Private quake briefing to provide answers

Canterbury's aftershock sequence will decay over decades rather than years, says scientist Kelvin Berryman.

The seismologist was speaking at this afternoon's briefing by GNS Science to Christchurch City councillors, MPs and the media on the latest quake science.

The meeting was called after concerns about the frequency and magnitude of aftershocks centred on Pegasus Bay since December 23.

Berryman said the aftershocks would continue for "many decades" but become imperceptible. "It is not a one to five-year period; it is a period of many decades."

He said much of the built-up stress had been released in the Christchurch area, "but around Canterbury there may well be an ongoing sequence over a period of a few decades".

He said remnants of the 1968 Inangahua quake were continuing to this day.

"They are still there, but they are not detectable or stopping people from getting on with their lives,'' he said.

"It decays away to something that is imperceptible.''

The briefing paused briefly as an aftershock rocked the audience, with the experts laughing it off as a magnitude-3.3 to 3.5 shake.

However, it was quickly followed by a long, rolling magnitude-5.0 tremor.

Only half an hour earlier, the audience was told it would "almost certainly" get another magnitude-5.0 quake, as well as "quite a few" 4s and almost daily 3s.

Tsunami warning system within months

Mayor Bob Parker, also speaking at the briefing, said a tsunami warning system was likely to be in place in Christchurch within months, and he hoped by the end of June.

However, the audience was told that a probability of a magnitude-7.0 quake striking Christchurch and triggering a tsunami was "very low".

Berryman said quakes of  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 quake that struck on December 23 was close to that which had been forecast.

GNS Science had forecast a 50-50 probability of a magnitude-5.0 to 5.9 quake striking the region.

"So this was at or a little bit above what we had forecast," he said.

"The probabality of a 7 is low ... It's not zero, but it's very low." 

Kaiapoi Fault not reached by recent spate

Berryman said the quakes at a depth of 8km to 10km were in "very old rock" that 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 quakes 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,'' he said.

"There is amost certainly still a 5.0 out there and we would guess quite a few 4s and 3s on a daily basis. 

"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-6.0 quake hit close to the city,  liquefaction was likely. "Fives don't really produce significant liquefaction."

Larger quakes further away, such as a magnitude-7.0 atn Hanmer Springs or a magnitude-8.0 on the Alpine Fault may 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 quakes 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 the 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.

Cr 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 quake had altered the stress field across the greater Canterbury region .

Mayor Bob Parker said this week that 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.

- © Fairfax NZ News