A rubbish week
Christchurch Earthquake 2011
Yes, lots has been happening within the faultlines near Christchurch.
Is it half-time yet? Because it feels like having wandered on to the pitch of a rugby test match as a spectator.
Whomp, from out of nowhere, a crunching 7.1 tackle. Smash, a 6.3. Wallop, another 6.3. The hits just keep coming. So tell us Mother Nature, are we even halfway there?
It is difficult to read Christchurch's mood since Monday's latest smackdown. The collective response to last September's earthquake was shocked surprise. Stunned that it could even happen, followed by a heroic pulling together.
The actual damage? Well, it seemed bad at the time, however no lives were lost and any broken property was mostly well-insured.
By February, the mood was mostly frustration. The gripe was the rebuild had become bogged down in bureaucracy. Yet work was on the verge of getting started. Earthdams to shore up the banks of the Avon. A total of eight houses might have to be abandoned, but for the rest there would soon be a schedule for repairs.
Then quake two struck. Incredible damage this time. Collapsed buildings and 182 deaths. Liquefaction for kilometres. And round-the-clock TV coverage for weeks with the world media tripping over themselves to interview a stoic Mayor Bob Parker in his orange quake jacket.
Surreal was the word everyone was using. A place beyond stunned, yet also now tinged with a nagging familiarity.
Again there was the heroic determination, the adrenaline rush, the Cantabrian spirit. Many did leave town. But mostly temporarily, out of practicality. Emergency campervans went unused as the displaced found homes with friends and relatives.
Businesses and schools also did a remarkable job of finding alternative accommodation. It was not comfortable, but by May and June, the new normal was settling in. The western half of Christchurch was functioning, the eastern half surviving.
Once more, it was all about to crank up. Roger Sutton was starting his first day as boss of the Canterbury Earthquake Recovery Authority (Cera), a lively conversation about the future design of the town was taking place, central city demolitions were being given deadlines, insurers were going to be sorted, those crucial decisions about the rebuild of the suburbs were about to be announced.
Then wallop. Monday lunchtime's 5.6 tickler followed by the 6.3 rock and roller.
It was odd. The immediate reaction being heard was that this time people must be calling it quits. A third big quake was too much.
No power, no water, no sewers. The familiar trifecta. Liquefaction which was even more liquid. And now the hillsides were properly giving way, the half standing heritage buildings were completely down.
Parker was back in front of the TV cameras in his orange jacket, reading out emergency information. It was deja vu.
Yet by Monday's late news bulletins - and I was a little outraged by this at the time - it was just another day. A further massive out-of-the-blue earthquake was simply first item up. Then it was on to other news, like the ash cloud cancelling air traffic.
But Tuesday morning dawned still and sunny. The realisation came too that this was damage on top of damage. There were no trapped people in fallen buildings. There might be shredded nerves everywhere, however, the new normal for Christchurch was still much as it was.
People's jobs and schools had already been relocated. Those grimly camped in their broken homes were still camping. Another 70 central city buildings had been buckled past repair, the sunken suburbs had sunk even lower, yet this made no essential difference to the general equation for residents.
Those who might have found it easy to flee Christchurch would have shot off to Australia or somewhere a good quake ago. Those remaining are still tied simply by the need to sort out their homes and earn a living. And by their family connections, social networks and personal histories.
Like it or not, the majority are bound to the fate of the city, whatever that is. So the task is to stand up, dust down, move on.
Sutton, speaking on Tuesday after day-long briefings with Prime Minister John Key and Earthquake Minister Gerry Brownlee, confessed: "I went home last night and I did feel a bit depressed. My local road is in St Martins and it was all this same stuff again. New big holes in the road.
"However, today people are going, gee, we've had another big one. But life goes on, I haven't been hurt. Overall I think it's surprising how people are looking fairly positive. People are annoyed, they're upset, but they accept we are in a seismically active place right now."
True enough. But if the harsh reality is we are still in the midst of a larger event, as has been echoed by Parker and Brownlee on several occasions this week, the question is how many more times?
