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The view from Cashmere of dust over Christchurch after the Dec 23 aftershock - Source: Supplied, G Walmisley -
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Canterbury residents are being urged not to be alarmed by predictions that aftershocks could be recorded in the region for many years to come.
Seismologists from GNS Science today briefed city councillors, MPs and the media on the latest quake information for Canterbury.
GNS Science seismologist Kelvin Berryman told ONE News that magnitude three aftershocks are likely to go on for a "number of years", and that magnitude one and two aftershocks "could go for 30 years".
Canterbury Earthquake Recovery Authority Chief Executive Roger Sutton said people should not be alarmed by the predictions.
He said "the reality is very little damage occurs after a five".
"We may get a few more sixes we're not sure about that," he told ONE News.
"But even then most of Christchurch doesn't suffer any more damage when those sorts of events happen."
Magnitude 7 quake risk
Berryman said the probability of a magnitude 7 quake in Christchurch is "low" but "not zero".
Berryman said the magnitude-6.0 quake which struck on December 23 was close to what had been predicted.
GNS had forecast a 50-50 probability of a magnitude-5.0 - 5.9 striking the region, "so this was at or a little bit above what we had forecast."
However, he went on to describe the recent spate of quakes as "very rare".
"The probabality of a 7 is low. . . it's not zero but it's very low," said Berryman.
He said quakes of a magnitude-6 did not produce tsunamis of any significance.
But if a quake of magnitude 7.0 hit, "don't wait to be told by Civil Defence to move off the beach."
Berryman said those living in seaside suburbs should "self evacuate".
However, a tsunami was more likely to be triggered by a quake off the coast of South America, and New Zealand would have 12 hours' warning of its arrival.
Christchurch Mayor Bob Parker told the meeting that the city council was committed to putting in a tsunami warning system.
Many faults
Berryman said the quakes at a depth of 8 to 10km were in "very old
rock" which had broken up into many faults.
The recent spate of quakes centred off the Christchurch coast were reaching towards the relatively large Kaiapoi Fault "but they're not there yet".
The Kaiapoi Fault may be up to 30km long, and potentially capable of producing a magnitude-7.0 quake but the current earthquakes were not "anywhere near" it.
"Our expectation is [the current sequence of quakes] will go into the same decay sequence as we saw post-February and post-June.
"There is amost certainly still a 5.0 out there and we would guess quite a few 4s and 3s on a daily basis," Berryman said.
"We are progressing into a period where quakes are not damaging but they can affect people mentally.''
Thousands of aftershocks
Research seismologist Stephen Bannister said the ground
accelerations of the recent earthquakes were less than the city had
suffered early last year.
In the February quake ground accelerations of more than 2G were felt in many places, including Heathcote Valley.
"As we move through the June event ... we still got high horizontal acceleration in some areas, and lower acceleration in the city centre,'' said Bannister.
"As we move to the current events in December, we are having much lower acceleration through the city.''
More than 9500 shakes had hit Canterbury since the magnitude-7.1 quake on September 4, 2010.
Mayor Bob Parker this week said he was keen to provide information on the "unusual" quake sequence.
"People are asking more and more questions around, 'What is going on?"' he said.
"People have said to me that they feel like they're not getting the full story.
"I don't believe that is the case, but I also believe that it's important for our scientists to front up for us to have a chance to get a full briefing as a community."
The briefing was not open to the public.