Dunedin residents 'should not be complacent'

While the chance of a serious earthquake hitting Dunedin is one in 1000, the odds are better than winning Lotto and citizens "should not be complacent", geologist Prof Richard Norris says.

Should such a quake occur, parts of South Dunedin, Mosgiel and the Taieri would "inevitably" be engulfed in liquefaction, the hill suburbs would suffer landslips and rockfalls and many heritage buildings would be reduced to rubble.

Prof Norris, of the University of Otago geology department, put such images to the Mornington Probus group at a meeting yesterday.

The city's most active fault-line, Akatore, which runs from Taieri Mouth to Dunedin, last broke about 1000 years ago, and the chance of it happening again is less than one in 1000, he said.

Researchers knew a lot about the fault-lines around Dunedin, most of which were not highly active - the Titri Fault has not moved for 90,000 years.

"But given Christchurch's experience, we should not be too complacent," he said.

The September 4 quake which shook Christchurch and its subsequent devastating aftershock on February 22 both occurred on an old fault-line.

"It's likely the fault had not moved for 15,000 years," Prof Norris said.

Even if extensive, and expensive, seismic research had been carried out on the fault, it probably would not have been given a high degree of significance.

The quake was "typical" of most in the South Island as it occurred on a fault which was not recognised.

Once a fault like that broke, strain was released in its vicinity, but pressure built at either end of it, evident from the pattern of aftershocks Christchurch experienced after September 4.

Between January and February 22, those aftershocks were increasingly closer to Christchurch city and seismologists knew a large quake could not be discounted.

Prof Norris believed the area around the fault was "being shortened", culminating in the deadly 6.3-magnitude quake.

"The Port Hills have been thrust over the lower-lying areas of the city," he said.

Christchurch's "seismic hazard" was "about double" Dunedin's, but should one occur, the impact would be similar, but of a different nature.

Soil liquefaction was likely at St Kilda, parts of St Clair, Mosgiel, and the Taieri, including the airport.

The hill suburbs, along with the Peninsula roads and parts of Kaikorai Valley, would likely suffer rockfalls and landslips, the severity of which would depend on ground moisture.

There "is virtually no chance" of liquefaction on the hill suburbs and the bulk of the city centre and North Dunedin "because there is nothing to liquefy".

The city largely sits on solid rock, which would result in less violent shaking than experienced in Christchurch.

However, Dunedin's higher proportion of historic brick buildings meant damage would potentially be worse.

"A lot of the images we've had [of the Christchurch quakes] have been masses of piles of brick and that's because brick is not a very good material in an earthquake.

"I shudder every time I look around Dunedin."

Older wooden houses fared better as they are "incredibly resilient" and "flex" in quakes.

"We should not be building with brick, in my view, in New Zealand."

Prof Norris believed New Zealanders needed to be aware of the nation's quake susceptibility - it sits on two tectonic plates, the Pacific and Australian - and while the Christchurch earthquake decreased the probability of earthquakes in that area, the rest of the country was still at risk.

"We live with earthquakes."

- ellie.constantine@odt.co.nz

 

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