The Alpine Fault

In the South Island of New Zealand, the boundary between the Australian and Pacific tectonic plates can be seen on land. This is unlike the North Island boundary, where a subduction zone is under water off the east coast.

Spot the fault

You can pick out the line of the Alpine Fault on this satellite image of the South Island. Snow on the mountains of the Pacific plate contrasts with the lower land of the Australian plate.

Alpine Fault movement

The Alpine Fault is called a strike slip or transform fault. The Australian plate is sliding horizontally towards the north-east, at the same time as the Pacific plate is pushing up, forming the Southern Alps. The mountains are rising at 7 millimetres a year, but erosion wears them down at a similar rate.

The horizontal movement along the fault is not smooth, as both sides are locked together. When tectonic forces overcome this locking, the fault slips, jumping up to a distance of 8 metres at a time. These large earthquakes don’t happen very often – the last one was nearly 300 years ago.

Why are New Zealand researchers interested?

Researchers are studying the Alpine Fault to investigate past earthquakes, mountain formation and the structure of the Earth’s crust.

A lot of research is being done to find out about earthquakes in the past (called palaeoseismology), as they may help indicate when to expect one in the future. The work involves scientists from several disciplines working together, using different methods, such as:

• detailed mapping and satellite surveying
• digging trenches to find buried evidence, such as landslides
• dating trees buried by landslides using radiocarbon dating and tree growth rings (dendrochronology). Some trees survive landslides, but the event is marked by unusual growth rings. Whole forests that have grown back after an earthquake can be dated, too.

Using techniques like these, scientists such as Mark Yetton of the University of Canterbury have found out that major earthquakes happened on the central Alpine Fault in 1100, 1450, 1620 and 1717. This is not a regular pattern, but enough to suggest there is a high probability of a large earthquake in the next 50 years.

Other New Zealand universities, GNS Science and overseas scientists are also interested in the Alpine Fault. Current research includes:

• detailed mapping
• using GPS to study small movements of nearby minor faults, and to measure growth of the Alps
• using seismic data to find out how the many minor earthquakes in the area are linked to minor faults and the main Alpine Fault
• using ground penetrating radar to observe hidden evidence of uplift and horizontal movement.

Keith Machin, Teaching Fellow at the University of Canterbury, helped visiting Swiss scientists study the Alpine Fault. Holes were drilled for explosives, and when the explosives were set off, a multi-channel seismograph recorded the seismic waves. The way the waves are reflected and transmitted tells much about the rocks and structures near the fault. This method can even indicate rocks rich in high-pressure water, the source of hot springs like those at Hanmer Springs.