Seismologists at GNS Science regard this earthquake, large though it is, as an aftershock relating to the magnitude 7.1 Darfield Earthquake that struck Canterbury on 4 September 4, 2010. The technical reason for this is that the epicentre is adjacent to the existing aftershock zone.
It has ruptured over a length of about 17km on a near vertical plane slightly inclined to the south and between 3 and 12km in depth. It is more or less parallel to the E-W trending Greendale Fault that ruptured in the Darfield Earthquake. It may be thought of as an eastern extension but it is clearly dislocated from the trend of the Greendale fault and stepped to the south.
No, this is not an aftershock. This is a new earthquake marching down the fault. But who am I? They are also tossing the old canard that this is a 'safety valve' releasing steam. Ha! As well they claim this whole area hasn't had an earthquake for 16,000 years. blah.