THE EARTHQUAKE THAT hit Christchurch, New Zealand was waiting to happen according to the Australian Seismological Centre.

The director of the centre, Kevin McCue, told The Age newspaper that the 6.3 magnitude earthquake that killed at least 65 people was an aftershock from the 7.1 earthquake that hit there last September.

Why did yesterday’s 6.3 magnitude quake cause more damage than the 7.1 quake last September?

Because yesterday’s quake had its focus only four kilometres from the surface, the damage was much worse than last autumn’s where the focus was 10km underground. The epicentre was also 40km away from Christchurch city centre, while yesterday’s was just 10km from the heart of the city.

McCue said:

I hate to say it, but this is the aftershock they had to have. This is what would normally be the biggest aftershock after a magnitude seven earthquake, although it’s unusual that it’s taken so long after the main shock.

Why did yesterday’s ‘aftershock’ of the September quake take so long to occur?

Last September’s quake was the biggest in New Zealand in nearly 80 years. There were quite a few mild aftershocks, as shown in this list from the US Geological Survey, all of which would have served to further weaken the fault line that didn’t entirely break in September.

Is there worse to come?

In terms of quakes, there may be more aftershocks but none as big as yesterday’s. McCue suggests that it was the “main aftershock, which typically were one magnitude smaller than yesterday’s quake”.

But the structural damage caused by the quake may lead to more problems. There is the obvious damage caused to buildings and infrastructure, and of course, in the death toll. The cathedral in Christchurch was a striking example of how the city has been devastated – you can see the pictures here in our earlier story. Damage caused by the earthquake to terminals at Christchurch Airport means international flight will be closed until at least tomorrow afternoon. Some domestic flights may resume tomorrow morning according to airport spokesperson Monique Oomen.

But there is also the danger, geologist Dick Beetham has been telling local media, of landslides around the Avon and Heathcote rivers. The soft earth there will have become dislodged by the seismic activity and there is a risk that it will slide down to the river basin, damaging more homes.

The timelapse map here shows the quake and aftershocks represented as dots as they hit yesterday – change the results time to ’24 hours’ to see the full intensity of the quake.