Big earthquake risk put at 23 per cent

PAUL GORMAN
Last updated 05:00 31/05/2011

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Canterbury has a nearly one-in-four chance of a third major earthquake of up to magnitude-7.0 during the next year, officials have been warned.

The forecast from Government research agency GNS Science for those charged with helping rebuild the damaged region will come as news to the public, who are still waiting to find out what scientists have discovered below Christchurch and the Canterbury Plains.

Christchurch City councillors and community board members were told of the risk of another large quake at a meeting of the Canterbury Earthquake Recovery Authority (Cera), chaired by Earthquake Recovery Minister Gerry Brownlee, last Wednesday night.

Incoming Cera chief executive Roger Sutton said his advice from GNS Science was for:

A 23 per cent chance of a magnitude-6.0 to 7.0 quake in the next year, dropping to a 10 per cent risk the following year.

More than 90 per cent chance of a magnitude-5.0 to 6.0 shake in the next 12 months, falling to more than a 70 per cent chance in the following year.

Those figures might not have seen the light of day for at least another couple of weeks if Cr Sue Wells had not included them in her blog at the weekend.

"It's now thought that we are looking at a longer-term seismic series," she said in her blog.

"Nobody was able to answer questions about Mercalli scale or G-force [gravitational acceleration] or depth, so you can take those figures for what they're worth. What we are seemingly certain of is that you shouldn't put your Blu-Tack away just yet."

Wells told The Press yesterday it was made clear at the meeting they could share the forecast figures with the community.

At the start of February, GNS Science said there was a 25 per cent chance of an aftershock of magnitude 6.0 or more affecting Christchurch or Canterbury during the next year.

Only three weeks later, the magnitude-6.3 quake at 12.51pm on February 22 killed 182 people and caused massive damage.

Nobody died in the September 4 magnitude-7.1 quake, which ripped open a 30-kilometre gash across the Canterbury Plains between Hororata and Rolleston.

One focus for researchers over the past month has been the aftershock-rich "gap" between the eastern end of the Greendale Fault and the western end of the Port Hills Fault.

Scientists want to know what is going on between the two, with some concerned that any faults in the gap may rupture and link the two bigger faults.

GNS Science spokesman John Callan confirmed the figures given to Cera and councillors were calculated by hazards modeller Dr Matthew Gerstenberger at the Crown research institute.

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Callan said it was important to remember the figures related not just to Christchurch but the whole Canterbury aftershock zone, from Sheffield in the west to Rangiora in the north, Lincoln in the south and across Banks Peninsula.

Gerstenberger said the longer-period aftershock forecasts given to Cera were based on earlier projections and used the same computer model that calculated possible quakes over the next few weeks.

The GeoNet website is predicting between one and 10 quakes of magnitude 4.0 to 4.9, and zero to two of magnitude 5.0 and above, in the period from May 19 and June 18.

- The Press

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