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The peak ground acceleration (PGA) in central Christchurch exceeded 1.8g (i.e. 1.8 times the acceleration of gravity),[31] with the highest recording 2.2g, at Heathcote Valley Primary School,[3] a shaking intensity equivalent to MM X+.
The acceleration occurred mainly in a vertical direction,[30] with eyewitness accounts of people being tossed into the air.[33] The upwards (positive acceleration) was greater than the downwards, which had a maximum recording of 0.9g; the maximum recorded horizontal acceleration was 1.7g[34] The force of the quake was "statistically unlikely" to occur more than once in 1000 years, according to one seismic engineer, with a PGA greater than many modern buildings were designed to withstand.
Mmm, even as I write, another aftershock rolls through the house...