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.
Most of the quakes we felt on Sunday were of short duration, but the 4.9 one which hit at 10.37am was as big as any we have felt in our lifetime. It's not an experience that we are keen to repeat any time soon.
We haven't ventured into the central city since Sunday although we may do today. Everything has gone quiet again, but now we are starting to understand the uncertainty and stress that Cantabrians have lived with since September 4th, not knowing nwhen the next quake will come from, how big it will be, and whether it will be the straw that breaks the camel's back for buildings already weakened by 4085 earthquakes since the Big One.