Last week's rare earthquakes reminded Aucklanders they are not immune from natural disaster. Jamie Morton explores what might happen in New Zealand's biggest city in the event of an eruption or major quake.
It starts with an earthquake.
As they did on the afternoon of March 17, tens of thousands of Aucklanders take to Facebook and Twitter to express their amazement that their seemingly quake-free city has had a rumble.
GeoNet, New Zealand's official quake recorder, is showered with notifications from excited residents.
The keenly felt shake has historians making comparisons with the city's strongest known quake - in 1891, which toppled a chimney in Onehunga and smashed crockery.
But the quakes continue - and the scientists begin to worry.
Somewhere in Auckland, a volcano is rousing from a long slumber - its reawakening marked by the shakes of breaking rocks as a batch of magma makes its way 80km to the surface.
Around the site, volcanologists scan for ground deformation and changes in chemistry.
The smell comes a few days, weeks or months later. Pungent sulphur dioxide pours out of the ground as activity within the volcano escalates.
Civil Defence planners look at evacuation arrangements - how and when will hundreds of thousands of Aucklanders be moved, and where?
When the threat is real enough, the order is given and the process begins.
Welfare centres across Auckland - large stadiums and venues - begin to fill up while hundreds pour into smaller evacuation centres.
No one is home as the eruption finally comes, wiping out an area of 3km to 4km around the vent with explosions, scoria falls and lava flows.
But eventually the volcano quietens down again, and Auckland gets back to business. Regional GDP has taken a hit, yet a localised ashfall - much smaller than large plumes that could be blown from the major volcanoes to Auckland's south - lessens the disruption to the airport.
Despite last week's dual 3.1 and 3.9 quakes below Motutapu Island - which led to a 300 per cent increase in survival kit sales - scientists and authorities remain convinced Aucklanders would more likely see a volcanic scenario like this rather than a major quake.
While the region is criss-crossed with faults, just one - the Wairoa North Fault south of the Clevedon Valley - is listed as active on a GNS Science database.
"In the Alpine Fault in the South Island, there's been a recurrence of a big earthquake once every 200 to 300 years," GNS earthquake geologist Russ Van Dissen explained.
"By comparison, the Wairoa North Fault, by the limited work we've done, suggests it makes big quakes every several tens of thousands of years."
Auckland was considered within the lowest earthquake activity areas largely because the quake-making Tonga-Kermadec subduction zone lay around 300km away.
Even the Wairoa North Fault - Auckland's only known active fault - had shown no evidence of rattling in the past 10,000 years.
"The earth isn't being stressed because it's far away from the plate boundary and there's not a lot of energy being dumped into it," Mr Van Dissen said.
Outside the city, the Kerepehi Fault in the Firth of Thames has been cited as a potential source of a damaging earthquake.
In the event of one, geological surveys have found that Auckland's buildings would not be significantly affected by the liquefaction that plagued Christchurch, thanks to the region's generally hard volcanic rock or ancient mud and siltstones.
Auckland Council has listed reclaimed areas around the port, floodplains of the Kumeu and Kaipara Rivers and the Manukau lowlands as potential areas for localised liquefaction.
Mr Van Dissen could not rule out there being more faults around Auckland city, although he said scientists still factored in random "floating quakes" when calculating hazard probabilities.
"Is there one right under the city of Auckland? It would be really unlikely."
But Dr Jan Lindsay said that although the probability of a major quake any time soon in Auckland was low, the "risk" was still classed as relatively high given an earthquake's potential impact on the city.
Dr Lindsay, an Auckland University volcanologist, is one of the key figures in the inter-agency Determining Volcanic Risk in Auckland (Devora) project.
Underground monitoring scientists' best predictor
It aims to better understand the Auckland Volcanic Field - containing around 50 different volcanoes - and its potential impact on people and infrastructure.
But a question mark hangs over where or when the next eruption might occur within the field - or what might spark it.
"If you just look at the Auckland Volcanic Field, it's been active for around 250,000 years and there has been about 50 to 55 volcanoes formed in that time, so that means on average there's one about every 5000 years - that's a basic statistic we can do. However we can see in Auckland there are periods when there is more activity than in others."
Rangitoto - which gave Auckland its most recent eruption about 600 years ago and has erupted twice - has also thrown a spanner in the works for scientists attempting to spot patterns or forecast probability.
"For example, Rangitoto's most recent eruption was right next to [Lake] Pupuke, which is thought to be the oldest one - so there's no obvious northward or southward trend," Dr Lindsay said.
Scientists draw three possible conclusions - that Rangitoto's activity was an anomaly and has now ceased, that the entire field's activity has now moved to Rangitoto, or that future eruptions may occur at Rangitoto and elsewhere in the field for the next 500,000 years.
