Rather than a magnitude 6 aftershock centred several tens of kilometres west of the city on the recently revealed Greendale Fault, the fault rupture was almost under the city, about 5km below the Heathcote to Lyttelton road tunnel in hard volcanic rock that snapped violently at 12.51pm that day. The angle of the previously unknown fault under the Port Hills directed the energy released by the earthquake straight back at the city, with severe shaking in eastern parts of Christchurch.
Richard Cosgrove/Stuff
Eighteen people died in the PGC building on Cambridge Tce after the February 2011 earthquake struck Christchurch. Those ground motions had been thought to be among the most extreme in the world to date.
CARYS MONTEATH/Stuff
Destroyed shops on Manchester St in the Christchurch CBD after the February 2011 earthquake. Study co-author Yoshihiro Kaneko said the earthquake was a rare event that produced very strong ground motions, and some unusual site effects. At the sites examined in the study, the ground motion recorders were working accurately, but the concrete slab foundations in the buildings they were kept in partly loosened from the underlying soil, exaggerating the shaking. A station at Heathcote Valley Primary School, near the epicentre of the quake, recorded a vertical peak ground acceleration of 2.2g, or just over twice the force of gravity.