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Christchurch quake rebuild hampered by aftershocks

A series of severe aftershocks is continually undermining efforts to rebuild the earthquake damaged New Zealand city of Christchurch, engineers told NCE this week.

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Devastated: Many buildings have still not been rebuilt

Major shocks occur again

On 23 December 2011, three major shocks measuring magnitude 5.8, 5.3 and 5.8 hit New Zealand’s third largest city.

Consultant Opus International’s office director Paul Eastwood is assessing public buildings for damage and has been doing so since the initial devastating 6.3 magnitude quake struck last February, seriously damaging many key buildings (NCE 22 February 2011).

“We’re in uncharted territory”

Consultant Opus International’s office director Paul Eastwood

Eastwood said the two largest aftershocks were recorded last June and measured 5.5 and 6.0 respectively. He said the aftershocks have been unpredictable because they have not followed the usual pattern.

This uncertainty is undermining the rebuild effort.

“We’re in uncharted territory,” said Eastwood.

Reconstruction is being led by the Canterbury Earthquake Recovery Authority (Cera), which was formed shortly after the February earthquake.

Business properties yet to be assessed

While Cera has made progress assessing residential properties − to date it has examined close
to 200,000 − it has yet to make the same progress with business properties.

In its initial assessment, Cera determines whether the location of the damaged properties is suitable for rebuilding with modifications such as strengthening or ground improvement. It will also decided whether buildings require further, detailed engineering assessment.

Latest figures from Cera show that 645 commercial buildings have been demolished, and a further 200 have been partially demolished across the city − 69% are in the Central Business District.

“The city centre is still in a state of deconstruction,” added Eastwood. “The impression is that the officials still haven’t made up their minds about the rebuild yet.”

“The city centre is still in a state of deconstruction”

Consultant Opus International’s office director Paul Eastwood

He added that this uncertainty, along with the continued threat of aftershocks is causing the recovery to stall.
In addition to these issues, there is still uncertainty about what the reconstruction plan will look like.

Last August, Christchurch City Council produced a new plan for a low rise city centre − which it hopes will make it more resilient in future earthquakes.

This was ratified in late December, but there is no clear commitment to timescales for its delivery or a firm start date.

Call for more workers

However, in late December there was a call for more construction workers to help with the rebuild. A report produced jointly by the New Zealand Department of Labour and regeneration organisation Canterbury Development Corporation says there is strong evidence that demand for construction related workers will be high during the rebuild, with 24,000 needed during the peak reconstruction phase.

But estimated demand for workers could vary significantly depending on the pace of recovery and the emergence of damage information as insurance claims are assessed, says the report.

Alternative scenarios still suggest many thousands of workers will be needed at peak but numbers could vary from 13,000 to as many as 48,000 additional construction-related workers.

Peak reconstruction activity looks to be some way off as the city still faces the threat of seismic activity.
New Zealand’s equivalent of the British Geological Survey, GNS Science said that Christchurch was likely to experience more magnitude 5.0 or 6.0 earthquakes in the coming weeks, but that the larger quakes are likely to be further away from the city.

Tremors moving east

The earthquakes have been moving east, according to GNS Science. The earthquake sequence began west of Christchurch with the 7.1 magnitude Darfield earthquake in September 2010.

This caused relatively little damage despite its size. The tremors have been following a fault-line running under Christchurch and are now occurring in Pegasus Bay, east of the city.

But concerns about whether strong quakes could create major disturbance of water have been played down. GNS Science earthquake scientist Kelvin Berryman said it was unlikely that tremors in Pegasus Bay would cause tsunamis.

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