South Island still being reshaped

17:00, Jul 20 2013

The South Island is still buckling and shifting to the rhythms of the earth nearly three years after titanic seismic forces began reshaping Canterbury.

High-precision satellite readings gathered from afar afield as the Southern Alps and analysed by scientists as part of an Earthquake Commission-funded project show that despite the apparent settling down of aftershocks, the ground keeps moving.

While it's 14 months since the last magnitude 5-plus earthquake shook Canterbury, the longer-term effects of the barrage of quakes are continuing to affect the landscape.

In the past 18 months, North Canterbury has been budged about 10mm further east towards Pegasus Bay compared with Christchurch, Banks Peninsula and parts of the region further south.

But researchers say the overall movement of the crust across the quake zone is slowing down, which they believe is a good sign for Canterbury residents.

On Friday, the country's biggest quake of recent months, a magnitude 5.7 event centred in Cook Strait at a depth of 8km, frightened Marlborough and Wellington residents. A second quake of 4.5 magnitude followed hours later.


GNS Science spokesman John Callan said there were several active faults under the seabed in Cook Strait.

Seismologists say post-seismic deformation in Canterbury is quite different from "slow-slip" or "silent" quakes that have been discovered in parts of the North Island, such as Poverty Bay, Hawke's Bay and offshore of the Kapiti Coast, over the past decade using global positioning system technology.

University of Texas research scientist Laura Wallace said the most recent GPS data recorded across what has become known as The Gap near Christchurch showed regional movement had slowed by 50-70 per cent from an earlier survey.

The Gap, an area around Prebbleton and Lincoln southwest of Christchurch, is a physical void between the eastern end of the Greendale Fault and the western end of the Port Hills Fault.

Scientists have calculated there is a shortfall in the amount of energy released by quakes there compared with that released close to the two big faults on either side.

Some scientists have advocated monitoring of The Gap to discover whether hidden and shallow faults there could generate a quake of magnitude 6 or more.

However, recent attempts to secure research funding for ongoing work have failed.

Wallace said it was clear The Gap and the surrounding region was still moving as a result of the long quake sequence.

"Basically there was six to 10mm of movement between January 2012 and January this year across the region from south of the aftershock [zone] to the region north of the aftershocks [zone]. This contrasts with a slightly faster 10mm or so movement across that region in the nine months between June 2011 and March 2012.

"So The Gap area is still adjusting, but at a slightly lower rate than it was before. [It's] interesting, but not super exciting, which is good for you guys," Wallace said.

GNS Science seismologist Caroline Holden believed The Gap was not as potentially hazardous as previously thought. "There's faults everywhere, but if they were to be going off they would have gone by now, although the risk of a [magnitude] 5 is still quite high."

The last aftershock of that size was a magnitude 5.2 on May 25, 2012. GNS Science forecasts there is a 61 per cent chance of a quake of more than magnitude 5 in the aftershock zone in the next year.

"The ground is still trying to settle down but the rate is now a bit smaller. It is a horizontal relative motion, where the northern side of the aftershock zone is moving roughly east-northeast relative to Banks Peninsula.

However, compared to regional deformation before the whole earthquake sequence started in September 2010 it is still a lot more active."

‘The September 2010 quake was 7.1 magnitude and the February 2011 quake, which killed 185 people, was a 6.3 magnitude.

Sunday Star Times