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Aftershocks rattling Canterbury and unnerving its inhabitants are likely to last for "weeks" rather than days but will get less frequent and probably weaker, says a Wellington seismologist.
"As time goes on, the likelihood of very large ones is reducing. But we really can't be sure about this, and I'm just aware that people in Canterbury are understandably getting very edgy and irritated and anxious and scared...but actually, a week isn't that long," Dr Warwick Smith, of GNS Science, said.
He said the number of aftershocks - an estimated 395 by midday today, including 10 above magnitude 5 - was expected after a magnitude 7.1 quake, and would not be stopping any time soon.
"We're looking at weeks, I'm afraid."
He said there was still a possibility of an aftershock larger than those of up to 5.5 experienced so far "but the chances of this happening are decreasing by the day."
GNS duty seismologist Brian Ferris said there had been 200 felt aftershocks reported by midday today, including 10 between 5 and 5.5 and 70 between 4 and 4.9. The largest has been 5.5.
Scientists had installed extra instruments in the region to measure the aftershocks and try to learn more about the fault they never knew was there, Dr Smith said.
"Remember, this was a fault that we didn't know existed. It's down beneath a few hundred metres, probably, of gravels and the Canterbury plains are flat because of all the alluvial material that's come down the rivers and spread out... and the geology that's causing this is deep beneath that. We've not had a look at it, we can't, but what we have to do is try and learn more about the process.
"From these little aftershocks that are occurring, we can learn a bit more about what's happening down below."
Seismologists are able to estimate aftershock patterns from previous earthquakes, but it was "very variable", Dr Smith said. Over time, estimates would become more accurate of how long the aftershocks would continue.
Aftershocks were a consequence of the stresses developed in the rock surrounding the main earthquake rupture. As they occurred, the stress expanded from either end of the main rupture, creating an expanding area of aftershocks that occurred at reduced frequency.
In this way, the aftershock zone had expanded progressively further east and west of the visible fault rupture and was now over 60km-long.
Dr Smith said the Canterbury earthquake and its aftershocks had been extremely well recorded by the permanent network of GeoNet seismic instruments operated by GNS Science and funded by the Earthquake Commission. But scientists from Victoria University of Wellington, GNS Science, and Stanford University in the United States had installed portable instruments to increase the amount and precision of data recorded.
- NZPA
Christchurch City Council service update:
* The CBD cordon remains in place. Police, engineers and Civil Defence staff are assessing the area. The Cordon will be lifted progressively and updates will be available during the day.
* The Central City and Akaroa libraries have opened today, with normal hours today and over the weekend. All other libraries are closed until further assessments have been completed.
*All Council recreation and sport centres and swimming pools remain closed until further notice.
* The Bus Exchange is not operating today, but buses are running. Visit www.metroinfo.co.nz for more information and times.
* The tram is fully operational.
* It is still expected that city water supply will be completely restored by the end the weekend. An impressive 280 pipes have been fixed in less than six days - the equivalent of a year's worth of repairs.
* For updated information on road closures visit http://www.ccc.govt.nz/thecouncil/newsmedia/mediareleases/2010/201009089
* All Banks Peninsula cemeteries are closed, as are the Bromley and Linwood cemeteries, as headstones and monuments are being assessed for safety. Some burials are continuing, but people are advised to check with their funeral directors.
* It is now safe to drink tap water in Christchurch City and Banks Peninsula and the boil-water notice has been lifted.
