Why so few casualties in Canterbury quake?

There are some striking similarities between the 7.0 magnitude earthquake which devastated Haiti in January and today's 7.1 shock in Canterbury, but also a big difference: 230,000 people died around Port-au-Prince, and in Christchurch only a couple of seriously wounded people known so far.

But the different impacts don't have much to do with the earthquakes themselves, the MSNBC website reported.

"The main difference is that New Zealand has a lot of experience with earthquakes, and they have good construction codes to make sure that their buildings withstand a strong earthquake," said Paul Caruso, a geophysicist at the US Geological Survey's national earthquake information centre in Colorado.

In contrast, Haiti - one of the poorest nations in the world - had virtually non-existent building standards, or at least, few that were systematically enforced.

Though the January quake was the biggest to hit the island of Hispaniola since an 8.1 event in 1946, scientists recently determined that the surprisingly strong Haiti quake occurred along a fault line that was previously undetected.

Similarly, Canterbury's quake was also a complete surprise.

GNS Science said: "At this stage it appears the earthquake has not occured on a known fault."

Both quakes involved strike-slip faults - meaning most of the shaking was side to side.

Both were shallow, usually a bad sign of a damaging quake (13km deep for Haiti, 10km for Canterbury), and each was centred 40km from a big city.

Dr Caruso said seismologists were still trying to reconstruct the exact cause of today's quake.

"We think that this is a very complex event," he said. "We think that the main shock may have consisted actually of three earthquakes."

Another major reason why the casualties were so great in Haiti, and so few in New Zealand, had to do with the time of day. Haiti's big quake event came at 4:53 pm, when a lot of people were moving round the city, but Canterbury's was at 4:36 a.m., when most people were home in bed.

That's not a bad place to be, Dr Caruso said.

"One of the worst things you can do in an earthquake is run outside, because stuff is falling down," he said. "If people stay in their houses, they're a lot safer."