From Stuff.co.nz
A magnitude-4.7 earthquake hit Christchurch at 8.04pm, hours after a magnitude-5 quake interrupted a special briefing on aftershocks.
The earthquake was centred 20km east of Christchurch at a depth of 11km.
The magnitude-5 quake, which struck at 2.20 this afternoon, was centred 5 kilometres underground and 10km north-east of Christchurch.
Only a minute before it struck, there was also a magnitude-3.5 shake, which was at a depth of 10km and centred 20km east of the city.
The briefing by GNS scientists to councillors, MPs, and the media, was called following concerns about the frequency and magnitude of aftershocks centred on Pegasus Bay since December 23.
The second bigger quake struck about an hour after scientists told the briefing that the city would definitely be hit by another magnitude 5.0 tremor.
Both this afternoon’s quakes were centred in the Pegasus Bay area.
Twitter users said the magnitude-5 shake was a “long wobble” and “lengthy shake”.
QUAKES TO DECAY OVER DECADES
Kelvin Berryman of GNS Science said the sequence of quakes will decay over decades rather than years.
“Christchurch city has had a lot of quake activity. A lot of the stress must have been released in the city area, but around Canterbury there may well be an ongoing sequence over a period of a few decades.”
Berryman said there are still some imperceptible remnants of the Inangahua quakes in 1968.
“They are still there, but they are not detectable or stopping people from getting on with their lives. We are getting into a period that is damaging to people’s confidence. It is mental rather than physical.”
“These are very upsetting events rather than dangerous or damaging.”
Berryman stressed the sequence would decay over the decades and become imperceptible.
“It decays away to something that is imperceptible”
“It is not a one to five year period, it is a period of many decades.”
Earlier, mayor Bob Parker told the briefing that a tsunami warning system was likely to be in place in Christchurch by the end of June.
The audience was told that a probability of a magnitude-7.0 quake striking Christchurch and triggering a tsunami was “very low” but further liquefaction remained a possibility.
Berryman 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.
Berryman said the recent spate of quakes was “very rare”.
However, the magnitude-6.0 which struck on December 23 was close to that which had been forecast.
GNS had forecast a 50-50 probability of a mag-5.0-5.9 striking the region. ” So this was at or a little bit above what we had forecast.”
“The probability of a 7 is low. . . it’s not zero but it’s very low,” said Berryman.
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 almost certainly still a 5.0 out there and we would guess quite a few 4’s and 3s on a daily basis,” said Berryman.
“We are progressing into a period where quakes are not damaging but they can affect people mentally.”
LIQUEFACTION RISK REMAINS
Berryman said if a magnitude-6 quake, close to the city, struck then liquefaction was likely. “5s don’t really produce significant liquefaction.”
Larger quakes further away, such as a magnitude-7.0 in Hanmer or a magnitude-8.0 on the Alpine Fault may also cause liquefaction in Christchurch.
The audience was told that the amount of liquefaction silt spewed out of the ground meant some parts of Christchurch were now “hundreds of millimetres” lower than they were two years ago.
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 have hit Canterbury since the magnitude-7.1 quake on September 4, 2010.
Councillor Helen Broughton asked the scientists if all 9600 shakes were “aftershocks” or if three or four could be classified as earthquakes.
Bannister said the thousands of events could all be be described as “aftershocks” to the September 4 earthquake. This Darfield earthquake had altered the stress field across the greater Canterbury region .
Mayor Bob Parker said earlier 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.”
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