'Hidden' faults pose quake danger

JOHN EDENS IN QUEENSTOWN
Last updated 05:00 07/03/2011

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A magnitude-7 rupture of an unknown fault similar to the quake that devastated Christchurch is possible in Otago or Southland, geologists say.

The 7.1 quake in September was caused by an unmapped system 40km west of the city, while the 6.3-magnitude event – part of an aftershock sequence – was also an unmapped fault.

Scientists and researchers know hidden faults exist but mapping or detecting dormant structures is next to impossible.

Otago University's Professor Richard Norris said Otago and Southland's myriad faults had an average length of 25km, often displaying erratic or irregular movement and could be dormant for thousands of years.

The Akatore fault near Dunedin has not ruptured for 100,000 years, while the Pisa system near Cromwell has been dormant for between 20,000 and 50,000 years.

The fault that caused the September quake in Christchurch was dormant for 16,000 years.

"We have to be careful we do not become too complacent, it's very difficult to pinpoint the risk of one structure," he said.

Seismic hazard mapping of known faults coupled with disaster management plans were important, Mr Norris said.

"But it doesn't tell you where the next earthquake is going to be.

"The September fault in Christchurch, even if it had been known, it hadn't shifted for 16,000 years.

"A lot of these faults show long periods of quiescence, there's so many of them, which one breaks depends on which one is the weakest," he said.

GNS Science tectonic geologist Dr Rupert Sutherland said it was a question of the level of preparedness in areas where unknown faults were likely.

New Zealand building codes recognised the possibility of unknown faults but the probability of a rupture was low.

"It's true the Christchurch quake specific fault was not mapped or known but we knew a magnitude-7 earthquake was possible. That's also true for Southland and Otago."

GNS Science geologist Mauri McSaveney said the main seismic risk in the South Island was the Alpine Fault.

"A lot of the others [faults], we don't know what the risk is, they don't rupture as often as the Alpine Fault."

The 7.1 Canterbury quake was produced by a complex unmapped four-fault system – the Greendale Fault – which caused a 22km-long surface rupture and up to 4m of horizontal movement.

February's 6.3 event was a rupture on an east-west fault between Halswell and Sumner, a movement large enough to increase the height of parts of Port Hills by half a metre.

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- © Fairfax NZ News

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