Violence of tremors stuns experts

Yesterday's quake knocked out a further portion of the western wall of Christchurch Cathedral. Photo / Geoff Sloan

Earthquake experts were surprised by the violence of the newest tremors beneath Christchurch, with reports showing ground acceleration far greater than expected for magnitude 5 and 6 quakes.

Four quakes of magnitude 5 or greater hit yesterday, the largest one a magnitude 6 which occurred 10km north of Lyttelton, and only 6km deep in Christchurch's hard basaltic rock.

The faultline of the largest quake was estimated to be 7km to 8km long, and its rupture caused the earth to slip about a metre.

University of Canterbury geologist Mark Quigley said the magnitude 5.8 earthquake generated ground accelerations of 1g in New Brighton, on the eastern side of Christchurch.

This meant the buildings in eastern suburbs were thrown up and dropped with the same acceleration a stone would have if dropped from a person's hand.

More significantly, the threshold for liquefaction occurring was around 0.17g to 0.2g.

"So the largest quake was five times the ground acceleration required for liquefaction," said Dr Quigley.

"Even though the first earthquake seemed slightly smaller, at a 5.8, the proximity to the eastern suburbs but also something about the way the fault ruptured generated extremely high ground accelerations."

The tremors were an expected part of the aftershock sequence that has been rumbling under Canterbury since the Darfield quake on September 4 last year.

A statistical analysis published by GNS Science on December 15 showed there was a 50 per cent chance of a tremor between 5.5 and 5.9 occurring in the next year.

Dr Quigley: "It's not surprising. As much as we want to move on, with faults like these ones that are locked up for five, ten thousand years ... three months without a major earthquake is the blink of an eye in geological terms."

GNS Science seismologist Ken Gledhill said it was not a classic aftershock sequence because the earth was taking slightly longer than expected to shake out the largest quakes. After a magnitude 6 quake a region can expect around 10 magnitude 5 aftershocks.

"The thing about the deeper rock around Christchurch is that it's quite firm, even though you've got the river gravels on top. So when you break it, it breaks quite violently. And when it has been locked for so long, it takes a bit longer to get rid of all its stress," said Dr Gledhill.

He warned that tremors would continue during the weekend before tapering off next week.

There have been 31 earthquakes of magnitude 5 and above since September 4 last year.

Yesterday's quakes
* 1.58pm, magnitude 5.8, 20km northeast of Lyttelton, 8km deep
* 2.06pm, magnitude 5.3, 20km east of Christchurch, 10km deep
* 3.18pm, magnitude 6.0, 10km north of Lyttelton, 6km deep
* 4.50pm, magnitude 5.0, 20km northeast of Lyttelton, 10km deep