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As almost everyone knows – 91 per cent of the population, according to a survey last year – an earthquake is the most likely disaster to strike Nelson.
The city and Richmond straddle one fault system. Another, the giant Alpine Fault, seen as the most likely source of a devastating South Island quake, runs within 40 kilometres of Nelson at its nearest point.
Still more faults lie to the west and triggered two of the worst earthquakes to rattle this country in recorded history – the 1929 Murchison quake (17 dead) and the Inangahua one in 1968 (three fatalities), both within 200km of Nelson.
In the various charts and tables scattered through the region's civil defence and emergency management plan, there's no room for doubt about the No1 threat from nature.
It is still a question of if, not when, a truly serious earthquake strikes this region – civil defence calculations suggest there's a 40 per cent chance of a "destructive" quake hitting Nelson sometime during the next 50 years.
The region would most likely be affected by a rupture on the Alpine Fault, even if it happened hundreds of kilometres away – and the science suggests such a rupture is due any time.
Less is known about the regional faultlines, such as the Waimea-Flaxmore system, which runs along the edge of the city, but major movement there could be catastrophic for the region.
Nelson geologist Mike Johnston says that on "very, very rough averages" it is thought to move every 6000 years – and the last known movements were 6000 years ago.
The complication is, the averages could be out by 2000 to 3000 years; so it may never happen for those of us alive today.
The popular line in the earthquake forecasting business is that the big one could happen this afternoon, and despite the converse possibility that it may never happen in our lifetimes, a catastrophic earthquake is the only threat which gains an "extreme" risk rating in the civil-defence plan, given the consequences it would have for the region.
Death and injury for starters, of course. Potentially huge landslides, including across the highways in and out of the region.
The port knocked out of action, the airport badly damaged, its runway destroyed, vital infrastructure crippled, isolated communities such as Golden Bay cut off for weeks, food supply lines blocked. Everything that's taken for granted, in other words, lying in a heap.
"We think there would be loss of life and we think there would be lots of injuries," says Nelson-Tasman civil defence emergency management officer Roger Ball.
"Quantifying that is impossible. It really depends on the time of day it happens – for example, if it happens at night then the chances of those numbers being higher reduces because people are safe in bed and not driving around. But if they're at work there's more opportunity for nasty things to happen ... Most people get injured or die when things fall on them."
He won't be drawn on where would be the worst places to be – "there are so many variables" – but one obvious thing in the city's favour is the lack of high-rise buildings; and, for modern buildings, the high standard of building work demanded by the most recent version of the Building Act, to withstand at least moderately-destructive quakes.
How many earthquake-prone buildings we have, however, is unknown. The region's local bodies are working on a database, as required by the act, but it is an enormous job.
Doing anything about all those which are regarded as high risk will take years. Plus, the exercise does not affect most houses.
One troubling feature of Nelson is the hilly geography and what appears to be an ancient history of massive landslides, probably caused by big shakes long ago.
"We just don't know how they'll react to an earthquake," Mr Ball says. But it seems certain they would be joined by more landslides. Earthquake planning assumes that all highways would be blocked by landslides on the Whangamoas and on any of the steep, twisting stretches towards Murchison.
Closer to the coast, the immediate geological hazard would be liquefaction, a spooky phenomenon which affects certain soils when they become saturated – waterlogged sand, for instance – making them behave like heavy liquid during the shake.
The result can be huge damage to any structures on or in them. Mr Ball says the reclaimed land around Port Nelson and the airport are identified as vulnerable to the phenomenon (although Dr Johnston is not so sure the port is vulnerable).
Both could be severely damaged; the port might even require resurveying before larger draft vessels would be allowed in, closing off one of the critical supply lines, particularly for fuel.
As with most cities in this earthquake-prone country, many of the pipes and cables carrying vital infrastructure run across the faultlines which crisscross the countryside, or are attached to vulnerable bridges.
The operators of these "lifeline utilities" have legal obligations to keep them functioning "to the fullest possible extent" during and after earthquakes.
More than 20 organisations are on the list of bodies which have a direct responsibility for lifeline utilities around Nelson-Tasman, including electricity, water, wastewater, telecommunications, roads, petrol and gas; as well as Radio New Zealand, TVNZ, and the airport and port companies.
The region's critical electricity substations, at Stoke and Kikiwa, are close to faultlines, and while they might be sufficiently engineered to withstand a bad earthquake, the greater weakness is the hundreds of kilometres of lines and pylons carrying power from the Waitaki hydro scheme in South Canterbury.
A quake "might affect the substations themselves, or just bring pylons down," Mr Ball says. "You can see how we've got to be careful with speculation ... Power could be restored within days, or it could take a week or two. Or it could be sporadic."
Among the most critical lifelines in the immediate aftermath would be telecommunications and, "we've very vulnerable on telecoms", Mr Ball says.
"People think they will be OK because they have mobiles, but they run down quickly," and so do the cell-tower batteries which provide back-up power to the cellular network.
"Civil defence would rely on radios and satellite phones; maintaining communications would be the top priority, ahead even of keeping the power on and water supply functioning
"Without that [communications] we can't build a picture. After that, electricity is usually considered the most important because nothing else works without power."
Many of the scenarios envisaged in planning reflect how quickly society becomes reliant on changing technology or business management.
How would services as basic as sewage pumping, or as vital as hospital care, cope without computers?
Where would we get food from when the heavily centralised (out of Christchurch) system of restocking supermarket shelves is crippled?
"We would expect to be closing supermarkets, and opening them on a restricted basis with security," Mr Ball says, at least until road links could be re-opened.
All this should start to bring into focus Civil Defence's mantra about the need for people to have a three-day supply of food, water and basic medical equipment at home.
Self-sufficiency is not so much about people fending for themselves in the rubble, but keeping the pressure off the authorities in the aftermath.
The survey that found 91 per cent of people recognised the risk posed by earthquakes, also asked, "do you have an emergency kit or any emergency supplies in your house".
Fifty-eight per cent did; 42 per cent did not, which, Mr Ball says, "isn't good enough".
One thing that gives him heart, though, if he should find himself helping co-ordinate Nelson's recovery from a calamitous earthquake, is the nature of this community.
"I think we're fortunate that our population is of a size where people aren't anonymous, where neighbours will help each other, where people will pull together.
"I think that's probably going to be the thing that would get us through something like this.
"Because it will be a shambles, the response will be a shambles. But history tells us in New Zealand that people do pull together."
- © Fairfax NZ News