Seismologist had hoped time for large shock had passed

Rescue workers search for survivors through debris in Christchurch. Photo / Getty Images
Rescue workers search for survivors through debris in Christchurch. Photo / Getty Images

A seismologist says today's devastating Christchurch earthquake was the type of common large aftershock which he'd hoped the city had avoided.

John Ristau of the Institute of Geological and Nuclear Sciences (GNS) said the earthquake which struck at 12.51pm which measured 6.3 on the Richter scale was still classified as a large aftershock in seismological terms to the 7.1 earthquake which rocked Canterbury on September 4.

However, today's quake felt stronger to many in Christchurch than the September quake as it was shallower (5km compared to about 10km in September) and its epicentre was closer to the city (10km southeast of the city, compared to 30km west of the city around Darfield in September).

"There have been large magnitude aftershocks already very close to Christchurch, such as the Boxing Day one, and there have been others that were larger than magnitude five that were very close to Christchurch.

"Right from the start we've said that the general rule of thumb for major earthquakes like the Darfield earthquake is that the largest aftershock is about a magnitude unit smaller than the main shock," Mr Ristau said.

"Up until this point we hadn't seen that magnitude six aftershock, and we kind of hoped that by now it wasn't going to happen anymore.

"It's a very big aftershock, but it's still an aftershock."

Mr Ristau described today's shock as "very shallow" and said the fact it was closer and shallower would explain why the damage in some parts of Christchurch was greater than in September.

"It's closer, plus if you had buildings that were already weakened by the 7.1, then one like this would come along and finish the job."

Several more large aftershocks have been recorded, among them a 5.7 magnitude quake eight minutes after the 12.51pm quake, and others of up to magnitude 5.5, and he said more were likely to be on their way.

Mr Ristau said the quake had been felt as far north as Tauranga, while there were reports of swinging lights in Pukekohe, south of Auckland.

- NZPA

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