Casting my mind back to Plate tectonics classes 20 years ago, energy release to achieve equilibration is what earthquakes are all about. The audible bang is only one of the several ways in which an earthquake releases energy. The others are heat and movement. If the sound did indeed precede the shake, it'd prove that the energy released travels faster through the air (in the form of sound waves) than through the crust (as motion waves). I forget which is which.
As the North Island sits right atop two plates which are trying to go in opposite directions it is never going to be a stable place to live. I recall my lecturer in 1977 predicting that the North Island would cease to exist within a century - apparently the two plates have been stuck together for way too long to be healthy.
As regards your question - hows about "backslide". (Must admit I didn't have to think too much about that one - thanks for the leg up!!) Will think further on the matter now that the geology lesson has been delivered.
stales
