Canterbury has history of shaking

BY PAUL GORMAN

People are asking, understandably, if that was "The Big One".

After such a traumatic and damaging event, it would certainly be nice to think so.

Over the next few weeks the nerve-racking aftershocks will slowly decrease in frequency and intensity, and the frightening events of the pre-dawn hours of Saturday will become memory and part of the region's history.

Until we have another large jolt, that is. Any perception that Canterbury is at lower risk of devastating earthquakes than, say, Wellington or the east coast of the North Island have been smashed.

A handful of significant and major earthquakes have shook the region over the past 150 years and there are faults whose pedigree is well-known and feared not all that far from central Christchurch - the Alpine Fault, the Hope Fault, the Porters Pass Fault and the Springfield Fault.

The 7.3-magnitude North Canterbury earthquake of September 1888 caused extensive damage and knocked the top off Christ Church Cathedral; a 7.1 shake in March 1929 was centred in Arthurs Pass; and 6.7 and 5.9-magnitude quakes in the Canterbury high country in June 1994, followed by a raft of aftershocks, gave Christchurch residents a scare.

Canterbury is seismically a volatile part of the country. As well as the Alpine Fault on the other side of the Main Divide, there are active faults running along the front of the foothills, at Porters Pass and near Mt Grey, all capable of generating highly damaging and potentially life- threatening quakes.

In recent years earthquake scientists have warned of new faults they were finding under the Canterbury Plains within 30km of the centre of Christchurch.

The discovery of these "blind- thrust faults" under the river gravels of the plains, together with Saturday's magnitude 7.1 earthquake, show that out of sight should not mean out of mind.

Scientists, who are admitting they were surprised by the size of the early-morning tremor, might need to tear up the geological maps and the risk assessments and start again.

GNS Science geologist Dr Hamish Campbell said blind faults, by their very nature, caused violent motion when they ruptured.

Compression between rocks on either side of the fault caused one side to ride up over the other, resulting in a large degree of friction.

"One side rides up over the other - it's difficult to achieve. You are trying to shunt a huge thickness of continental crust against another continental crust.

"You get the big break, but there's always stress left in the system, which means you get the aftershocks as the rocks adjust to their new situation."

The Bam earthquake in southern Iran on Boxing Day 2005 had been close to a blind fault and killed 5000 people, he said.

Canterbury University geologists were closely studying the faults under the Canterbury Plains.

"They've done some very fine work in identifying a lot of subtle features, warps and folds, which relate to active faults at depth under the gravels. They have moved in the last few hundreds of thousands of years and generated a little bit of topography.

"They may be hidden, but these faults are big and cut deep, right through 20 to 30 kilometres of the crust."

New Zealand could expect a magnitude-seven quake once a decade and a magnitude-eight quake once a century, Campbell said.

The logarithmic magnitude scale meant an eight was 32 times more powerful than a seven.

Geotech Consulting geologist Dr Mark Yetton said the earthquake could not have occurred at a better time than early on a Saturday.

"It's the best time it could ever happen. There was even a weekend afterwards to come to terms with it.

"During the week we would have been counting fatalities, the number of people injured or killed by falling debris in the city," Yetton said.

"As soon as you move above seven, you are really talking about a significant earthquake. A 7.5 would have trashed the place."

The weekend's earthquake had a precursor in an 1869 shake that was only magnitude five but was extremely shallow and had its epicentre on an active fault under Addington.

"That was the strongest ever felt in Christchurch. It shows that, even under the city, there may be faults we know nothing about.

"I think what will happen is we will now see some money spent on good-quality modern seismic surveys of gravel layers and the crust below that. That has been needed for quite a while in the consideration of the possibility of these blind-thrust faults in close to the city. All those sort of things are worth thinking about."

The response from emergency services, Civil Defence, local authorities, Mayor Bob Parker and Prime Minister John Key had been impressive, Yetton said.

"It's been a superb earthquake response, with the work done between 1995 to just last year and the series of lifeline impact studies. The first one done was for Christchurch city.

"Organisations like Orion, Telecom and the city council have quietly done a lot of good work on infrastructure to make it ready for this sort of thing. We could have been much worse in terms of preparedness, lack of response and the time of the event."

So, to answer the original question - was it "The Big One?" Well, yes and no.

Yes, in that scientists spoken to say it would need to be a simply massive event to top the violence of the motion residents experienced in Saturday's quake.

But no, in that the "Big One" traditionally expected - a major rupture of the Alpine Fault with a likely quake of magnitude eight - is still in the wings, although because of the greater distance from Christchurch the shaking would probably be less vigorous but longer-lasting.

Yetton said there was no room for complacency.

"My feeling is it isn't 'The Big One'. Yes, there's damage, but it is spread around the city and by no means everywhere."

- © Fairfax NZ News