Magnitude is used to measure earthquakes. When you have low magnitude earthquakes, it is likely there will be minimal or no damage in buildings (even taking to account other factors such as "site subsoil class", being "near [a] fault", "importance of a structure" ).
As the magnitude goes up , then people will be taking account other factors.
There is not much we can do about the other factors like subsoil class when the tall buildings (like the ones in Christchurch CBD) are already there. Like I said before, engineers should make sure buildings can withstand at least an earthquake with the magnitude of previous quakes.
If you look this article : http://www.bbc.co.uk/news/science-environment-12540504
It says that:"
In fact, a magnitude 6 earthquake possesses 32 times more energy than a magnitude 5 quake, as seismologists use a logarithmic scale to record these natural disasters.
a gap of two steps, from 5 to 7, represents an earthquake nearly 1,000 times stronger."
Japan had a 9.0 and NZ had a 7.1. Their buildings held up better. Is it because of other factors? Like I said, it could be. It is my opinion that it is probably the code.