What's the risk of a major earthquake, volcanic eruption, tsunami or wildfire in New Zealand – and how might it play out?
The next big one
A major Alpine Fault earthquake is likely within our lifetime.
Caroline Orchiston was a third-year geology student when she saw it. “It was one of those moments I’ll never forget,” she says. “When it’s described to you what it actually means – where these rocks have come from and the power required for that to happen – it’s quite stunning.”
After five years in the mining industry, that memory led her back to the Alpine Fault and her current role as science lead of Project AF8, in partnership with Civil Defence emergency teams. Three potential scenarios have been modelled of a magnitude-8 shake caused by a rupture along the fault, and agencies throughout the South Island are working on a co-ordinated response plan to cope with what could be a national disaster of almost unimaginable proportions.
Unlike a volcanic eruption or a tsunami – or the gradual nature of sea-level rise – there’ll be no warning. And the next big one is already more than a year overdue. The Alpine Fault generates a major earthquake on average every 300 years; the most recent was in 1717. “They’re remarkably regular through time,” says Orchiston, who’s based at Otago University as deputy director of the Centre for Sustainability. “What that tells us is there’s an inevitability about the next quake and we’re a little on borrowed time.”
One of her projects over summer is to talk to local kaumātua to see if any oral histories have survived of the 1717 earthquake. So far, no accounts have been recorded.
The main AF8 scenario, and the most testing one because of the number of people affected, starts in Milford Sound and intensifies as it moves north, with severe shaking right up to Nelson – creating a 400km rupture along the western side of the Southern Alps. Another places the epicentre to the north, near Greymouth, pushing southwest; the third is a bilateral rupture midway along the fault that radiates in both directions.
Although there’d be no damage in the top half of the North Island, shaking would be felt widely across the whole country. Along the West Coast, landslide dams would threaten settlements downstream, and rockfall tsunamis could be devastating for places like Milford Sound. Cut off by road, with some Alpine passes closed for well over a year, isolated communities would face power and communications blackouts.
In centres like Queenstown, a top priority would be evacuating thousands of tourists, who quickly become a burden on the local population. “We saw that in Kaikōura,” says Orchiston. Despite such a nightmarish vision, she doesn’t view AF8 as some kind of doomsday project. “Evidence shows communities that are cohesive and engaged do better after a disaster. New Zealand is high risk for this stuff and the work that’s being done now will make a difference.”
The Earthquake Commission has just funded a project exploring the potential for quakes in low-seismicity areas, with an initial focus on Otago. Christchurch was considered relatively low risk before 2010, and new fault lines are still being found.
In the capital, a GNS Science study puts the estimated probability of a rupture of the Wellington-Hutt Valley segment of the Wellington Fault during the next 100 years at about 11%. More people could be killed by the resulting tsunami than the earthquake itself, and many thousands of homes would be at risk from landslides in the hills.
The idea that three days of supplies will be enough to see you through in a disaster is unrealistic, says Orchiston, who recommends having an emergency kit (including food and medication) that will sustain the family for the first couple of weeks.
She turned down a good job offer in Wellington because of the earthquake risk, although she still regularly visits for work. “It’s an occupational hazard,” she says. “I love Wellington, but I’m always grateful when the plane takes off and it hasn’t happened.”
A wave of destruction
Rising sea levels aren’t the only threat faced by people living on the coast.
When tsunami scientist William Power moved house, he swapped a home 50m from the Wellington fault line for one more than 100m above sea level. “I did look at a nice house [on the coast] and think that if I needed to respond to an event, I didn’t want to be worrying about what was happening to my family down on the beach.”
A senior geophysicist at GNS Science, Power was lead author of a 2013 report updating New Zealand’s tsunami hazard, modelling peak water heights around the entire coastline, including waves of up to 10m that could strike Northland, Great Barrier Island, parts of East Cape and the Wairarapa.
On average, a dangerous tsunami hits the shoreline every 40 to 50 years. In 1947, two earthquakes north of Gisborne caused run-ups of up to 11m above sea level. Power says it was lucky the waves struck a sparsely populated stretch. “If the same thing happened now in the summer holiday season, it could be a very different story in terms of casualties.”
There’s nowhere on the coast that doesn’t face some degree of risk, but the three main scenarios are:
- A tsunami is triggered by an offshore earthquake in the Hikurangi or Kermadec subduction zones, with potentially catastrophic consequences for the entire east coast of the North Island from the Far North to Wellington, and spilling into Marlborough. The threat to Auckland would be “somewhat mitigated” by buffering from the Coromandel Peninsula and Great Barrier Island.
- An earthquake in South or Central America sends a tsunami across the Pacific Ocean, affecting the whole coast of New Zealand, with Christchurch, Banks Peninsula and the Chatham Islands among the hardest hit.
