This quake was a lot less strong than the one in Chile last year, but still killed a lot of people. From what I understand, Christchurch is built on sandy, silty soil. When a quake hits an area like that, the water table rises and the soil turns to a quicksand-like mess of glob. The technical name is "soil liquifaction." It's what made one of the big quakes in California a few years ago as severe as it was.
My adopted daughter lived through the big quake in Chile, so she's just looking at the strength number and doesn't understand why a smaller quake killed so many people. She doesn't understand the differing soil dynamic, nor does she comprehend that a 6.5 quake is not exactly a "tremor," as she calls it.