Why did modern Wellington buildings fail in Kaikoura quake?

The building housing Statistics NZ, which was badly damaged by Monday's 7.5 earthquake. Photo / Mark Mitchell

Statistics New Zealand boss Liz MacPherson was unusually blunt for a Wellington bureaucrat as she assessed the serious damage to her department's headquarters from this week's earthquake.

"I am asking the same questions that I am sure you are asking," she told her staff in a Facebook post. "How is it that a building that is as new as Stats House, with the [earthquake] code rating it had could suffer this sort of damage. I'll continue to ask those questions."

Many people, from the Prime Minister down, are asking those questions too as the damage to supposedly well-constructed modern buildings becomes more apparent in the aftermath of Monday morning's quake.

Overall Wellington council inspectors have so far found 60 buildings of concern with signs of structural damage, and 28 at risk of part of the building falling down.

Concrete beams in Statistics New Zealand's 2005 waterfront offices were ripped from the outside of the building causing the floors to partially collapse, according to owners CentrePort. It could be a year before staff return.

About 1200 Defence staff have reportedly been told they face a similar wait to get back into their headquarters, Freyberg House, built in 2007. John Key has already observed that "questions obviously need to be asked" about how such a new building could fail.

Auckland University structural engineering Professor Jason Ingham is adamant that the floor collapse at Statistics House never should have happened. If the earthquake had struck on a Monday morning instead of just after midnight, he told the Herald, then people would probably have been killed.

Ingham added that buildings on reclaimed land seemed to have been hit the hardest, but this should not have caused major problems in itself. The general experience from the Christchurch earthquake was that most well-designed modern buildings performed well.

Yet at this stage both the Wellington buildings appear to have failed in an earthquake well short of the strength they were designed to withstand.

An initial assessment of the seismic impact in Wellington was 0.3G or ground acceleration, which measures the intensity and speed of the shaking.

Both buildings should have been able to cope with three times that force. While that reading may turn out to be an underestimate - the Kaikoura earthquake lasted an unusually long time and was later upgraded from 7.5 to 7.8 - it clearly didn't rank as the major event that Wellington has long planned for.

Politicians will need to tread a fine line when the reports on Wellington building failures come in. As several experts have observed, we have a tendency to be too complacent about earthquakes when they are not happening and too quick to push the panic button when they do. The Government initially overreacted to the danger posed by older brick buildings in the wake of the Christchurch earthquakes, with legislation that could have led to the pointless destruction of many historic buildings in very low risk areas.

It needs to make sure that this time any changes are aimed firmly at the right target.