Isaac Davison

Isaac Davison is a NZ Herald political reporter.

Quake - a year on: 5500 shocks, many more ahead

Rescuers search for survivors on top of rubble at the CTV site. Photo / Geoff Sloan
Rescuers search for survivors on top of rubble at the CTV site. Photo / Geoff Sloan

Five thousand earthquakes after the devastating February 22 tremor, Canterbury's aftershock sequence has a recognisable personality.

Seismologists looking at 12 months of data have found that the frequency and size of the aftershocks is similar to the seismic aftermath observed in eastern Californian and Tasmanian events, where the tremors lasted decades but were increasingly smaller in scale.

Since the original rumble in September 2010, 10,000 quakes have rattled Canterbury. More than 5500 of these have occurred since the deadly February quake re-energised the sequence, and 214 of these tremors were magnitude 4 or larger.

Natural hazards research platform manager Kelvin Berryman said the Darfield aftershock sequence was expected to last for 30 years, with tremors tailing off gradually until they were unnoticeable.

Cantabrians repeatedly asked whether they were experiencing a particularly vicious sequence, in particular when a magnitude 5.8 earthquake occurred two days before Christmas.

Seismologists say the Darfield sequence was not violent compared with historical records. In contrast, Sumatra is still experiencing moderate-size quakes eight years after the magnitude 9.1 quake which caused the Boxing Day tsunami in 2004.

But unlike the Sumatran sequence, the Canterbury quakes were limited to a smaller geographic area, and have been particularly hard on Christchurch's central city.

"We had the misfortune of the quakes migrating into the city and staying in the city for a while. But now we're seeing them migrating away," said Dr Berryman.

GNS Science's latest computer modelling shows there is an 8 per cent chance of a 5.5 to 5.9 magnitude tremor in the next month and 2 per cent chance of a magnitude 6 to 6.4 quake.

Earthquake scientists the Herald spoke to felt they had regained the high ground after a difficult year in which their expertise was challenged by a public that was desperate for hard answers on the city's seismic future.

Seismologists, geologists and engineers relished the public thirst for their knowledge after the original Darfield quake in September, 2010.

But the huge death toll in the February tremor meant some frazzled residents vented their frustration at the scientific community, and asked why the huge quake had not been forecast.

A paper released last month said a week before the magnitude 6.3 tremor, computer modelling had shown a 25 per cent chance of "a magnitude 6 or greater earthquake occurring in the general aftershock zone of the Darfield earthquake in the next year".

GNS Science stood by its decision not to publicly announce this figure, because the projected earthquake could have occurred in six or seven different places, and an announcement could have needlessly alarmed residents.

University of Canterbury geologist Mark Quigley said scientists should always be challenged on their findings, but a lot of the criticism they faced after February was based purely on emotion.

"It got to this stage: 'Why couldn't you predict that one?' or 'What could you have done better?'. But I felt that we had done a very good job as a scientific community."

On the anniversary of the tragic quake, the public frustration surrounding quake prediction has dissipated, said Dr Berryman.

"There's still some tough questions, and there needs to be. But there's also some appreciation of the challenges of forecasting rare events. Many, many people in Christchurch are pretty tuned up on earthquake science these days."

Why city suffered so badly

* 60km to 80km of the earth's crust has experienced a stress change, meaning the aftershock sequence will last about 30 years.
* Horizontal ground motions 1.7 times the force of gravity were larger than expected.
* One reason for this could be the way the fault ruptured towards Christchurch (it had strong "directivity").
* Upper and lower layers of the earth separated during the quake, then "slapped" back together, producing very high impacts (known as the trampoline effect).
* The hard rock under Banks Peninsula may have compounded the effect of the quake by reflecting seismic activity back towards the city.
* There were "remarkably high" levels of stress built up under Canterbury's soil.
* High water tables trapped energy in the top layers of soil in some areas, boosting liquefaction.
* Liquefaction caused the worst damage to land and buildings, including many CBD high-rises.
* Deep-seated landslides caused the most damage in the southern Port Hills.
* Critical structural elements in buildings built between 1976 and 1992 failed.

- NZ Herald

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