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Geologist: Tremors not over despite big ones

Published: 9:42AM Tuesday June 14, 2011 Source: ONE News

Big aftershocks are not necessarily all over in Christchurch just because the city has had a few big ones, according to an earthquake expert.

Two weeks ago GNS Science predicted there was a 23% chance of a magnitude 6.0-7.0 quake and a 90% chance of a five or six magnitude quake hitting the area in the next 12 months.

Two quakes yesterday filled the brief - a 5.6 tremor at 1pm and a quake measuring 6.3, at 2.20pm.

There have been eight earthquakes measuring more than 5.0 since the catastrophic 6.3 quake at 12.51pm on February 22, which was itself an aftershock of the larger but less damaging 7.1 earthquake in September.

Geologist and earthquake expert Dr Mark Quigley of Canterbury University told TV ONE's Breakfast the models used to predict the aftershocks are based on aftershock statistics.

"Obviously that event has happened but it doesn't necessarily mean we're in the clear," said Quigley.

"We haven't lived through a complex event on faults like this before and we're learning as we go."

He said the only thing scientists can do besides statistical models is to look to other events around the world that they think are similar.

One of those areas is near the San Andreas Fault in south east California where there were two magnitude six quakes and a seven all within a year, Quigley said.

Seven years later they had another magnitude seven, within 40 to 50 kilometres of one another.

"That sequence happened in '92 and '99 so I'm not necessarily sure that we could say it's over yet in Christchurch," he said.

Quigley said other faults in south east California, away from San Andreas, have probably had very similar earthquake recurrence intervals to the ones in Christchurch.

"So they build up this elastic strain energy over thousands of years. Eventually when they go they seem to be quite complex ruptures that involve many faults that may go on for a while."

Quigley said "knowledge is power" and the more people know about quakes the more they can deal with things.

"We've been trying to answer the questions as best we can scientifically, and  we're going to continue to do so."

Brownlee defends release of predictions

The earthquake forecasts released a fortnight ago caused alarm among some Christchurch residents, but Earthquake Recovery Minister Gerry Brownlee has defended the release of data and its availability. "With all due respect I think people know to go to the GeoNet website, GNS perhaps could publicise it a bit more," Brownlee told Breakfast.

"Those stories about the probabilities were around two weeks after the last earthquake. So people were perhaps having a reflective moment, thinking about what it means.

"The good thing is that despite yesterday's events, we will be down to a risk factor of one in 10 in 12 months' time."

Last night on Close Up seismologist Martin Reyner said that after the September and February quakes, scientists looked at the rate at which aftershocks were occurring, and used that data to revise their forecast for probability in the future

He said yesterday's magnitude 6 quake will generate its own aftershock sequence and people can expect magnitude 3s "for months and years to come".

Quigley was asked if areas of Christchurch hit by repeated liquefaction could be built on again.

"I actually live in one of them and it's very sad for us, but we've had five bouts of liquefaction. I'm not one to make financial decisions, all that sort of stuff. It's a very complicated issue, but it'll happen again," he replied.

Brownlee said ironically the picture is now clearer for people in terms of rebuilding, especially in the eastern suburbs.

"I think there are some things that are blindingly obvious out of all of this, the question is, is what sort of position are those people going to be left in and that's what we are working on at this present time."

He said the government was yet to assess whether further financial assistance was required.

Reyner confirmed there are still aftershocks coming from the original quake, but said there is now a concentration at the eastern end around Christchurch.