The South Island, New Zealand sits astride the subduction zone where the Pacific Tectonic Plate, moving westward plunges beneath the Australia Plate that is moving eastward. The relative movement is about 45mm per annum.
At the surface this interface is a major fault line trending SW-NE, called the Trans Alpine Fault. In this photo, the surface of the Australia Plate is the low-lying coastal land in the foreground, whilst the Pacific Plate carries the uplifted Southern Alps. The fault scarp seen here is quite complex in that there are a series of linear faults which appear as linear etches along it face. Glaciers have flowed down valleys cutting the fault scarp into blocks.
The pressure generated by the movement of the plates against each other builds up until an earth movement releases that pressure. This release produces the shock waves of an earthquake.
While there have been many minor movements over the years of lived history, the last major quake was in 1717. It is anticipated that there will be another major, catastrophic quake oh this fault, in the near future. It has been surmised that there is around a 50% probability of such a quake in the next 20 years, the probability increasing at the rate of at least 1% pa.