Big aftershock more likely - GNS

By Nicholas Jones, Paul Harper

The dominant energy in Monday's aftershocks was horizontal, while in February's earthquake it was vertical. Photo / Thinkstock
The dominant energy in Monday's aftershocks was horizontal, while in February's earthquake it was vertical. Photo / Thinkstock

The chance of another large aftershock in Canterbury has increased following Monday's tremors, GNS Science says.

The agency estimates there is a 30 per cent chance of an earthquake between magnitude 6 and 6.9 in the next 12 months.

That figure is up from a one in four chance, which the agency predicted last month.

GNS Science seismologist Gill Jolly told Radio New Zealand there was a three in 10 chance of an earthquake in Canterbury similar to the magnitude 6.3 one which struck on Monday at 2.20pm, or the one on February 22, within the next year.

Ms Jolly said this morning's aftershocks - a magnitude 5 at 6.27am and a 4.2 five minutes later - fit what was to be expected after Monday's aftershocks.

"After an large shock like we had on Monday ... you will expect a lot of aftershocks of similar size and obviously a lot more smaller ones."

GNS Science is still working on determining whether Monday's aftershocks were on the same fault as the destructive February 22 jolt, Ms Jolly said.

"They look to be in a similar location and a similar depth to the February 22nd one and a lot of the aftershocks have been in that same area around Lyttelton and the Port Hills. Seismologists have been working very hard overnight to locate all the aftershocks which have been occurring."

More aftershocks rattle Christchurch

Christchurch was shaken awake this morning by a magnitude 5 aftershock.

The tremblor, at 6.27am, was centred 10km southeast of Lyttelton at Port Levy at a depth of 6km.

GNS Science said the quake was widely felt across Canterbury.

A 4.2 magnitude earthquake struck soon after, at 6.32am, at a depth of 5km, 10km east of Diamond Harbour, also near Port Levy.

The quakes were part of a flurry of shakes overnight for the battered city, with half dozen smaller jolts, the largest of which was a magnitude 4 at 9.41pm, 20km southeast of Christchurch at a depth of 10km.

So far there have been no reports of further damage to the city.

This morning about 3000 Christchurch households will have spent a second night without power after Monday's two big quakes in the city.

Authorities warned people to wrap up warmly against the cold as efforts to restore power continued.

Most of those who remain without power were in pockets of the eastern suburbs around the Avon River, and in the hill suburbs, which suffered the greatest ground movement during Monday's 5.6 and 6.3 magnitude quakes.

An initial assessment indicated damage from the aftershocks was much less severe than in February, lines company Orion said.

And the death of an elderly man in a Christchurch aged care facility after Monday's second big earthquake may not now be quake-related.

The 88-year-old man fell during yesterday's 2.20pm 6.3 magnitude quake and lost consciousness. He was seen by a doctor and seemed to recover, but died early yesterday.

But police said the man had a number of health issues and his death, which had been referred to the coroner, was not necessarily linked to the earthquake.

The size of Monday's two biggest earthquakes was revised upwards yesterday, as scientists revealed which areas around Christchurch were hit hardest.

GNS seismologist Bill Fry said it was likely that stresses from the magnitude 5.6 earthquake at 1pm triggered the magnitude 6.3 earthquake at 2.20pm.

"Technically this makes the magnitude 5.6 a foreshock, or precursor."

The force of shaking at ground level, or the peak ground acceleration, during the magnitude 6.3 earthquake ranged from 212 per cent of gravity in Sumner to about 30 per cent of gravity in the CBD.

"In comparison, much of the Christchurch CBD experienced greater than 50 per cent of gravity shaking during the February magnitude 6.3 earthquake," Dr Fry said.

Sumner may have suffered intense shaking because of "topographic effects" that happen when landforms focus seismic waves like light through a lens.

Widespread liquefaction - particularly in the eastern suburbs - was to be expected, as shaking as low as 20 per cent of gravity caused liquefaction in February.

People in Christchurch would have felt Monday's earthquakes differently to those in February and June because the dominant energy in Monday's aftershocks was horizontal. The dominant energy in February's earthquake was vertical.

Dr Fry said Monday's earthquakes occurred on a fault about 3km south of the Port Hills fault that ruptured on February 22.

Smaller aftershocks began in the same area four days after September's earthquake and there had been six aftershocks of magnitude 4 or greater since February.

Scientists have said the high number of aftershocks since September's magnitude 7.1 earthquake meant they were in "unknown territory" but Dr Fry said there were precedents.

"It's not unprecedented globally. It is unprecedented in Christchurch because our historical record is so short. But sequences like this happen around the world.

"These sequences typically happen in places where you have high stress build-up and low recurrence rates, when the crust is particularly strong."

Dr Fry said such sequences eventually tapered off but could take a while to do so because of the stress build-up.

He said the aftershocks were within GNS forecasts.

"The long-term implications aren't any different than they were before [Monday's aftershocks]."

University of Canterbury geologist Mark Quigley had muck from liquefaction come through the floorboards of his Avonside house on Monday.

He said because the faults in Canterbury had recurrence intervals of several thousands of years or more scientists were learning as they went.

"There wasn't any evidence of previous earthquakes on these faults that we could see ... we've never really had a sequence like this in New Zealand."

Meanwhile, a laser scanner was sent to Christchurch from Wellington on Monday night to map, in 3D, fresh damage on the coastal cliffs in the Sumner and Redcliffs areas.

- NZ Herald

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