Odds of another major earthquake lessening

Posted 19 Aug 2011 by MediaStuff Popular
Posted in Earthquake Facts , Media
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The chance of a major earthquake in Canterbury in the next year has dropped.

GNS Science quake-probability figures released yesterday show the chance of a magnitude-6 to 6.4 quake over the next year is 10 per cent, down from 15 per cent in July's estimates.

"Each day the probabilities decrease by some amount and that will continue to happen as long as no larger events occur," GNS seismologist and hazard modeller Dr Matt Gerstenberger said.

June 13's magnitude-6.3 quake would have slightly increased the probability of a major quake, but that had since dropped.

However, the chance of a magnitude-5 to 5.4 aftershock in the next year was still at 82 per cent.

"It certainly would not be a surprise if there was another [magnitude] 5 to 5.4," Gerstenberger said.

A shake of that magnitude could cause reasonably strong ground shaking, depending where the earthquake was centred, its depth and other factors.

GNS's revised figures showed: An 82 per cent probability of a magnitude-5 to 5.4 aftershock – down from 85 per cent a month ago.

39 per cent of magnitude 5.5 to 5.9 quake – down from 45 per cent.

10 per cent probability of a magnitude-6 to 6.4 quake – down from 15 per cent.

A 5 per cent probability of a magnitude-6.5 to 6.9 quake – down from 7 per cent.

The chance of a magnitude-7 or higher quake stayed at 2 per cent. GNS said there had been a 0.5 per cent decrease in probability, but the figure had to be rounded up.

The area affected is from Hororata to large parts of Banks Peninsula, and from Kaiapoi to Lincoln.

Up to yesterday afternoon, the last quake over magnitude 5 was an early-morning magnitude-5.1 quake on July 22, 12km deep, between Dunsandel and Charing Cross.

According to the GeoNet website, it was the 29th quake of magnitude 5 or higher to hit the region in the previous 10 months, including the September 4 magnitude-7.1 main shock.

GNS's figures are based on research in earthquake-prone areas around the world and specific observations in Canterbury since September.

While the probability of a major quake was dropping, GNS said a major aftershock, about magnitude 5.8 or higher, could spark a resurgence of earthquakes.

Statistically, a magnitude-6.9 jolt is roughly 100 times less likely than a magnitude-5 quake.

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