- A megathrust earthquake in the Puysegur subduction zone south of New Zealand generates a tsunami that strikes the southwest corner of the South Island, from Fiordland to Southland.
A tsunami from South America would trigger the early-warning system and take 12 or more hours to make landfall here, but a local tsunami caused by an earthquake offshore might take as little as 10 minutes. If the ground starts to shake for more than a minute or violently enough to knock you off your feet, people in coastal regions should self-evacuate to higher ground or at least 1.5km inland.
Tsunami evacuation maps are available on council and civil defence websites, and it pays to be familiar with your escape route, says Kate Boersen, project leader for East Coast LAB, a collaborative programme based in Napier that combines scientific research with community engagement. In Japan, some 3000 school students in one coastal city survived the disastrous 2011 tsunami by following a well-practised emergency evacuation procedure. In what became known as “the miracle of Kamaishi”, only five school-age children were among the city’s 1000 dead, and none happened to be at school the day the quake hit.
“They knew their drills,” says Boersen, who’s run tsunami education workshops along the coast. “Be prepared. Know your route and practise it. We’re a coastal nation that loves going to the beach, so at any time you could be in an evacuation zone. Think about where you are and where you could go to be safe.”
In the worst-case scenario, a magnitude-8+ earthquake caused by a rupture in the Hikurangi subduction zone – New Zealand’s largest plate boundary fault – could not only cause severe damage on land but generate a tsunami similar to what was experienced in Japan. “It’ll depend on the earthquake, where the tsunami occurs and where it’s heading, but we’d most likely be looking at international assistance. It would be something New Zealand hasn’t experienced in living memory.”
While tsunamis are relatively infrequent, Power thinks it’s a hazard that needs to be taken into account not only by coastal homeowners but when planning for vulnerable communities, such as the location of retirement villages or childcare centres. “We don’t want people developing or living in places where they can’t evacuate fast enough. It’s balancing the freedom of people to live where they like versus picking up the pieces afterwards – and social questions around who pays for picking up the pieces and how we allocate risk.”
Under the volcano
Up to half a million people could be evacuated from Auckland if subterranean rumblings warn a new volcano is about to blow.
Volcanoes always “put their hands up” before erupting, says GNS Science volcanologist Brad Scott. “Our challenge is to recognise they’ve done that.”
In Auckland, the first warning that superheated magma is smashing up through the Earth’s crust will be when a network of seismographs start detecting local earthquakes. By then, the rising magma will be 15-30km below the surface. That might give as little as five days warning before a 54th cone is added to the Auckland Volcanic Field.
By projecting the earthquakes from the city’s volcanic monitoring system to the surface, it should be possible to get a rough location of the eruption site and guide planning to mobilise an emergency plan to evacuate everyone within a 5km radius – up to a fifth of the population. “As soon as it looks like something is rumbling under the city, very, very high levels of preparation need to be put into place very, very quickly,” says Tom Wilson, an associate professor in disaster risk and resilience at Canterbury University. “There’s serious potential for substantial loss of life if the evacuation isn’t successful.”
That’s a grim prospect, given the gridlock out of Auckland on a typical holiday weekend, with a “shadow evacuation” inevitably adding to the chaos as people outside the exclusion zone also decide to make a run for it. But Wilson says scientists have worked with emergency services and civil defence to create a well-developed plan. “In such a dynamic situation, having those formal and informal networks in place will be critical.”
Last year, Wilson was involved in modelling an eruption in the Manukau Harbour near Māngere Bridge, causing complete destruction within 2.5km of the vent and long-term damage to essential services, with electricity outages and water restrictions lasting more than a year. The ashfall, like the liquefaction in Christchurch, would costs millions to clean up.
The Devora (Determining Volcanic Risk in Auckland) project is building a simulation of Auckland’s transport network, mapping congestion hotspots during an evacuation, and exploring alternative eruption scenarios. Key infrastructure is being rated according to a “fragility index”; for example, one PhD student is looking at how deep telephone cables would need to be buried to keep functioning if they were overrun by a lava flow.
It’s impossible to predict when or where the next big blow in Auckland will be. The wildcard is Rangitoto, the most recent volcano to erupt (about 600 years ago) and the first cone in the Auckland field to have erupted twice. “To make a human analogy, that’s a big personality change,” says Wilson. “What happens next, we’re really not sure.”
National parks create a natural buffer around Mt Taranaki and the trio of volcanoes in the central plateau. Taranaki has a major eruption on average every 300-500 years, most recently in 1655, and contingency plans are in place to block off the park and evacuate the rural community within a 15-20km boundary zone.
