The cluster of aftershocks that alarmed much of Christchurch and caused damage in parts of the city should die out by this morning.
More than two dozen earthquakes shook the city on a memorable Boxing Day.
The 4.9-magnitude quake at 10.30am was the 17th most powerful since the damaging 7.1 quake on September 4.
The ground directly below the city was sparked into action by the shallow magnitude 4.2 aftershock that woke many residents at 2.07am yesterday.
Aftershocks were only minutes apart at one stage late yesterday morning, but became less frequent in the afternoon.
GNS Science natural hazards manager Kelvin Berryman said that aftershock clusters, or swarms, often followed a major earthquake and could have a life of their own.
Because of that, they did not follow the usual rule in which the first quake was larger than subsequent aftershocks.
"Something as big as 4.9 is starting to be big enough to have its own aftershocks," he said.
"Obviously the ground is still a bit stressed up and needs to relax. These aftershocks ebb and flow and have a life of their own. We may still well have magnitude 5 quakes once a month for some time."
Aftershocks could go on for a year, but the latest cluster was likely to last only a day or two, he said.
Their shallowness and location below Christchurch and out towards the Port Hills indicated a link to the magnitude 5.1 quake near Heathcote on September 8, which had also caused alarm and damage.
Scientists have speculated over whether the recently revealed Greendale Fault continues east underneath the city.
GNS duty seismologist John Ristau said the quakes after 10.30am yesterday were all "aftershocks to an aftershock".
All the quakes were still technically aftershocks of the 7.1 magnitude one. Although the frequency and size of the quakes had declined in the past month or so, seismic activity was still higher than would be expected compared with long-term earthquake averages for Canterbury.
Yesterday's quakes had been felt strongly because they were all close to the city and were generated by rocks breaking in the first five kilometres below the surface.
KICK IN THE GUTS FOR CITY RETAILERS
Christchurch's central business district was strewn with police tape and littered with debris instead of happy shoppers scoring Boxing Day bargains.
Fresh cracks appeared in buildings and aftershocks loosened more bricks from already damaged structures.
Ballantynes department store was evacuated shortly after the 10.30am quake.
A crowd of bargain hunters had gathered in the City Mall to wait for the store to open and it was doing a brisk trade when The Press visited about 10.15am.
Managing director Richard Ballantyne said those inside left calmly when the shake hit.
"There was a loss of power and a good shaking, but luckily the customers and staff kept very calm and orderly," Ballantyne said.
An engineer spent four hours inspecting the building yesterday afternoon but it would have to wait for inspections by council engineers before it could be opened. "They are doing an inspection building by building, after which time they will make a decision," Ballantyne said. "We had an engineer go through our building and give it a provisional tick.
"Every day's trading is key and the Boxing Day sales are very important, both for the customer and for the business. It has shaken people's confidence again. It's not an easy time."
Central City Business Association manager Paul Lonsdale said the aftershock "could not have come at a worse time".
Even undamaged buildings were threatened by other unstable structures nearby.
"It's been hard enough getting back on our feet after the first earthquake and we were just starting to get some normality back into customers' psyche and getting a bit of positivity back and this is just a blow.
"It just reopens all the fear that people were just starting to get over."
Kathmandu store manager Andrew Veitch said there had been a steady flow of customers before the quake hit.
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