15 Feb 2016

Why does this keep happening to Christchurch?

1:44 pm on 15 February 2016

Just as Christchurch was about to mark five years since the 2011 earthquake that killed 185 people, it was hit again. But why does the city continue to be shaken by quakes?

Rocks tumbled from cliffs on Richmond Hill and Whitewash Heads creating a massive dust cloud as yesterday's earthquake hit at 1.13pm. It struck with with enough force to fling items off shelves and walls and push up liquefaction in Christchurch's streets.

The 5.7 quake that jolted Canterbury is one of thousands of aftershocks from the 2010 earthquake and most likely occurred on a previously unknown fault, an expert says.

GNS seismologist John Ristau said it was another in the sequence of aftershocks following the 2010 quake - albeit a very big one.

"All of this goes back to the September 2010 Darfield earthquake," he told Morning Report.

Since 2010 there had been "easily over 10,000 aftershocks".

GeoNet has recalculated the probabilities of aftershocks in the Canterbury region after yesterday's quake and now says there is a 63 percent chance of 5.0-5.9 quake in the next year - up from a 49 percent probability.

Dr Ristau said it was probable yesterday's tremor which occurred offshore, 15km from Christchurch, was on a fault that scientists did not know about.

"The problem when it comes to Pegasus Bay, of course, is that everything's under water and it becomes much more difficult to map any faults.

"NIWA has done some mapping out there but until we have a chance to look closer at exactly where this earthquake happened we won't know whether it occurred on any of the currently mapped faults although it's most likely that it did not occur on a fault that we knew about."

Christchurch quake, how it unfolded.

University of Canterbury professor of earthquake engineering Brendon Bradley said the sequence of earthquakes since 2010 had moved east.

New Brighton experienced the strongest shaking yesterday, but by the time it had reached the city, the force had halved.

"As each different part of the fault breaks it transfers stress in the corresponding direction and there is this general trend of moving eastwards."

As part of that trend, the latest earthquake was located offshore. "And because of that, fortunately, much of the city's been spared from some strong shaking."

Prof Bradley said the frequency of these quakes has decreased over time - and the chances of an 5.0-5.9 quake over the year had changed a little but not materially for the general public.

"We use these probabilities because we can't predict exactly when the next earthquake is so what matters for the general public is just be ready for the chance that we could have some level of shaking."

The 5.7 quake was the largest in the city since a quake in May 2012, and was on par with what was felt in the city in December 2011, when a series of strong shocks, including a 5.8 struck, Geonet said.

While the peak ground acceleration measurement was 0.4g - enough for liquefaction to occur - it was significantly lower than the 2.2g experienced in the 6.3 February 2011 quake.

The epicentre of the the 5.7 earthquake near Christchurch, 14 February 2016

The epicentre of the the 5.7 earthquake near Christchurch, 14 February 2016 Photo: Geonet

Liquefaction along Bower Ave

Liquefaction along Bower Ave Photo: RNZ / Sally Murphy

Otago University professor of earthquake science Mark Stirling said the quake was a part of New Zealand's very old fault system.

He said it was impossible to locate all of the faults in the crust, as New Zealand had a long history of fault movement.

"It's not like there's a new major alpine fault forming there or anything like that. It's a feature that's... probably very, very low activity - hasn't moved for a very long time."

Demolitions on hold

Demolitions of crown-owned properties on Christchurch's Port Hills have been put on hold following the earthquake.

Just over 400 hundred houses were red zoned on the Port Hill's after the February 2011 quake.

Land Information New Zealand said demolitions would not resume until all sites had been inspected and approval was given for work to continue.

Meanwhile Christchurch City Council staff are continuing to check the city's facilities and infrastructure.

The council said some facilities remain closed while engineering reports were done and a full list of the closures would be available later on Monday.

The council said the Hadlee Pavilion at Hagley Oval was operational and preparations for the New Zealand, Australia test match this weekend were continuing.

Quake a 'painful blow'

Prime Minister John Key said the quake was a painful blow but had not shaken confidence.

He told Morning Report the lack of damage to buildings showed the reconstruction of the city had been done to a high level.

"No one likes being part of an earthquake, it's an awful feeling an awful sensation and can claim lives - they're terrible things - but you can take a degree of confidence in Christchurch that your infrastructure is strong."

'Terrifying' cliff escape

Lifeguard Craig Jamieson has described the moment the cliffs in Sumner began crashing around a group of junior lifeguards when the quake hit.

Mr Jamieson, the life saving club patrol captain, was in a boat under the cliff face, showing the team of rookie guards as young as 12 how to jump off cliff faces and perform rescues.

He said it was "absolutely terrifying" to see a huge slab of cliff, as well as rocks as big as a microwave, slide and cascade near the children.

He told Morning Report three of them hid in a cave while two dived into the water, and he was amazed no one was hurt.

"How can five kids avoid so much rock and get back in the boat - that was insane."

Major roads, bridges open

Much of Christchurch is returning to normal today as major roads and bridges, which were checked yesterday, are all open.

Motorists are advised to take care on roads around North New Brighton and Parklands where there may be silt due to liquefaction.

Council rangers have closed the Summit Rd between Rapaki Gate and Mt Cavendish. Bridle Path and Rapaki tracks are also closed due to rockfalls in these areas.

There have been no reports of major damage at any schools though three are closed for the day while their buildings are checked: Christchurch Girls' High School, Aranui High School and St Thomas of Canterbury. The University of Canterbury has been cleared to reopen by structural engineers who have inspected key indicator buildings.

Power was out in some parts of the city yesterday but all is now restored.

Public transport is operating as normal and malls that were closed yesterday are expected to open today.

Council staff are due to resume checks on council-owned facilities this morning and engineers will inspect other buildings throughout the city for damage.

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