More big aftershocks likely, scientists warn

Civil Defence says the 5-storey Strategy Building has to be demolished. Photo / Simon Baker

The Christchurch earthquake claimed another building yesterday, and scientists reminded residents that the aftershock sequence was still in its early stages.

The 5.3 magnitude aftershock on Saturday night occurred at a shallow depth near Diamond Harbour in Lyttelton, and was one of the largest since the February 22 earthquake.

It was felt as far away as Timaru, cut power and water, and caused minor damage to buildings.

Six more shocks followed Saturday's tremor, registering between 2.9 and 4.1 on the Richter scale.

GNS geological hazard modeller Matt Gerstenberger said magnitude 5 quakes were not abnormal two months after a magnitude 6.3 quake.

Even a year after the February quake, four magnitude 5 aftershocks could be expected.

The shocks are gradually becoming smaller and further apart, but would not stop completely until around February 2013.

Larger aftershocks were also possible - the probability of an earthquake of magnitude 6 or greater within the next year was roughly 6 per cent.

Seismologist Steve Sherburn said the aftershock sequence, or "cloud", has virtually joined up with the pattern of aftershocks from the Darfield quake in September.

"We are locating events under Darfield, under Lyttelton, and all the way out to the coast. It is still a very active area."

Christchurch mayor Bob Parker said the most damage from the latest shock "was to our minds".

Residents reported on Twitter that their houses lurched, chimneys toppled and items had fallen from shelves.

The Strategy building on Victoria St, already damaged by the February tremor, was to be demolished after the aftershock made it crumble further.

Civil Defence controller John Hamilton said the building had begun to lean, and it was not safe to keep it standing.

Roads had been closed around the site so the five-storey structure - which housed a pharmacy, post shop, design company and offices - could be gradually pulled apart.

The Christchurch City Council reported widespread but minor damage as a result of the shock.

Twelve water mains burst, rocks fell in Sumner, and power supplies for 20,000 people were off for for two hours.

Supply company Orion said most of the cuts were caused by automatic switches responding to the tremor.

Further liquefaction and flooding happened in the eastern suburbs of the city, where about 250,000 tonnes of silt have been scraped from roads and gardens after February's quake.

North Queensland also experienced a rare quake on Saturday. The magnitude 5.2 tremor, unrelated to the New Zealand and Japanese events, was the state's biggest in 70 years.

There were no reports of damage or injuries.