Since February 22, an army of geologists and seismologists have been out surveying the landscape. Their reports are still partial, but a more detailed picture is building.
Kelvin Berryman of GNS Science says there are two ways of assessing the hazard of more large quakes.
One is purely statistical. You begin with the historical record of a region's probability of earthquakes and then overlay the typical decay curve for aftershock strength following a major shake.
The other is to find the actual faults and gauge their potential to act up. The dominating risk for New Zealand is, of course, the Alpine fault which runs down the mountain spine of the South Island and snakes out to sea past Wellington and the east coast of the North Island.
This is a well-worn groove between two continental plates that have been grinding past each other for hundreds of millions of years. Every 400 or 500 years or so, the Alpine fault slips in lengths of up to 400km, generating catastrophic plus-8 quakes. It also goes more regularly in smaller lengths to create plus-7s like the 1929 Murchison, 1968 Inangahua, and 2009 Fiordland earthquakes.
Berryman says the steady motion of the Alpine fault sets up a general twisting of the land to the side so that places like the Canterbury plains end up riddled with smaller faultlines. These can lie dormant for hundreds of thousands of years. And measuring at most some tens of kilometres in length, they are rarely much more than magnitude 6 in potency.
Berryman says Christchurch is considered a low-risk city so far as New Zealand goes because it is a reasonable distance from the Alpine fault. An 8 in the Southern Alps would diminish to a 7 or less 100 kilometres away. And locally, the ground has been quiet a while.
Yet there is a record of quakes in the region. In 1869, there was a 5.8, or thereabouts, beneath Addington that toppled chimneys and stone walls. In 1870, there was another similar near Tai Tapu. Then 1888 saw a big plus-7 near Hanmer, connected to the Alpine fault, causing rockfalls in Lyttelton and the top of the Christ Church Cathedral spire to tumble. There was another 6.8 centred near Cheviot in 1901 that created liquefaction in Kaiapoi and the cathedral to once more lose the tip of its spire.
Christmas Day 1922 produced a 6.4 at Motunau to the north of Pegasus Bay. And in 1987, a 5.2 out in Pegasus Bay rocked New Brighton enough to crack pavements. So there is a baseline of activity in Christchurch's past. And now the area has been stirred up, a chain reaction looks to be going on. Shifting strains in the bedrock are rippling out and an unusually rich sequence of aftershocks is being seen.
University of Canterbury geologist Mark Quigley says even the first 7.1 Darfield quake last September has itself turned out to be a network story. A loose collection of faults unzipped together to release thousands of years of strain.
"The rupture process seems to have involved four sources," Quigley says. It started with a 6.5 event near Charing Cross, then the main 6.9 at Greendale, leading to a smaller 6.2 out at Hororata, then finally a 6.5 back at the eastern Christchurch end. "It looks like a tip-to-tip rupture as the Greendale fault has gone very quiet since."
And now the chain reaction is continuing with faults being triggered further afield.
Jarg Pettinga, head of geological science at Canterbury University, says for any particular fault, it can work either way. Either their hidden strains are going to be relieved, or the transfer of stresses will load them closer to breaking point. So there has been a race to identify the Christchurch region's other possible faults so we can be warned. Multinational teams have been probing both Canterbury and Pegasus Bay with sonar-type instruments and gravity meters which can detect subtle changes in rock density.
A fortnight ago came the first reports. A boat survey discovered a new 25km fault off Kaiapoi. But while the fault might be large enough to generate a near-7 quake, it is not thought a tsunami risk and looks to have been inactive for a million years.
Far more of a concern, however, was the finding of the "CBD faultline" that is now believed to have generated the 4.9 Boxing Day aftershock - one felt strongly, being so near the heart of town.
Pettinga says the CBD fault reaches 10km from Hagley Park to New Brighton beach and perhaps a bit out to sea. Again, it is deeply buried under sediment and so must have been dormant a long time. Also, there is no certainty about whether it is the type that would go only in isolated segments or as a long line. Yet there is the potential for a magnitude 6 jolt if it were to go with a single bang.
