Latest quake more violent than first

16:00, Dec 27 2010

Violent ground-shaking in central Christchurch during the Boxing Day earthquake exceeded that of the September 4 quake.

GNS Science strong-motion sensors show peak ground movements – either from side to side or up and down – during Sunday's magnitude-4.9 quake at 10.30am reached 48 per cent of the acceleration of gravity at the Christchurch Botanic Gardens.

Christchurch Hospital motion peaked at 25 per cent of the acceleration of gravity and reached 22 per cent at Christ Church Cathedral.

During the magnitude-7.1 quake on September 4, peak ground accelerations recorded around the central city were between 15 and 20 per cent that of gravity.

Geonet manager Ken Gledhill, of GNS Science, said the high ground accelerations in Sunday's shake, along with its shallowness and proximity to the city centre, explained the extra damage to buildings.

While maximum ground-shaking in the area had been lower during the September 4 quake, it had lasted much longer and therefore caused more damage, he said. The shaking had not lasted long enough on Sunday to cause liquefaction.


Other peak ground accelerations on Sunday morning were: Shirley, 19 per cent of gravity; Cashmere High School, 14 per cent; and Heathcote Valley Primary School, 12 per cent.

Gledhill said the high reading at the Botanic Gardens was interesting. "There must have been some amplification effect there as the epicentre of those quakes was further to the southeast."

The Boxing Day magnitude-4.9 quake could have been a 5.0 and about the same size as big aftershocks on September 8 and October 19, he said.

"To be frank, the error in the measurements can possibly be 0.1 to 0.2 [magnitude]."

Sunday's aftershock cluster had died out as quickly as it began, but more such swarms were possible in the next few months, Gledhill said.

"There have been sequences like this out towards Darfield since September – not felt so much in the city – but the stronger aftershocks occurring nearer to the city also tend to be shallower, so unfortunately it's a bit of a double whammy for you in feeling them."

The Press