The entire city centre is severely damaged, with entire buildings collapsed, the Cathedral in ruins, cars crushed and many people still trapped under concrete blocks and bricks. This extreme damage is caused by the shallow depth of this earthquake: at a depth of only 4 kilometres and ten kilometres southeast of the city.

(From Earthquake Report) Reasons why the damage and casualties are higher in this earthquake as opposed to the Sep 4 earthquake:

1) Ground shaking higher (shallower hypocenter) and closer to higher population.

2) Midday earthquake where many workers and shoppers were in the city center with many verandahs and other masonry failed and hit large numbers of people. Many people were getting lunch at the time.

3) Office buildings were most affected, and being the middle of the day many workers were in these buildings.

4) Structures previously weakened by the mainshock (M7.1), were now affected greatly by this aftershock (M6.3).

For updates about the Christchurch earthquake go to Earthquake Report and USGS.

See also New Zealand blogger Grant Jacobs at Sciblogs for regular updates.