Quigley says the placement of the fault is bad news. However, there is reason to think its stresses have been relieved rather than heightened because, since February 22, it has gone quiet.
"We can see there is something in the subsurface that has slipped before, but has very long recurrence intervals. It switched on on Boxing Day yet has switched off again. And we can only look on that optimistically," says Quigley.
Pettinga agrees. Going quiet is not ominous as some might think, he says. It really is an encouraging seismic sign. And because the CBD fault has the look of a very old structure, there is no reason to attach any particular fears to it at the moment.
Pettinga has the responsibility for researching another of the big question-mark locations, the "gap" between the Greendale and Port Hills faults.
On the map, there is a temptation to see these two faults connecting to make a longer line. And the area around Rolleston, Lincoln and Hornby has seen a disturbing number of aftershocks that could be evidence of another concealed fault. Rolleston was hit by a 5.5 just on June 6.
Pettinga says seismic surveys have now been done in this area and so a definitive answer should be possible within a matter of weeks. However, he already believes that because the number of aftershocks has been dwindling, and because the ground has a different sub-soil orientation, there will instead prove to be just a patchwork of smaller cross-wise fractures in this gap.
"Given the patterns of activity, we're reasonably confident there isn't a single through-going fault that links the Greendale structure and the Port Hills fault," Pettinga says.
After a third surprise hit, Christchurch just wants the truth about what it needs to be worried about. And it seems like the news on two of the major concerns is going in the right direction, even if it is unlikely scientists can fully eliminate them.
But where does Monday's earthquake fit in? It was a shock that yet another fault, one unknown and which did not turn up in the recent surveys, could strike so close to the city again.
The first word was that this was indeed a new faultline some 2km to the south of the February 22 Port Hills fracture. Pettinga says it is still early in the analysis, yet there is a fair chance that Monday's quakes were instead simply further ruptures on the original Port Hill's fault - ones on a "splay" that forked off at a different angle towards the surface above.
"So they could be linked at depth in the subsurface," Pettinga explains. The different angles would explain why February 22 was felt as a fast up-and-down motion while Monday's was more a bucking side-to-side.
Pettinga says the reason why there was no warning of the fault's existence is that ground imaging equipment can only probe for faults buried under sediment layers and the Port Hills fault is part of the hard rock making up the old Lyttelton volcanic cone.
Since Monday, what scientists have been noticing is that the aftershocks have been radiating steadily around the sides of the cone, heading eastwards towards Port Levy and that side of the Banks Peninsula.
GNS Science researcher Martin Reyners, who has been tracking the progress, says even this is good news probably. Reyners says when the volcano was forming in a series of eruptions millions of years ago, it would have created a ring of small fractures around it. It now looks like these are mopping up most of the energy released by the Port Hills fault.
Certainly the activity is spreading away from Christchurch and is scattered enough not to appear to be focused on any further large faults.
Reyners adds the public should also know that scientists hold no fears for the volcano itself. "There've been a lot of calls about new hot springs appearing in the Port Hills and the water of Lyttelton harbour getting warmer." But Reyners says all the instruments show the volcano is inactive. "There's no magma bubbling up underneath because of all this."
So the evidence is that we are still in a seismically active period. Monday confirmed that. And because so much more energy was dumped into the landscape by the latest events, GNS Science has had to raise the odds to a 30 per cent chance of seeing another earthquake of between 6 and 7 in the current aftershock zone over the next year.
But on the other hand, says Pettinga, the total activity is in fact slowly subsiding as the landscape rebalances. And the patterns of activity are moving away from the city as far as we can tell.
Psychologically, this is not an easy story to adjust to. The fears will have to exist for a while yet. But it is also a fact of life that for the majority of Christchurch people there is just not the luxury of upping sticks and leaving the shaky ground to it.
So even after Monday, it is again a case of stand up, dust off, move on.
- © Fairfax NZ News
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