Jolts a lesson for capital buildings

Large number of fittings shaken lose in tremors prompts call for stricter standards.

Wellington's building stock performed well during the weekend's magnitude 6.5 earthquake, engineers say, but the injury toll would have been higher if the quake had struck on a weekday instead of a Sunday evening.

There's been no let up in aftershocks, with central New Zealand rattled by dozens over night - including a magnitude 4.3 quake east of Seddon at 6.25am.

There have now been about 1000 aftershocks since the main quake.

Experts have recommended that national rules for securing interior fittings and ceiling tiles were made stricter out of concern that voluntary standards had led to poorly restrained items which broke free in the quake.

Institute of Professional Engineers New Zealand chief executive Andrew Cleland said that assessment of the Cook Strait tremor found that its energy peaked at 20 to 35 per cent of new building standards.

There were 600 buildings in Wellington which did not meet 33 per cent of the building code, which meant they could partially or completely collapse in a moderate earthquake.

Mr Cleland said: "The earthquake was getting to a level where it looked like it potentially could have been big enough to be difficult for masonry ... and the masonry seems to have survived reasonably well."

He stressed that assessments had so far focused on the outside of buildings, and no major structural weaknesses had been found.

Mayor Celia Wade-Brown said the fact that only four minor injuries occurred during the quake showed that the city was "well-built and well-prepared".

But engineers emphasised the timing of the earthquake was fortunate. Like the Christchurch quake in 2010, the Wellington tremor struck on a weekend, when the central business district was mostly empty.

Beca technical director of earthquake engineering Richard Sharpe said that the risks of falling glass and masonry in the CBD would have been significantly greater on a working day.

"It really reinforces the adage that people should not exit a building quickly after an earthquake ... You could have had an accidental death in some places, there were a few bits of concrete ... which if it had caught you on the head it would have probably done you in."

Of the 42 people who were killed by falling masonry in Christchurch's earthquake in 2010, 38 were killed outside of buildings.

Office workers could also have been harmed if the quake occurred on a weekday.

Mr Cleland said that interior fittings such as water tanks, pipes and air conditioning ducts were displaced because they were poorly restrained - best illustrated by an image of dangling lights and fittings in the BNZ building on the city's waterfront. He said the standards for seismic restraints were voluntary, and could need regulatory tightening.

Cabinet will next week consider setting a 15-year limit for assessing and strengthening earthquake-prone buildings. But because Wellington was in a seismically active area, the council had already assessed 85 per cent of its 5482 earthquake-prone buildings.

GNS Science said that Sunday's quake was a once-in-a-decade event for Wellington or rarer. But geophysicists were also investigating whether the Cook Strait quake could have increased stress on the Wellington fault, which ran directly under the CBD and Lower Hutt.

Mr Sharpe: "There'll be no doubt if we get a Wellington fault event it will be quite a different scenario."