A major eruption would cut off the roading network and isolate New Plymouth, leaving it accessible only by sea or air, and have a massive impact on the dairy and petroleum industries.
But while 72 distinct layers of volcanic ash from Taranaki have been found in Auckland, none from White Island has left a trace on the mainland, suggesting a more localised threat. That would be little comfort for any tourists who happened to be in the wrong place at the wrong time, because with an active volcano like White Island there’s no “societally useful warning” before an explosive eruption.
One of Brad Scott’s focuses is on volcanic unrest in the eight caldera volcanoes in the Taupō Volcanic Zone: “supervolcanoes” that erupt rarely but with catastrophic power – collapsing into themselves rather than forming a cone. Tarawera was the most recently active, destroying the Pink and White Terraces in 1886, while Taupō still holds the record as the world’s most violent eruption in the past 5000 years.
Volcanic unrest in the calderas triggers unsettling earthquake swarms that can be felt in the surrounding districts and last for weeks to months. This is a key area of research for the Eclipse (Eruption of Catastrophe: learning to Implement Preparedness for future Supervolcano Eruptions) project on the Taupō Volcanic Zone: to try and interpret what’s happening underground, and what it might mean for the future of life on the surface.
In the line of fire
Wildfire danger is predicted to double or even triple in parts of New Zealand over the coming decades, with homes in semi-rural areas particularly at risk.
As the California wildfires consumed thousands of homes in November, killing at least 88 people, firefighting teams were making life-and-death calls about which properties could be saved and which should be left to the flames.
Those same risk assessments were made last year as the Port Hills burned above Christchurch. It’s a standard triage process, based on factors such as accessibility, water supply and the safety of the firefighters who’d be sent in, says Scion institute researcher Grant Pearce, a specialist in wildfire behaviour.
“There’s an expectation every house will be protected, but we simply don’t have the resources to do that,” he says. “It’s a conversation we need to have with property owners that if the worst comes to worst, we’re not going to be able to protect your property. If you want it saved, you need to start doing something about reducing the risk.”
While New Zealand hasn’t seen wildfires on the same devastating scale as in the US, Australia or parts of Europe, the number of days rated as a very high or extreme fire risk are projected to double or even triple in some areas by 2090 as climate change brings higher temperatures, lower rainfall, lower humidity and stronger winds.
Modelling by Scion shows conditions in regions already red-flagged for wildfires, such as Christchurch, coastal Otago, Marlborough and Gisborne, will worsen, while Dunedin, Whanganui, Hawke’s Bay and down the east coast to the Wairarapa and Wellington will likely see significant increases in fire danger. Last summer, Fire and Emergency New Zealand positioned “incident management teams” at Clyde, Invercargill and Dunedin for the first time, with helicopters on standby. Another 1°C rise in temperature, says Victoria University climate change scientist James Renwick, and the entire eastern stretch from south of Dunedin to the East Cape could be in the very high or extreme danger zone for up to six months of the year.
New Zealand isn’t infested by pests like the mountain pine beetle, which has ravaged plantations in Canada and the US, where dead trees are another fire risk. But other insects will thrive as temperatures warm, and native bush is already under attack from diseases such as kauri dieback and myrtle rust. Helicopter footage of the Russell State Forest, released by Forest & Bird in September, shows a “ghost forest” in severe collapse from damage by rats and possums.
Private homes have become increasingly vulnerable to wildfires as residential developments extend into semi-rural land, butting onto what Pearce describes as “flammable fuels” (vegetation and bush).
“You only have to look at the last few fire seasons, which weren’t particularly bad climate-wise, but we had significant losses in the Port Hills and Hawke’s Bay, and in the Coromandel near Whitianga, where a major fire took out a number of properties. We’ve gone from seeing one to two houses lost a decade to almost one to two a year,” Pearce says.
Fire and Emergency is pressing councils to take wildfire into account as a natural hazard; after the Port Hills fire, the Selwyn District Council incorporated the risk to property from wildfire into its district plan. Pearce would like to see high-risk areas identified (for example, on property LIM reports) and regulations targeting new subdivisions, from controls on construction materials and design to the provision of a firefighting water supply and setback distances from forest and bush to enforce a “defensible space”.
Homeowners should consider the fire risk of plantings close to the house and regularly trim away dead material and lower branches so it’s harder for flames to spread. A study at Lincoln University tested the flammability of 150 native species and found mānuka shrubs were “right up there with gorse”, says Pearce, who recommends fitting wooden houses in high-risk zones with external sprinklers.
“You don’t have to live in a concrete bunker, but think twice about building a house right on the top of the ridge, where you’ve got the best view, with flammable fuels right underneath you.”
These articles appeared as part of Safe as houses? The biggest risk to your greatest asset, published in the January 2019 issue of North & South.