depends on things that are inherently uncontrollable: mostly time, and weather. If it's raining, landslides will become more severe; if it's day time, tourists will be among the hazards.

What is more certain is that the effects of the earthquake will last decades. Liquefaction, that nasty process where solid land turns to sludge, will be widespread.

Greymouth and Queenstown in particular will be vulnerable. Even Christchurch, already ravaged by it years ago, could find the ground liquefied once more.

Some in Christchurch will tell you its disaster is still going; Insurance claims prolonged, land stripped of value, a regional economy still dusting itself off.

The possible long term ramifications are endless. The West Coast's sediment clogged rivers would aggrade and change course, rising in elevation and flooding low-lying towns routinely.

Aftershocks would continue for years. The conservation estate, stripped bare of vegetation, dealt a permanent blow. Regions built on tourism remaining empty for years.

The Canterbury and Kaikoura earthquakes have prompted consideration of these issues much more intently.

Experts admit that 10 years ago, we wouldn't have been prepared.

The work to prepare is under way, more so than ever, but one fact remains inescapable, gnawing at the brain. We're overdue.

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