Explaining the science behind quakes

DAVID WILLIAMS
Last updated 05:00 28/07/2011
ALL EARS: Hundreds of people attended the first of a series of free  lectures on earthquake behaviour.
IAIN McGREGOR/ The Press
ALL EARS: Hundreds of people attended the first of a series of free lectures on earthquake behaviour.

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The gash torn in the Canterbury Plains by last year's Darfield earthquake had the highest ratio of movement for its length of any recorded quake, University of Canterbury scientists say.

Last night, hundreds of people attended the first of six free earthquake lectures run by the university to explain the science behind the earthquakes that have rocked the region since last September.

Because of the lectures' popularity, many were forced to watch a live video in adjacent lecture theatres. Canterbury University geologist Mark Quigley said mapping of the 30-kilometre surface rupture had revealed the quake had the highest ratio of slip, or offset, to surface length of any earthquake anywhere else in the world.

The reason for the high displacement was the high level of stress on the fault and the low frequency of earthquakes, he said. "It takes a lot of elastic strain energy before they break."

Before the lecture, Professor Jarg Pettinga, head of the university's geological sciences department, said the faults that had activated since September were likely to "link up" in the coming tens of thousands of years and form a continuous fault zone.

He said Cantabrians should expect to feel aftershocks for the next year or more, but considering the vigorous earthquake sequence since last September, the aftershocks should now progressively decline.

"We're not through it, I don't think you'd want to kid yourself that it's all over.

"At the same time we have seen significant activity along the entire fault zone system from east of Christchurch to near Hororata, and at the end of all this that system will have substantially relieved the stress levels that have existed along those faults."

Pettinga did not rule out earthquakes on other unknown, or "blind", faults.

Quigley said it would not be surprising if the region experienced another strong earthquake, similar in magnitude to the Darfield quake, in the next decade.

Last night's audience emitted gasps when they saw maps comparing Canterbury's seismicity in the decade leading up to September 4 and the dense cluster since.

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- The Press

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