Post-quake preparedness a priority for town

GNS Science's Kelvin Berryman.
GNS Science's Kelvin Berryman.

One of the important lessons from the Canterbury earthquakes has been that the impact on communities goes far beyond lives saved or lost. JOHN McCRONE reports.

Would you build a petrol station and motel right on top of an earthquake fault? And not just any old crack in the ground, but straddling the magnitude 8 Alpine Fault?

Franz Josef township on the West Coast highway must surely take the prize as the single most hazardous spot to live in New Zealand. The Alpine Fault cuts straight through the very centre of this glacier country tourist stopover.

When the fault goes - and it is only a matter of time - the town will literally be ripped in two, the separate halves travelling nine metres in opposite directions at a rate of knots.

Not good news for the Allied service station, Alpine Glacier motel or any of the other buildings sitting directly astride the gap. Nor that great for those built up to its lip, such as the police station that was put up just a few years ago.

So what is Franz Josef to do? Move back a decent distance, entirely relocate, cross its fingers and trust the Alpine Fault holds out for another few generations, making it someone else's problem? The current odds are 30 per cent the fault will rupture in the next 50 years.

The town has of course known for a long time that it is bisected by the fault running the length of the South Island - but anecdotally rather than officially, says GNS Science earthquake hazard expert Robert Langridge.

It has not been written into local maps and planning policies he says, so people have been allowed to continue building across it even during the past 15 years. "I've been on several field trips with a car-load of geologists and we go, 'Aw yeah, the fault runs right through the petrol station.' And then we kind of chuckle. But on the other hand, we know it's not really acceptable," says Langridge.

The township has some hard decisions to make now that Langridge has delivered a blunt language report about exactly how exposed its residents and visitors actually are.

Langridge says the issues go beyond simply the risk to life if the petrol station is torn apart. After the big one, Franz Josef could be isolated for months by landslips, broken bridges and re-routed rivers.

'In terms of post-disaster preparedness, you've got to have a petrol station that is functioning so that people can get on with filling up their diggers and vehicles.' It is serious stuff. And Franz Josef is a microcosm of what is happening throughout New Zealand since the Canterbury earthquakes.

We always knew we lived on the shaky islands, but now the actual experience of a worst case scenario disaster is forcing a re- examination of the hazards to which we may have been guilty of turning a blind eye.

In Tekapo, for example, locals are looking at their sweeping foreshore - a tempting spot for town expansion - and realising that it has been probably carved out by mini-tsunamis, the slopping of the lake, when the Alpine Fault ruptures every 330 years or so.

In Wellington, a new Civil Defence report has pointed out that with just two hilly highways in, the city could be cut off for up to four months by a decent shake.

Christchurch was fortunate in being a flat city, ringed by access roads. Help could arrive from all directions, services could be swiftly reconnected, people and businesses were able to relocate. The same earthquakes in Wellington would likely force a mass evacuation, creating a far more permanent economic devastation.

GNS Science's Kelvin Berryman, manager of the national Natural Hazards Research Platform, says New Zealand has perhaps been rather lulled because it has been so long since a really major destructive event like Napier.

'Institutionally, we've probably forgotten some of the lessons because there's been 50 years of fairly benign earthquake activity,' he says.

However, scientists like himself now have people's attention. And as the information continues to pour out about what actually happened in Canterbury - how the land behaved, how people responded - the understanding of quake hazards can be that much more fine-tuned.

'To be realistic, the honeymoon period where earthquake hazards are going to be in the front of everybody's minds is going to be very short. We'll be overtaken by other events before long. But at least there's a window of about two or three years to embed some better processes around the country,' Berryman says.

The Canterbury earthquakes are certainly still yielding up their secrets.

Canterbury University's head of geology Professor Jarg Pettinga says they will be about the best understood anywhere because in the 1990s a former university seismic engineer, John Berrill, installed a network of ground motion sensors right across Christchurch and out into the surrounding Canterbury Plains.

The network of 30 accelerographs was there waiting to record the Alpine Fault, but ended up perfectly placed to capture everything that has happened over the past two years. So along with the satellite imagery and other evidence, there is a huge volume of data to be analysed and gradually a very detailed picture is emerging.

For instance, says Pettinga, recent 3-D modelling by an Oxford University collaboration has found that the original September 2010 Darfield earthquake was not the result of a single fault, but a chain reaction of some eight fault segments that unzipped in quick succession.

