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Aftershock Sequence Unusual, Seismologists Say




Last night's magnitude 6.1 aftershock in Fiordland came as a mild surprise, seismologists said today.

" After the August 22 magnitude 7.1 earthquake in Fiordland, we anticipated getting an aftershock of about magnitude 6," said seismologist Warwick Smith of the Institute of Geological and Nuclear Sciences Limited (GNS).

" A magnitude 6.2 aftershock occurred within two hours of the main shock. But it's unusual to get two aftershocks of roughly this size in an aftershock sequence," Dr Smith said.

No two aftershock sequences were exactly the same, but they generally followed a commonly observed pattern.

" The rule of thumb is that the biggest aftershock is generally about one unit magnitude smaller than the main shock. It's rare to see two aftershocks of this size in a sequence.

" It's not possible to attach any significance to this aftershock sequence other than it's slightly unusual."

Aftershocks were expected to continue in Fiordland for some weeks, or even months. But they would gradually become less frequent.

The biggest aftershock in the Fiordland sequence is still the magnitude 6.2 jolt that occurred two hours after the main shock. Since August 22, the have been 19 aftershocks of magnitude 5 or greater.

Within two days of the main shock on August 22, GNS seismologists had deployed eight portable seismometers near the epicentre at Secretary Island, at the entrance to Doubtful Sound, to record aftershocks.

Close study of the aftershocks would reveal valuable information about the size and orientation of the "fault break" in the crust, and indicate if the main shock and aftershocks had affected any nearby faults. ENDS

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