Quake lessons for Dunedin, prof says

Tim Davies.
Tim Davies.
Civil Defence assumptions about the likelihood of a major earthquake in Dunedin need to be reconsidered following Kaikoura’s seismic shake, a University of Canterbury professor says.

The call came as ODT Insight found authorities in Otago still had work to do to prepare for a magnitude 8-plus Alpine Fault earthquake, which scientists estimate has a 30% chance of striking within 50 years.

Associate Prof Tim Davies, a specialist in natural hazards and disaster management, told ODT Insight last month’s Kaikoura earthquake should shake assumptions about where large earthquakes could occur.

Seismologists have found at least 10 faults combined to produce Kaikoura’s 7.8-magnitude quake, although the largest of them — the Hope Fault — had hardly ruptured at all.

That could have implications for the Civil Defence response in a place like Dunedin, which has traditionally been considered at low risk of  earthquakes, despite the presence of nearby faults, he said.

"Until now, the perception has been ‘OK, we know where the major faults are. These are where the major earthquakes are going to be. We can prepare on that basis’.

"In reality, if we want to be prepared, we need to prepare for something like the Kaikoura event happening basically anywhere," he said.

Otago Civil Defence and Emergency Management  (CDEM) regional manager Chris Hawker agreed, saying the potential for local faults to act unpredictably was "absolutely" being taken seriously.

The lack of knowledge about faults had previously been underscored by the Canterbury earthquakes, triggered in 2010 by the previously unknown Greendale Fault, and again by last month’s Kaikoura quake, he said.

Otago CDEM’s approach now was to assume something "really bad" could be generated by any of Otago’s faults and plan accordingly, he said.

"Our biggest single problem is we have only been on this planet for a nanosecond.

"Really, we think we know something until we’re proven wrong."

Otago was home to a network of faults, including several — such as the Akatore, Maungatua, North Taieri and Titri faults — that ran close to Dunedin and were considered capable of generating strong shaking.

GNS Science,  in co-operation with the University of Otago,  was already studying the Titri fault, which runs  within 10km of the city centre and is believed to have the potential to produce a magnitude-7.2 earthquake.

GNS spokesman John Callan said last month’s Kaikoura quake was a reminder "large magnitude earthquakes are not necessarily limited to single, major plate boundary faults".

The Kaikoura quake’s complexity would have implications for modelling seismic hazards  but it did point to  a significant increase in risk, he said.

The faults that ruptured were  known to have produced large earthquakes previously, and the national seismic hazard model already included the potential for multi-segment ruptures in the area. Further research was  under way before the Kaikoura earthquake, he said.

Sarah Stuart-Black, the director of the  Ministry of Civil Defence and Emergency Management, said it was "well established" New Zealand had undiscovered faults, "and that our fault networks can behave unpredictably".

The civil defence framework was designed to be flexible "for this very reason", she said.

"We know that there could be large aftershocks following the Kaikoura earthquake so it is important that we are prepared for all possibilities."

chris.morris@odt.co.nz

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