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Four thousand aftershocks in six months

By Paul Harper

Canterbury has been rattled by about 4000 aftershocks in the past six months. Photo / Simon Baker
Canterbury has been rattled by about 4000 aftershocks in the past six months. Photo / Simon Baker

Around 4,000 aftershocks have rattled the Canterbury region since February 22, according to GNS Science records.

The latest was a magnitude 4.2 aftershock this morning, six months to the day a magnitude 6.3 aftershock struck Christchurch, killing 181 people.

This morning's quake struck at 8.38am, and was centred 10km southeast of Lyttelton, in Port Levy on Banks Peninsula, at a depth of 10km.

GNS Science said the quake was "widely felt in Christchurch", with reports it was felt strongly in Bexley, Burwood and Redcliffs.

Since the devastating quake at 12.51pm on February 22, there have been at least 3,985 aftershocks of any size in the Canterbury region.

The number is likely to be even higher, with many of the smaller ones not recorded by GNS Science.

Duty seismologist Caroline Holden said at least 1,023 of those were over magnitude 3.

"They are so shallow and so close, they would be felt every single time, at least by one person, unless they are in the middle of the Canterbury Plains," she said. "It's pretty crazy."

After a quiet week in Christchurch last week, when snow was more of a disruption than tremors, Christchurch has had three magnitude 4 quakes in as many days, with a magnitude 4 and a 4.2 hitting on Saturday.

"If you look at an average fortnight I think it is pretty standard," Holden said. "There was a bit of a quiet time last week ... but it just caught up in the weekend."

Since the initial magnitude 7.1 earthquake on September 4, there have been more than 8,300 earthquakes in the Canterbury region, with 28 over magnitude 5 and two over magnitude 6.

After another magnitude 6.3 earthquake on June 13 the likelihood of another big one increased, Holden said, but the odds have since decreased again.

According to GNS Science, there is a 26 per cent chance of up to two quakes magnitude 5 to 5.4 between August 15 and September 4. Within that same period there is a 6 per cent chance of a magnitude 5.5 to 5.9, a 3 per cent chance of a quake 6 to 6.4, and less than a per cent of a chance of a magnitude 6.5 and higher.

In the next year, there is an 82 per cent chance of a up to five quakes measuring between 5 and 5.4, a 39 per cent chance of up to two quakes 5.5 to 5.9, a 10 per cent chance of a quake between 6 and 6.4, a 5 per cent chance of a magnitude 6.5 to 6.9 and a 2 per cent chance of a jolt measuring 7 to 7.9.

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