Canterbury's still on the move

PAUL GORMAN
Last updated 05:00 05/09/2012

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Canterbury is still moving and settling in response to two years of earthquakes.

High-precision readings from satellites show the land below Christchurch and the eastern Canterbury Plains is still deforming at an average rate of several millimetres a year.

The land movement is not just a direct consequence of aftershocks but is known as "post-seismic deformation", which could be due to slow fault-slip around the edges of the faults that ruptured in the main Canterbury quakes.

Work by GNS Science geophysicist John Beavan and others using satellite radar images and global positioning system (GPS) measurements has found:

Small but clearly detectable land deformation east of the Greendale Fault since September 4, 2010.

Deformation around Christchurch for several months after the February 22, 2011, quake.

Displacement of up to 10 millimetres a year across the region since the June 13, 2011, quakes.

Beavan said central Canterbury "appears to still be adjusting" to the release of stress from the region's quakes.

However, the levels of movement had surprised the research team.

"It's changing less than we expected. After quakes like this, there's typically a lot of post-seismic deformation," he said.

"The more well developed faults often have a mix of brittle and less-brittle patches. The brittle parts break in a quake and put more stress on the more ductile patches.

"The increased stresses cause those patches to slip and this slip can take place over weeks or months. That can account for 20 to 30 per cent of the movements seen in the earthquake itself.

"But in this case it's small, just a few per cent, and we think that's because the crust is so strong in this area."

Other researchers have concluded the extreme ground accelerations caused by the quakes were due to the strength of the rocks that ruptured.

Beavan said changes to underground water bodies as a result of a quake led to surface deformation for days to weeks as the water flowed back to equilibrium.

Longer-period surface deformation could also happen as a result of the movement of more flexible rocks deeper down where temperatures were higher, he said.

The movements in Canterbury were "matters of tens of millimetres".

"The region appears to still be adjusting," he said.

"It does seem there is still some deformation going across the zone from south of where the aftershocks are to north of where they are, of about 10mm per year, measured from June last year to March.

"This is small but it's a lot more than before the quakes."

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In the four months between the February and June quakes, sites near central Christchurch moved east and south by more than 10mm in the first two months before decaying to lower rates close to June.

- The Press

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