NZ website stuff.co.nz seems to have lost the link to this 14 Sep 2010 article quoting GNS experts during a lull in the Darfield earthquake aftershocks.
A couple of 5.0 shocks could be on way
BY PAUL GORMAN
Last updated 05:00 14/09/2010
Canterbury could get at least two more aftershocks of magnitude 5.0 or higher, but the risk of a bigger earthquake is fading by the hour, GNS Science says.

The figures are based on hazard modelling that also calculates there could be another 20 aftershocks of magnitude 4.0 or bigger by this time next week.

Geological hazard modellers Matt Gerstenberger and Annemarie Christophersen have produced the figures based on the normal pattern of aftershocks expected in New Zealand after a quake of magnitude 7.1.

Gerstenberger said the aftershock figures had been updated daily, and it had been a "textbook" sequence.

"When taken as a group, aftershock behaviour tends to follow predictable patterns, and we can have a good idea of how many aftershocks of a particular magnitude range we can expect and when they may occur," he said.

"However, within that group, individual aftershocks can be random and unpredictable, which makes it impossible to know when or where any single aftershock will occur."

Only half of the anticipated number of aftershocks of more than 5.0 in magnitude had occurred, with 10 recorded up to the last one on Wednesday morning.

"There is still a chance that an aftershock occurs that is larger than the ones so far observed. This might have its own aftershocks and could change the model," he said.

GNS spokesman John Callan said there was now only a remote chance of a magnitude 6.0 aftershock.

The calculations suggested there could be up to six aftershocks between magnitude 4 and 4.9 today and again tomorrow, and up to five each day from Thursday until Sunday.

Aftershocks, ranging in magnitude from 3.2 to 4.2, continued to rattle Canterbury yesterday.

KiwiRail is keeping a watch on the Midland Line, where the new Greendale-Rolleston fault scarp crosses it in Railway Rd.

Spokeswoman Ruth Larsen said the 7.1 quake on September 4 put a kink in the railway tracks, which were fixed, before a sharp aftershock on Monday night last week moved the tracks again "in exactly the same spot", needing a second repair.

Aftershocks since then had not damaged the line, although frequent inspections were being made and speed restrictions had been in place.

The Midland Line has about 16 train movements a day, including the TranzAlpine and coal trains to Westport.

From GNS website 23 Feb 11
http://www.geonet.org.nz/news/feb-2011-christchurch-badly-damaged-by-magnitude-6-3-earthquake.html
The city had been comparatively lucky with both the location and timing of last September's magnitude 7.1 Darfield (Canterbury) earthquake; the location of this one within 10 km of the city and at a shallow depth of 5 km during the middle of a working day has resulted in destruction, injuries and deaths.

Shaking intensity in the city was much greater for this earthquake than the magnitude 7.1 earthquake for any of its other aftershocks. The highest shaking was recorded at Pages Road Pumping Station at 188 %g, with readings of 127 %g at Heathcote Valley Primary School and 107 %g at Hulverstone Road Pumping Station. This is due to the proximity of the epicentre to the city and the shallow depth.

Seismologically, this is classed as an aftershock because of its relationship to the ongoing activity since September last year. Its occurrence was always statistically possible, but the long time interval and slow decrease in general activity had made it less likely. Unfortunately, it has happened after all and in a location that has brought the worst result.