Wherever the next blow falls, an eruption would cause certain havoc - and devastate an area stretching kilometres from the vent.
"If a volcano produced an explosion crater, that could be 1-2km across, and everything in that would be destroyed," Dr Lindsay said.
"And if that does happen and you have a big base surge or pyroclastic flow, one of those could travel three, four, sometimes 5km."
In Auckland, these powerful blasts usually travelled 1-2km, but contingency plans allowed for a 3km radius.
"We then have a 2km buffer area, and beyond that an area which should also be evacuated just in case."
Response efforts would focus on the area directly affected by an eruption - and how many people would be indirectly affected by having to evacuate.
But it was unlikely an eruption would result in deaths, said Clive Manley, Auckland Council's group controller and manager of Civil Defence and emergency management.
"In our scenario for a volcanic eruption, we don't believe there would be casualties because the best advice we get is that we will have time to safely evacuate people."
Even a three-day lead-up would still give plenty of time, he said.
There was also no scenario of an eruption taking out the whole of Auckland.
"It would be a bit like the power crisis, if you like - there would be an area where people would have to move from somewhere else."
Any decision to evacuate would come after advice from the Auckland Volcanic Scientific Advisory Group, which includes leading scientists from agencies and universities across the country.
Mr Manley said authorities would need to gauge whether magma was on its way towards the surface, or the activity was likely to be limited to rumbling.
"That's the uncertainty that exists at the beginning of any activity - is this for real, or is it going to die away again?"
Authorities had mapped out various location-specific scenarios, allowing them to work out population bases in different areas at various times of the day.
"I think the unknown would be how long it will go on for - if you have a brief eruption, then yes, you'll have a devastated part of an area, but the economy will return quite quickly."
Research by Market Economics estimated that in a worst case eruption near the CBD, the Auckland region could suffer a 47 per cent reduction in gross domestic product.
But this could be reduced by 40 per cent if businesses had effective preparedness measures in place.
New Zealand would suffer a 14 per cent decline in GDP, however relocated business could bring about a 3 per cent rise in GDP in the rest of the country.
Ultimately, Civil Defence's message to Aucklanders was simple.
"Regardless of the threat, you are going to have to either leave your property quickly, or you are going to be staying in your property with reduced services because you don't have power.
"If you actually manage yourpreparedness around that, then you are going to be equipped for anything - whether it's volcano, power outage, flood or whatever."
HIDDEN VOLCANIC BLAST STUDIED
Destructive blasts known as base surges are one of the most fearsome weapons in the volcanic arsenal - yet little is known about how these have fired in Auckland's explosive past.
A new study, to be published this year, seeks to shed more light on this phenomenon within the Auckland volcanic field and provide planners with an invaluable tool for mapping scenarios.
Base surges occur when magma is turned to steam or gas and blasts laterally during an eruption.
Capable of travelling up to 180m per second and with temperatures as high as several hundred degrees, these surges sweep over land in a devastating mixture of gas and rock.
Many of the volcanic centres in the Auckland volcanic field have produced such effects, their evidence remaining in surge deposits.
But because the field ranges over urban Auckland, little is physically exposed and the distances these blasts have travelled isn't well known.
Now, a team of US and New Zealand scientists with the Determining Volcanic Risk in Auckland project are using ground penetrating radar at South Auckland sites Maungataketake, Waitomokia and Crater Hill to learn more about base surges.
The images could help scientists model what direction pyroclastic currents would travel - and the power they pack. The data could also be used to model potential impacts on buildings and infrastructure at other centres in the field.
Study co-author, University of Canterbury vulcanologist Dr Darren Gravley, said it had been hypothesised that a surge could fan out 5km from an erupting Auckland volcano.
"That's the kind of dashed radius around one of these in terms of the volcanic hazard, but it's not based on actual evidence - it's based on looking at rings in similar-sized volcanoes around the world," he said.
"So what this study has been seeking to do is find out the limit of these base surge deposits at a couple of examples around Auckland, and try to refine that a bit more."
Researchers would also gauge impacts as the surges moved away from the volcano.
What You Need:
All homes should have getaway kits, emergency survival items, a first aid kit and food and water for at least three days. You should also have essential emergency items in your workplace and in your car.
Emergency Survival Items:
• Torch with spare batteries or a self-charging torch.
• Radio with spare batteries.
• Wind and waterproof clothing, sun hats, and strong outdoor shoes.
• First aid kit and essential medicines
• Blankets or sleeping bags
• Pet supplies
• Toilet paper and large rubbish bags for your emergency toilet
• Face and dust masks
• Food and water for at least three days
Source: Civil Defence.
Learn more about natural hazards in your area, what to do in an emergency and how to prepare at getthru.govt.nz.