Stresses along New Zealand’s plate boundary break the crust into separate fragments or blocks that move relative to each other. The boundaries of these crust fragments are known as faults. Minor ruptures occur along these faults all the time, but when a great deal of stress builds up and is released all at once, the result is a significant earthquake.

The force of the collision between the Pacific and Australian Plates has broken the plate boundary into many fragments, which are able to move independently. Like the plates, these fragments also move against each other along their own fault lines, causing friction and stress to build up along their boundaries.

Apart from very deep movements (deeper than 600 km), all New Zealand earthquakes occur on fault lines. Even so, for most earthquakes, the fault does not break the surface. It can only be detected by analysing the energy released by the earthquake as it reaches the Earth’s surface in the form of waves. These seismic waves are released whenever stress has built up and movement occurs along a fault boundary.

Faults can be anywhere from a few metres to a thousand kilometres long. One of New Zealand’s most well known and widely studied faults is the Alpine Fault. It is over 600 km long and is responsible for some of the largest earthquakes in New Zealand’s history.

The Greendale Fault is one of the faults responsible for the Darfield earthquake in September 2010. It was hidden under the river gravels of the Canterbury Plains until it was reactivated after probably several thousand years without any movement. The fault line can be clearly seen from the air in this GNS Science video, taken the day after the Darfield earthquake in 2010.

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