Because the fault line in question, the Alpine Fault, runs diagonally across the island from the northeast to the southwest, widespread death and destruction is likely when the quake hits. The Alpine Fault is the largest active fault in New Zealand, running from Blenheim to Milford Sound, a distance of about 480 kilometres. It is essentially the boundary where the Pacific Plate slides past the Australian Plate at a rate of about 4 centimetres a year. Its vertical movement has been enough to build major mountains of the Southern Alps, stretching up to 3750 metres at Mount Cook. The pressure gradually builds up until the fault ruptures the rocks in a major earthquake, much as happens in California.
Seismologists can unravel the past using a combination of four techniques.
Excavations of ruptured areas allows dating of sheared rocks. Organic material buried by earthquakes can also be dated. A major event can clear large areas of forest, so the timing can be established by estimating the ages of trees. Finally, some trees will survive but suffer damage, so the growth rings of very old trees can indicate major disturbances. Together these techniques show that there were major ruptures along the Alpine Fault in 1717, about 1620 and about 1450.