Australasian Science: Australia's authority on science since 1938

Christchurch’s Unknown Fault

By Stephen Luntz

The earthquake that caused billions of dollars of damage to Christchurch, including to the University of Canterbury, occurred on a previously unknown fault line.

If that is not enough to cause alarm to those who thought their houses were safe, the head of the University’s Geology Department, A/Prof Tim Davies, says that most earthquakes occur on fault lines that were unknown previously.

“New Zealand is on an interplate margin,” Davies says. “Any active plate boundary will have very high stresses, with motion taking place along the cracks.

“A stressed region will have a lot of faults. The bedrock at Canterbury is buried under several hundred metres of gravel so there is no chance of observing the fault directly, unlike in the mountains where the practised eye can see them.”

Davies adds that the magnitude 7.1 earthquake caused a bulge in the landscape, and that there are a number of similar such bulges around the region. “It’s only just occurred to me that these slight bumps may have been caused by ancient earthquakes,” he says.

The good news is that while there has been a normal pattern of aftershocks, a quake of similar size so close to the city is considered unlikely. However, it is possible that the quake on 4 September could transfer stress to other fault lines, triggering quakes there. For instance, a major shock on the Alpine Fault has been anticipated for some time. Such a quake would be larger than the one Christchurch experienced, but is further from populated areas.

Davies says the campus “came off remarkably well” with very little structural damage. Classes were cancelled for 2 weeks, but it took only 1 week for staff to return to their offices.

The unexpected location of the fault has raised the question of whether the building code needs reform. “Building design is based on New Zealand’s Probabilistic Seismic Hazard Map, which is based on anticipated shaking from known faults,” Davies says. The discovery of the new fault may require tougher standards for buildings in Christchurch, and suggests that other areas previously thought to be safe may also require re-examination.