Quake 'pretty much a bullseye': building expert

A building engineering expert says yesterday's devastating Christchurch quake was "pretty much a bullseye''.

Prof John Wilson, chair of the Australian earthquake loading standard and deputy dean of engineering at Swinburne University of Technology in Melbourne, said the quake struck right at the heart of Christchurch.

"It was so close to Christchurch that we weren't surprised to see significant damage, at that close range the level of shaking is quite severe.''

The magnitude 6.3 earthquake struck at 12.51pm, and strong aftershocks have shaken the city since.

"We expected the older buildings with unreinforced masonry to suffer - their masonry is heavy, brittle and vulnerable to earthquake shaking,'' Prof Wilson said.

"In general the contemporary buildings performed well, although a few contemporary buildings have collapsed, which did surprise us.''

Since the mid-70s New Zealand buildings have been designed for earthquake resistance.

"What's more, the standard of design has still been improving over the last 20 years or so - which is why most buildings performed well, with the exception of a few buildings that were severely damaged or partially collapsed.

"The immediate challenge is to allow the Urban Search and Rescue (USAR) teams to respond and rescue in what is a very hazardous environment with continuing aftershocks,'' Prof Wilson said.

It was possible to make earthquake-proof buildings by using 'base isolation' - the building was built on springs and de-coupled from the ground, Prof Wilson said.

"It adds about five per cent to the building cost but makes it totally secure. The design has been around for about 30 years, though really became more common about 15 years ago. It's widely used in California and Japan, which see so many earthquakes, but is also used in Wellington for buildings like Te Papa.''

University of Calgary professor of geology Dr Melissa Giovanni said the reason the damage was so much greater this time was because buildings were already weakened by September's magnitude 7.1 quake, so yesterday's quake was much more devastating.

"Another reason this earthquake was more damaging is that it was closer to the surface, and so the movement would be more intense than in a deeper quake where the earth can dissipate some of the energy.

"The fault that broke, we call an oblique thrust, which means there was some horizontal movement and some vertical movement. So the initial movement probably felt like a lift as the earth moved vertically, and then later you would feel vertical and horizontal movement,'' Dr Giovanni said.

At least 75 people died in yesterday's quake, with the toll expected to rise.