New Zealand sits right on the cusp of a plate margin between the Australian and Pacific plates. Both plates are moving in similar directions but at different speeds, and there there is a convergent relative velocity of 23mm/year between the plates, due to convection currents circulating towards each other.
Below: Diagram showing relative plate velocities, with proportional arrows
It is estimated that the earthquake is a 7 on the Richter scale, but is clearly far less destructive (I on Mercalli scale "Generally not felt by people unless in favorable conditions". Typical fast release M7 earthquakes would rank roughly X, described as "Many well-built structures destroyed, collapsed, or moderately to severely damaged"). The reason for the unusual devastation for an earthquake of such energy is that rather than the plates slipping over a period of days, in New Zealand the earthquake has been ongoing since January 2013 and "will most likely continue for several months".
The focus of the earthquake is only 40km below sea-level and acts in the vicinity of Wellington, a medium-sized city of 395,600 inhabitants.
Below: Cross section diagram of subduction
Tectonic Plate velocity data:
Earthquake information, Subduction diagram:
New Zealand fish fossil:
Thomas Ruddell, 19/07/2013