There were always going to be aftershocks, but this was almost the worst possible combination of factors, and it happened much later than anyone imagined.
Seismologists had started hoping the worst was over, with aftershocks since September 4 diminishing in size and number.
Then came Tuesday's 6.3 magnitude aftershock. GNS Science's natural hazards manager, Kelvin Berryman, said it would have been worse only if the epicentre had been under the city centre.
Scientists are now grappling to understand how the quake caused the strongest ground shaking measured so far in New Zealand. "This does seem to be very high for a magnitude 6.3."
The highest shaking was recorded at Pages Rd pumping station, at Bexley, north of the city. It measured 1.88 times the force of gravity.
Dr Berryman said seismologists were poring over the data from Tuesday's quake, trying to explain why its force was so devastating. Shallow earthquakes resulted in more powerful ground shaking, but he said there were still measurements to be worked through on the size, location and depth of the quake.
"There's a bit of uncertainty still in the earthquake's epicentre and its depth. It may move by a couple of kilometres."
The shake was "not beyond expectations", as seismologists had warned since September 4 that an aftershock of about magnitude 6 could occur.
However, they had tried to be "cautious" and had been optimistic that such a big earthquake would not occur because of the time that had passed since the original 7.1 earthquake. It was large and late, he said.
Tuesday's earthquake occurred on a previously unknown fault line, and resulted from stresses in the Earth's crust resulting from the Darfield quake, which did not feel so powerful because it happened 40km west of the city and at a depth of 10km.
Seismologists had been watching the aftershocks move along that fault line closer to Christchurch, and they could see now that the result had been another area of stress under the Port Hills.
There had been many big aftershocks since 12.51pm on Tuesday, but this was good as they were relieving underground stress. "There's a lot, and they're quite big for the size of the 6.3."
The aftershocks were not moving towards the city centre, which was also a good sign.
Melbourne University's principal research fellow in earth sciences, Gary Gibson, said all earthquakes in the Christchurch region would be shallow, making the effect worse than those from a similar, but deeper quake.
Christchurch was not on the plate boundary between the Pacific Plate and Australia-India Plate, but near related secondary faults resulting from a bend in the plate boundary.
- The Dominion Post
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