Seismologists Revise Canterbury Quake Details Slightly

The size of the earthquake that hit Canterbury early today has been revised to magnitude 7.1 with a focal depth of 10km to 12km, GNS Science said.

The epicentre remains at about 30km west of Christchurch, or 3km to 5km southeast of Darfield. Seismologists have revised the quake's details after additional earthquake recordings were included in their calculations.

Duty seismologist at GNS Science, John Ristau, said aftershocks were likely to continue for days, even weeks. Typically the largest aftershocks occurred within the first 48 hours of a large earthquake, Dr Ristau said. They generally declined in frequency and size over time.

"A rule of thumb for a large earthquake at a shallow depth such as this is that the largest aftershock will be about one unit of magnituide lower than the main shock," Dr Ristau said.

Damage from today's earthquake, which struck at 4.35am today, is considerable particularly in Christchurch. Seismologists are describing it as New Zealand's most significant earthquake, in terms of damage, since the 1931 magnitude 7.8 Hawke's Bay earthquake.

Seismologists say a foreshock of about magnitude 5.4 occurred a few seconds before the main shock. Both shocks occurred in slightly different locations. Seismic energy from the two shocks became entangled making it difficult to pinpoint the size, location, and depth of the main shock.

There are several known active faults under the Canterbury Plains and in the Canterbury foothills, but at this stage it appears the earthquake has not occurred on a known fault.

Geologists and seismologists from GNS Science are heading to Canterbury today to study the physical imapct of the earthquake. They will also deploy portable seismometers to record aftershocks. The scientific response is being co-ordinated under the GeoNet Project within GNS Science

ENDS

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