When it comes to earthquake damages, magnitude is not everything.
The February 22, 2011, 6.3-magnitude earthquake that struck the South Island of New Zealand was less powerful than the September 3, 2010, 7.1 quake that struck to the west of Christchurch. Black circles on the diagram below from NASA (the source of this information) represent earthquakes in the region between these dates. Red circles show the locations of the magnitude 6.3 quake and aftershocks on February 22 and the morning of February 23. Larger circles represent stronger earthquakes. Yellow shows urban areas, including Christchurch.
In fact, the USGS characterized the February 22 event as an aftershock of the quake on September 3.
The Darfield earthquake in September 2010 caused no casualties, even though it had a higher magnitude. Besides striking closer to a major population center, the 6.3-magnitude Christchurch earthquake had a depth of just 5 kilometers (3 miles). The New Zealand Herald reported that, whereas the Darfield quake happened in the early morning hours, the February 22 quake struck at the “worst possible time” of day—at the lunch hour when city streets were crowded with shoppers, diners, office workers, and school children. Moreover, some of the buildings that collapsed may have been weakened by the September 2010 quake.