What's Next?

PAUL GORMAN

Last updated 11:08 12/03/2011

 

As Christchurch starts to come to terms with the deadly pounding it received from an earthquake on a previously unknown fault, the question on many people's lips is, will there be a third big one?

Everything changed a few seconds after 4.35am on Saturday, September 4, last year. A concentration of events under the Canterbury Plains, as thousands of years of strain reached breaking point, generated a damaging magnitude-7.1 earthquake.

Rocks adjusting to that have since generated about 5000 aftershocks, among them a magnitude-5.1 quake four days later near Lyttelton that told scientists there was something, a small fault, under the Port Hills worth investigating.

That was the precursor for the second major quake centred in much the same spot, the 12.51pm February 22 shallow magnitude-6.3 event, which has destroyed much of central Christchurch and may have killed up to 200 people.

However, a third significant aftershock is bothering scientists. On Boxing Day at 10.30am, another small, hidden fault ruptured pretty much under the city's central business district. The magnitude- 4.9 quake it generated caused considerable damage to some old buildings and weakened others.

The question is, does that signal something else yet to come?

If that's the bad news, the good news is this: if the fault that caused the Boxing Day quake does generate a bigger quake, some scientists believe it is unlikely to produce anything as high as a magnitude 6.

Other good news is that, just because a fault is there, it does not necessarily mean there will be an earthquake.

Rocks along the fault already have to be considerably weakened before they will break and move, and that might not happen for hundreds or thousands of years.

It's fair to say, though, that among everything else the scientists are trying to decipher, the threat from this Boxing Day structure looms large.

Are worried residents getting the full picture?

It's an ethical nightmare for scientists. Should they wait until they know almost all the facts before they say anything?

We had certainly been warned after the 7.1 quake that there was a chance of an aftershock as large as one degree of magnitude lower, about a 6. While there were plenty of magnitude 5s, as the weeks passed the threat of that big aftershock receded.

Was it a convenient complacency that made us feel it was not going to happen?

The fact is that as recently as the start of February, Government research institute GNS Science was saying there was a 25 per cent chance of an aftershock of magnitude 6 or more affecting Christchurch or Canterbury during the next year.

Judging by the way the February 22 quake shocked the population, that message, if it was conveyed, does not appear to have been conveyed very well.

So, what can science tell us about what is happening under our city?

Scientists are caught in the middle. Everyone is looking to them for answers, but science, by its careful nature of analysis and discussion, cannot easily provide rapid responses.

There is also a very evident political angle to all of this.

GNS Science natural hazards manager Dr Kelvin Berryman has been briefing Cabinet, Minister for Earthquake Recovery Gerry Brownlee, the Earthquake Commission and the Christchurch City Council on the science.

As head of a Government research platform spanning Crown research institutes and universities, he cannot afford to go public with loose theories or break consensus views.

He talks frequently about the need for a consistent message and is painfully aware that people in Christchurch are already scared, "so we don't want to wind them up any more".

With new information, interpretations can change quickly. The initial thinking by geophysicists was the February 22 quake had its genesis on a sub- surface rupture between Halswell and offshore of Sumner that could have been up to 17 kilometres long.

That changed to an 8km to 10km-long fault finishing about Cashmere and now, with further satellite information, the latest thoughts are there could be two parallel, smaller faults, one with more vertical motion and the other with predominantly sideways motion.

They believe the small fault that generated the December 26 quake is separate from these Port Hills faults.

"Boxing Day was always thought to be a downtown, more or less CBD location," Berryman says. "But we think that with further, more detailed analysis of subsurface structures, its final location may be a few kilometres further north.

"When we say it was under the [Christ Church] cathedral, we really mean 5km or so beneath the downtown CBD area, although it has a plus or minus on it of a few kilometres in the location as well as depth.

"I think the Boxing Day quake is a concern, but we would prefer it be on the backburner for the moment. We can't say anything useful at the moment, except wind people up.

"We will be working on that quake. By raising the topic, you are raising doubts."

Berryman says there has been no Government directive to keep quiet about the risk from that fault.

The current thinking is there may be a couple of faults lying southwest-northeast between the Greendale fault and the Port Hills structures, which Berryman says is preferable.

"If it was a continuous structure, there would be the potential for bigger earthquakes because the slip along the fault could be greater."

It is not unusual to have a very large aftershock months after the initial quake.

"Six point three is a big aftershock - it's a big one and a late one, although overseas, they have sometimes been up to two to three years later.

"The change in regional stress levels is infinitesimal, though, compared with what it takes to actually break the rocks. Unless rock is quite close to failing, it won't be ruptured by those stress levels."

Which could be a strong factor in staving off any more large central Christchurch quakes for many years to come.

As an outsider, visiting Pennsylvania State University geologist Professor Kevin Furlong believes the best science does not come from such consensus.

Unlike New Zealand's small pool of experts, Furlong, who has been a visiting fellow at the University of Canterbury and University of Waikato since the September quake, has the luxury of being able to talk more freely.

"Healthy debate is very good and not generated by a consensus viewpoint. There hasn't been enough discussion and that is unfortunate.

"It could be a bit to do with the size of the country. There aren't enough people in any one field to be able to challenge each other. In the United States, you have these debates all the time, but there's enough of us and you can fight it out. It leads to a better result."

He says the magnitude-4.9 Boxing Day quake was followed by an almost textbook aftershock sequence along the fault. "It really stood out. It's north of what ruptured on February 22. I look at that and say, 'Is that important?'.

"We know faults have finite lengths. So here's a magnitude-5 basically that has triggered enough little aftershocks on a fault plane about 5km to 6km long. That tells me probably the whole of the Canterbury region potentially has lots of these 5km to 10km-long faults. The trouble is when they connect up.

"We know from February 22 that 10km is, unfortunately, more than sufficient to be devastating. So it's important to know where these 8km, 10km, 15km fault segments are."

Clouds of aftershocks with magnitudes of only 1 or 2 "light up" potentially earthquake- generating faults, he says.

- The Press