Fault off Kaiapoi may spark marine quake

PAUL GORMAN
Last updated 07:40 15/11/2012

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A large offshore fault near Kaiapoi could generate what the National Institute of Water and Atmospheric Research (Niwa) calls a major marine earthquake.

Speaking at the New Zealand Coastal Society conference in Auckland yesterday, Niwa principal marine scientist Philip Barnes used the recently discovered southern Pegasus Bay fault as an example of a previously unknown submarine quake hazard.

The fault is similar in size and shape to the Greendale Fault, which ruptured in the September 4, 2010, quake.

Scientists said at the time that the undersea fault had the potential to generate a quake of between magnitude 6.0 and 7.0, but the tsunami threat from it was low.

About 200 newly identified active faults, including those responsible for the Canterbury quake sequence, have increased the number of faults in the revised national seismic hazard model to about 530.

The greater the number of faults, and the more information about them, in the catalogue, the more thorough the understanding about the magnitude and frequency of future quakes.

Barnes told the conference that the large Pegasus Bay fault was not the biggest fault in the bay but the latest significant feature to be discovered.

Extensive sea floor surveying by scientists on the research ship Kaharoa after the February 22, 2011, quake revealed a roughly east-west- oriented fault off the coast near Kaiapoi that was about 25 kilometres long.

The marine survey of sediment and bedrock below Pegasus Bay covered about 800 square kilometres.

As well as uncovering the new offshore fault structure, it confirmed the location of marine faults near the mouth of the Ashley River.

Barnes told The Press that scientists had begun referring to it as the "central Pegasus Bay fault".

"In terms of an expected magnitude of anything it might generate, we can only really do our best to estimate it in terms of the mapped extent of the fault," he said.

"With our best early work, we thought it was one large structure. But with more advanced work we suspect it is segmented into a couple of sections, and they might not rupture together. So that means it could be like other quake sources typical of that area, and we might be looking at low magnitude 6 [quakes].

"It's large enough to generate a moderately large earthquake."

As faults went it was not particularly active and appeared not to have moved for more than 100,000 years, Barnes said.

"It has an extremely low slip rate on it, very much in line with other southern Pegasus Bay structures, of 0.1 to 0.2 millimetres a year. At national levels, that is very low."

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Barnes said the country's next major quake could be centred offshore.

"It highlights the need for coastal marine investigations of active earthquake faulting, for both ground-shaking hazards and tsunamis," he said.

The seismic hazard model is used to inform future engineering design.

- The Press

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