The first to go was a cross-wise section, the Charing Cross Fault, that by itself would have amounted to only about a 6.5 shake.

But this then pressed on the centre of a complex line of fracture - the Greendale Fault - running down from the Canterbury foothills towards Christchurch, triggering a cascade to both east and west which built to a magnitude 7.1 earthquake until either the energy was spent or the faulting was halted by a change in terrain.

Pettinga says the Alpine Fault - the crunching joint between the Pacific and Australian continental plates - is a single frequently polished break. It fractures often enough to be worn smooth. This is why it is possible to point to the individual buildings in Franz Josef which are bang in its firing line.

The Canterbury Plains however are a maze of ancient fissures and folds that get loaded up with the twisting lateral strain the Alpine Fault exerts on the whole east coast of the South Island. The Alpine Fault disperses about 75 per cent of the seismic energy and the other 25 per cent goes into generating local earthquakes.

'Over geologic periods of time, these Canterbury Plain faults will get some width associated with them because they will sometimes rupture on one bit, sometimes on a different plane right next to it.' Pettinga says the modelling now suggests the February 22 Port Hills earthquake was produced by a linking up of three fault segments, June 13's 6.3 was the result of another pair, and then December 23's 5.8 off New Brighton Beach was just a single fault.

Pettinga fingers its dotted trace on the map. You can see how last December's is now looking like a seaward continuation of the CBD Fault - the buried line found after a seismic survey last year which alarmingly runs all the way under the central city from Riccarton out to Aranui and New Brighton beach.

This section stirred on Boxing Day 2010 with a sharp 4.9 shake, yet thankfully switched off again following February 22. 'The rupture of the Port Hills Fault did seem to relieve the stresses in the city area as it has gone dormant since then,' says Pettinga.

However, it does appear to have been a narrow escape. 'It is looking as though the two sections are lined up - they may be two branches of a historic fault lying quite close to each other, but not quite linked.' In hindsight, if both had lit up together, there was the potential for a February-level quake even more directly under tall buildings and centres of population, Pettinga admits.

On a positive note, the energy unleashed by the Darfield earthquake does now seem to have worked itself across Christchurch and safely out into Pegasus Bay, he says. And once any local faults have gone, it will take thousands of years to build up the same levels of strain again.

It is this kind of fine detail about how the land has behaved that is going to be so useful in judging earthquake hazards in the future, Pettinga says.

For example, knowing how the stresses of one quake radiates through the ground to trigger another much further away could well have given Christchurch warning of what was to come after Darfield.

'Because we've got this world- class recording system in place, we've got an incredible record of the seismicity over some 22 months."

'We've captured the aftershock data with a high degree of precision and we can now back- analyse to see whether future stress modelling may allow us to do a much better job in terms of forecasting what will happen over a period of months in a large earthquake event.' Probably even more valuable will be the ability to match the strength of shaking to the actual topography of the ground.

Pettinga says one thing learned is the way the aquifer under Christchurch International Airport acted as somewhat of a trampoline shock absorber for that area. And by contrast, how other landscape features, such as the ridgelines of the Port Hills and its loose sediment filled basins, worked to focus the energy of the earthquakes.

A sudden slowing and bunching of the shock waves as they passed from hard rock to unconsolidated gravel was the reason the Heathcote Valley suffered the most off-the-scale ground accelerations in February, says Pettinga.

'In the future, we should be able to identify areas prone to very strong ground shaking due to these kind of particular site effects,' he says.

There could be a micro-zoning of a city based on this knowledge, a fine-tuning of building codes that would make New Zealand cities both safer, and also save money because the precautions could be more closely tied to the actual levels of risk.

GNS's Berryman says the Canterbury experience has definitely created some new questions. The cascade effect is one that seismologists want to understand better - especially in relation to the Alpine Fault.

Knowledge about the Alpine Fault itself tightened up recently with the June publication of a paper by a team led by Berryman. The group managed to find a hillside by a creek near Milford Sound that bore the marks of 24 Alpine Fault quakes dating all the way back to 6000BC.

Berryman says this continuous record shows the fault indeed keeps to a remarkably regular schedule. The average time between movements is 330 years, with two-thirds of all the quakes happening within 260 to 400 years.

