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Scientists search for fourth fault

PAUL GORMAN
Last updated 05:00 14/02/2011

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Scientists hope to soon find a possible fourth fault site near Christchurch.

Experts are considering various hypotheses regarding the September 4 earthquake.

One GNS Science hypothesis suggests at least three faults ruptured in close succession, with a possible fourth fault – running between West Melton, Sandy Knolls and Burnham – also involved.

The generally accepted trigger for the magnitude-7.1 quake was a hidden, "blind thrust" fault lying at depth in a broad northeast to southwest zone close to Charing Cross.

When that fault ruptured, it activated the sleeping Greendale Fault further south, ripping open a nearly 30 kilometre-long gash across the Canterbury Plains.

Another blind thrust fault to the southwest of Hororata is then thought to have ruptured underground, followed quickly by what could be a fourth fault.

Satellite radar images, GPS (global positioning system) calculations and computer modelling showed the presence of the third and possibly fourth faults angled across the Greendale Fault.

GNS seismologist Dr Caroline Holden said she was looking closely at whether the fourth fault existed.

Her original model was that the 7.1- magnitude quake could be accounted for by an about 6.3-magnitude quake lasting two to four seconds on the Charing Cross fault, followed by a 6.9 shake lasting seven to 18 seconds on the Greendale Fault and a close-to-6.5 jolt of 15 to 18 seconds near Hororota.

"I am still confident with the three-fault model, but am refining it to be able to spot any major misfit in the seismograms recorded east of the Greendale Fault, where we feel a fourth fault might be," she said.

"One of my colleagues is refining the location of the aftershocks in order to get a sharper image of the faults involved in the Darfield quake and I hope to be using his findings in the next few weeks to place this fourth fault accurately, instead of just guessing with the seismograms."

GNS geophysicist Dr John Beavan is more convinced the fourth fault exists and does not rule out further faults.

His original calculation of the rupture sequence was slightly different.

He put the initial quake at magnitude 6.5, followed by shakes of 7.0 and 6.2 on the second and third fault.

He believed the last rupture, on the fourth fault, generated a quake of about 6.5.

Beavan had been re-analysing the post-quake GPS data.

"The next step ... is to see if I can tie down the fault or faults that were active near the eastern end of the Greendale Fault."

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