Anthony Doesburg: Wellington's 'big one' likely to take some time

Some people might be prepared to rely on barking dogs or restless cats to warn them of an impending earthquake. In Wellington, where the threat of an overdue "big one" has long had authorities on edge, they're taking a more scientific approach.

It's Our Fault is a seven-year project aimed at getting a better understanding of when the city might next be hit by a big quake and what effect it might have. The project has just passed the halfway stage and the early signs are Wellingtonians can relax a little.

GNS Science seismologist Russ Van Dissen, the scientific leader of It's Our Fault, says geological and geophysical data collected over three years around the capital and in Cook Strait suggest calamity might not be about to strike after all.

"The perception now is the Wellington fault is not overdue to rupture," says Van Dissen, who is co-ordinating about a dozen studies by researchers at NIWA, Victoria and Canterbury universities and GNS.

"None of the studies we've done suggest it's dead; it's a major active fault - but it's not in the phase where it's overdue."

That will be a relief to the $3.5 million project's funders - Wellington City Council, the Earthquake Commission and ACC - which are presenting the results to date in the capital today. With the city apparently unlikely to be shaken to bits any time soon, infrastructure owners and insurers will be able to go about their business with greater confidence.

Not that anyone should get complacent, Van Dissen says. "We haven't been given a pardon - the sentence has just been delayed."

The Wellington fault runs through the CBD and up the Hutt Valley and, with the Wairarapa and Ohariu faults, is one of three key pressure points under scrutiny. Those are by no means the only faults in the region - there are about 50 in total.

The reason for so much seismic activity is that Wellington sits on top of the boundary between the Pacific and Australian tectonic plates, that are moving into and against each other at a rate of about 40mm a year. That was well known before the present project. The fresh light cast by the new research is the speed of ground movement around the various faults, more accurate timing of past fault ruptures and a detailed map of Cook Strait faults.

A range of technologies have been used to collect data on the land and seabed. Seismology has been given a leg-up by the network of global positioning system (GPS) satellites put into space by the US military.

GPS receivers allow measurement of ground movement of just a few millimetres a year - light years better than the old process of using a theodolite to gauge the movement of survey marks relative to each other, which was so imprecise that it was necessary to wait decades between measurements to get a meaningful figure.

In contrast, working out the timing of past quakes is surprisingly low-tech. A digger makes a trench a few metres deep across a fault, and radiocarbon dating is used to age samples from either side of deformed sections to establish when the disturbance occurred.

Seabed mapping, carried out by NIWA, is done by "boomer" seismology - a matter of firing blasts downwards from a vessel trailing a string of sensors that measure the echo returning from the sea floor. Faults show up as ridges, just as they do on land.

Interpreting the data is complex, but made more doable by improved computer power. Based on the region's dozens of faults and known ground movement, a synthetic seismicity model was developed to investigate past quake behaviour.

Van Dissen doesn't want to speculate when the crucial Wellington fault might next go off. But rather than being overdue, the new data suggests it may not be for 300 years. For one thing, the last big quake is now thought to have been more recent than earlier believed.

Also, the research suggests the time between ruptures of the Wellington fault is longer than previously thought. And the final piece of good news is that an 1855 rupture of the Waiararapa fault appears to have de-stressed the Wellington fault.

The region is still vulnerable, but can work on making itself less so, Van Dissen says. "If we choose to build resilience into our communities now, we have time to reap the benefits."

Does he see any point in monitoring animal behaviour for advance warning of the big one? "I don't doubt that there are phenomena that animals are more perceptive of than humans. I think the problem is because we don't know what to look for, we can't rely on it."

The scientific picture, though, is becoming reassuringly clear.

Great shake

* The 1855 Wairarapa earthquake that hit Wellington was the most powerful recorded in New Zealand - magnitude 8.2.

* Many buildings were damaged and some Lambton Quay properties were flooded by tsunami waves. Parts of the city's CBD are on land that was below sea level before 1855.

Source: TeAra.govt.nz

Anthony Doesburg is an Auckland technology journalist