Weekend's aftershock cluster 'forecasted'

Posted 10 Oct 2011 by MediaStuff Popular
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The magnitude-5.5 aftershock that struck Christchurch last night, just as the fourth Rugby World Cup quarterfinal kicked off, was normal, a GNS seismologist says.

GNS Science duty seismologist Caroline Ashenden said the weekend's cluster of 13 shakes, including a magnitude-4.8 tremor on Saturday afternoon, were "within the realm of forecast aftershocks".

GNS scientists expected Christchurch to be struck by one or two magnitude-5 quakes and about seven magnitude-4 shakes between September 13 and October 12.

Following predicted patterns, the city had felt eight magnitude 4s within the month and then last night's magnitude-5.5 shake.

Ashenden said the 13 aftershocks in the city over the weekend could trigger up to a dozen more this week.

In a usual aftershock sequence, 10 magnitude 4s and one magnitude 5 were expected for every 100 magnitude-3 quakes that hit.

"It's not out of the ordinary, and the quakes are following the same pattern as expected," she said.

"As we progress through the afteshock sequence, the frequency of earthquakes are decreasing, but we are still going to get the odd larger, significant earthquake."


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