The smallest gap was just 140 years, the longest stretched out to 510 years. But with the last earthquake taking place in 1717, this is why the current odds of another stands at 30 per cent within the next 50 years.

Berryman says a lot of people's thinking has been focused on the immediate impact of a magnitude 8 earthquake - what it will be like for Queenstown, Franz Josef, Hokitika and others most likely to be hardest hit. But now there is the further worry of the chain reaction that perhaps should be expected.

Depending on which direction the fault breaks, says Berryman, it will load up the landscape with an energy that will have to be dispersed in a series of knock-on events.

'If it ruptures North to South towards Milford Sound, then that's bad news for Fiordland. Most of the forward energy will be directed down there. But if it ruptures from South to North, then it will impose an additional stress to the ends of all the Marlborough faults.' So quite possibly, in the same way that the Darfield earthquake has triggered a sequence fanning out into Pegasus Bay, this could cause a succession of large earthquakes through Kaikoura, Blenheim and other areas lasting 40 or 50 years, Berryman says.

The point is not to worry people, but to have good information on which to base decisions. And right now, geologists are not even sure which direction the Alpine Fault normally does break.

Berryman says as much as learning about the way the land reacts, an important lesson from Canterbury has been realising the potential social and business costs of a major event.

He says the old hazard models emphasised saving lives and strengthening individual buildings. Now there is a need to consider the effects on whole communities and the national economy - how to minimise the costs and disruption of a recovery. 'It's been a huge wake-up call that has certainly sparked a whole bunch of thinking around economic resilience.'

Berryman says such has been the shock of the quakes that in some quarters he is seeing almost an over-reaction. 'We've now got some officials worrying about liquefaction and weak soils in their cities where they haven't really got the level of earthquake risk to make it a problem.'

Or the fears need some calibration. Wellington has become so dominated by its talk about really big earthquakes, says Berryman, that it is possibly not paying enough attention to the more likely medium-scale ones that would still cause serious disruption.

Likewise, Auckland is thinking about its volcanoes. However, from a risk-modelling point of view, maybe there should be more concern about the virtual certainty of Mt Ruapehu going again sometime soon.

'Ruapehu has an average 20-year return for a 1995 style eruption, and every 100 years you are going to get something even more significant. Imagine the ash coming out of that and a southerly heading towards Auckland. If you had to close down the airport for a long time, the economic impact could be pretty serious.'

Yet generally the Canterbury earthquakes are being correctly heeded, says Berryman. He sees changes being made that are building in a preparedness for all kinds of natural hazards.

'Some people may be just thinking about translating 'earthquake' all around the country. But it's irrelevant what kind of event it is if it's potentially economy- changing. The resilience implications are much the same whether it's mega-floods, tsunamis, volcanoes or even biosecurity."

So how is Franz Josef going to respond to its own specific dilemma?

Franz Josef Community Forum chair Marcel Fekkes, who runs luxury villas at the entrance to the glacier field, says as you can imagine there are some rather delicate negotiations taking place with the Westland District Council and other local authorities.

The problem is the risks were known, but never properly recognised. 'The police station was built only three years ago, but they put that right by the fault line as well.' Fekkes says the question now is what kind of solution will be acceptable - and who pays?

Langridge's report suggests a number of options, the most extreme being to abandon the town completely and move its people elsewhere. But a minimum plan is to create a fault avoidance zone (FAZ), a safety corridor some 200 metres wide, and gradually shift people back from the edge.

'It'll be like being red-stickered, but allowed to stay in the buildings,' says Fekkes. And town opinions are divided about this.

'Due to events in Christchurch, everyone has got a little more panicky about the risks,' he says. Yet faced with the prospect of having their properties blighted without any official recompense, many also would rather the whole subject was quietly dropped. 'For some, the best option is just to forget about it, I think.'

Fekkes says the hope is the local council will eventually accept blame for not preventing development and so come to the table with financial assistance to shift the petrol station and buy out the other properties. 'This whole process could take a couple of years,' he says.

GNS's Langridge says Franz Josef's case is certainly a tough one. But as the ripples from Canterbury's experience continue to spread, he is seeing fewer excuses, a greater willingness to respond to information about New Zealand's natural hazards.

"Yes, people are kind of accepting that what these scientists have been going on about for all these years might actually have something to it now."

The Press