Small Reduction in Quake Probabilities

MEDIA RELEASE from GNS Science

3 FEBRUARY 2012

Updated Modelling Shows Small Reduction in Quake Probabilities

Updated computer modelling shows a small decrease in the probability of aftershocks in Canterbury for the next 12 months compared to
figures released late last year.

The new figures, issued today, are derived from a revised computer forecast modelling technique updated from procedures used during 2011.

The revised model incorporates work by a panel of international experts who met at GNS Science in Lower Hutt in October to review the
Canterbury earthquake sequence and consider the best models to use to forecast future earthquake activity.

The scientists, from the United States, Australia and Ireland along with New Zealand colleagues, considered more than a dozen modelling
techniques for their robustness and appropriateness to the Canterbury sequence.

The end result is a composite technique that combines a number of different modelling methods to calculate earthquake probabilities.

“The revised modelling procedure, which incorporates earthquake activity in late December and January, involves a robust process that
includes additional understanding of the Canterbury earthquake sequence,” said Dr Kelvin Berryman, Manager of the Natural Hazards Research
Platform at GNS Science.

“Nothing has changed inside the earth to increase or decrease the risk of an earthquake. It’s just that we have used additional data and
incorporated feedback from international specialists to make adjustments in the way the probabilities are calculated,” Dr Berryman said.

“Earthquakes will continue to be a feature of the Canterbury region over the coming decades, and a combination of measures including land
zoning mapping and strengthened building regulations that incorporate forecast earthquake activity will provide for a more assured future.”

The updated figures based on the computer model show for the 12 months to 15 January 2013 there is:
An 82% probability of a magnitude 5.0 to 5.4 aftershock – down from 92% using the old model over the same period
A 39% probability of a magnitude 5.5 to 5.9 aftershock – down from 51% using the old model over the same period
A 13% probability of a magnitude 6.0 to 6.4 aftershock – down from 17% using the old model over the same period
A 4% probability of a magnitude 6.5 to 6.9 aftershock – unchanged from the old model
A 1% probability, or one in a hundred chance, of a magnitude 7.0 or higher – unchanged from the old model.

An important aspect of this modelling is that these figures are for the entire aftershock zone, and not just for Christchurch city. The zone extends
from Hororata in the west to the offshore region east of Christchurch, and from Kaiapoi in the north to Lincoln in the south.

ENDS

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