Greendale Fault still under stress
Aftershock activity moving back to the west of Christchurch is a sign stress still needs to be released from near the Greendale Fault, scientists say.
Since Saturday there have been half a dozen earthquakes centred to the east of Rolleston and Weedons, three of them of about magnitude 4.0.
Their focus close to the eastern end of the Greendale Fault is at odds with where most aftershocks have been in the past month, just off New Brighton.
American seismologist Kevin Furlong, a professor at Pennsylvania State University, said the area between the end of the Greendale Fault and "The Gap" closer to Prebbleton and Lincoln had been "popping off" since the September 4, 2010, quake.
"This fault zone sure seems to be an Energizer Bunny. Based on its behaviour in the past, it does seem to die out a bit and then come back again," he said.
"It is the one area that did seem to rejuvenate a bit after the June events.
"That is one of the intriguing aspects of these events. In general, once a fault has a good-sized event on it, and the associated aftershocks, they seem to be done.
"There hasn't really been any reactivation of the main Greendale rupture since over a year ago [and] similarly the February rupture on the Port Hills Fault had its aftershocks but has been pretty quiet since then.
"And neither the June event nor the recent December-January events has turned it or the main Greendale Fault back on.
"But the area west of the February rupture – typically a bit east of Rolleston – does seem to rejuvenate a bit each time another segment goes.
"How these will play out is unclear to me, but ... it is reasonable to say that it is an area of the crust that still has not released its stress to the same degree that the main Greendale Fault has, so it still is unstable.
"You guys can't buy a break."
Canterbury University head of geological sciences Jarg Pettinga said the latest quakes were where "significant stress concentrations" occurred after September 2010.
"There is clearly some complex subsurface stress accommodation occurring around this tip zone of the Greendale Fault rupture, and the ongoing activity does come and go from time to time,'' he said.
"That's been the pattern over the last few months."
He said it was difficult to say whether the offshore quake activity in recent weeks might have triggered the Rolleston aftershocks.
"Certainly, we have been seeing some transferred activity back and forth post-larger aftershocks from east to west of the city, but this has generally been with respect to adjacent faults in closer proximity to each other.