Scientists believe shaking in the recent earthquake was exacerbated by a trampoline-type effect that made the land under Christchurch bounce up and down.
GNS Science seismologist Bill Fry says the phenomenon is relatively rare, but has been observed five or six times in the city because of the high quality of the data recorded in the quake.
Dr Fry says the trampoline effect is evident when loosely connected layers of rock are subjected to lots of vertical movement.
He says it is similar to what happens when a person bouncing up and down on a trampoline gets out of rhythm, with their joints receiving an impact if they land as the trampoline is moving upwards.
Dr Fry says the trampoline effect would have magnified the vertical shaking caused by the quake.
GNS Science says the ground accelerations in Christchurch, the largest ever recorded for a New Zealand earthquake, were as much as four times higher than the highest accelerations measured in the magnitude 9.0 earthquake off Japan.
