Auckland low-risk for quakes, but no way to know for sure

There's no way to know for sure about what's rumbling under Auckland's surface. Photo / Richard Robinson

Auckland is deemed a low earthquake hazard in calculations by GNS Science (the Institute of Geological and Nuclear Sciences) - who provide the earthquake science on which building rules are founded. But, says Mark Stirling, GNS lead scientist in seismic hazard modelling, there is always the possibility.

The city is a long way from the nearest plate boundary, which runs north from Wellington through to Taupo then towards East Cape, and where heightened seismic activity could be expected. The location dictates an "intra-plate regime rather than an edge of plate regime", says Stirling, responsible for GNS's hazard modelling programme.

The nearest known active fault, Wairoa North, runs from the Hunua Ranges through Clevedon, about 30 km from the CBD. Its recurrence interval is assessed at one in 20,000 years. The only other known fault, Kerepehi East, runs from the Hauraki Plains into the Firth of Thames. Its recurrence interval is "in the thousands rather than tens of thousands of years" but its distance from Auckland makes it not quite so serious, says Stirling.

"In terms of probability, Auckland is the lowest risk of all our main centres in terms of the likelihood of strong shaking."

Then come the riders. Hazard modelling can forecast recurrence timeframes for earthquakes of various magnitudes but can't say when a very rare event, a big one, is going to happen.

The scientists rely a lot on surface expression - evidence of a fault in the landscape and to a lesser extent on seismicity. "Something has to have happened often enough for it to be expressed in the landscape. You can have active features that have no expression."

The two Canterbury quakes have highlighted the uncertainties. The recurrence interval for a quake on the Canterbury Plains was one every 16,000 years "but the model is incapable of saying it's going to happen in 2010 or 2011".

"There was no way of telling the Greendale fault (which ruptured near Darfield) was there beforehand unless some funding agency wanted us to do detailed geophysical analysis.

"Canterbury tells us to worry about unknown faults close to the city."

Auckland experiences low magnitude shocks often enough to show the region is not seismically dead. "There's always a possibility we could end up with a larger quake."

The most recent reasonable shudder, a 4.5 magnitude quake off Orewa in 2007, woke children sleeping in Mt Roskill.

Auckland has only two known faults but there may be more, as the previously unknown Canterbury faults suggest. "We haven't that much information on Wairoa North beause it is not moving very fast."

Faults considered dead may not be dead in future. And the relationship between seismicity and faulting is not one-to-one; seismicity doesn't always occur directly on faults. "The presence of seismicity tells you an area is tectonically active, just as volcanism indicates that there is crustal activity going on."

The area's low seismicity means it is less well researched than others. Stirling says there could be a case for further research, "based on Canterbury's experience and knowing any faults in the Auckland area are going to have very long recurrence intervals and may not have topographic expression."

"Certainly the distance from the plate boundary makes the probabilities of large shaking quite low.

"But tomorrow there could be a one in 20,000 year earthquake."

Auckland's new buildings - built to modern ductile (flexible) engineering standards - are designed to withstand shaking from a moderate (6.5 magnitude) quake with maximum ground acceleration (G-force) of 1.3g, reflecting its distance from the plate boundary and known faults. In Christchurch, the tolerance level is 2.2g and in Wellington 4.5g. But the ground acceleration on February 24 far exceeded the model.

Is there a case for higher earthquake tolerance levels for older buildings - currently 33 per cent of the new building standard - in light of Christchurch?

"You would need to have convincing evidence of the seismic hazard in the area relevant to buildings."

Stirling says a recent review has lowered the seismic hazard for the Auckland region "because of the virtual absence of faults."