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The Alpine Fault

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The Alpine Fault is where two great tectonic plates (the Pacific and Australian) plates collide. It is a dominant feature of the South Island. Movement along this fault has uplifted the Southern Alps and produced large earthquakes.

The Alpine Fault is where two great tectonic plates push together. Movement along this fault has uplifted the Southern Alps and caused large earthquakes.

The West Coast

One major geographic feature of the South Island of New Zealand is the Southern Alps, a mountain range caused by tectonic uplift along the Alpine Fault.

The Alpine Fault runs for about 600km up the spine of the South Island, and is one of the world’s major geological features. The Alpine Fault marks the boundary of the Pacific and Australian Plates.

How often does the Alpine Fault rupture?

This fault has ruptured four times in the past 900 years, each time producing an earthquake of about magnitude 8. Recent research by GNS Science has extended our knowledge of the Alpine fault earthquake record back through the past 8,000 years.

The investigation found the mean interval between large earthquakes on the fault is 330 years and two thirds of the intervals were between 260 and 400 years. As with many natural systems, there was a spread of intervals with the longest being about 510 years and the shortest about 140 years.

Movement on the Alpine Fault

Horizontal movement of the Alpine Fault is about 30m per 1,000 years — very fast by global standards. Each time it has ruptured, it has also moved vertically, lifting the Southern Alps in the process.

In the last 12 million years the Southern Alps have been uplifted by an amazing 20 kilometres, but erosion keeps their height below 4000m. The glaciers and rivers have removed the rest of the material and spread it out across the lowland plains or onto the sea floor. The rapid uplift also means that faulted rock from deep down has been brought to the surface, and can be studied by scientists.

When will the Alpine Fault next rupture?

There is no way of predicting exactly when an earthquake will occur. Scientists believe the Alpine Fault has a high probability (estimated at 30%) of rupturing in the next 50 years. This is based on how regularly it has ruptured in the past. The rupture will produce one of the biggest earthquakes since European settlement of New Zealand, and it will have a major impact on the lives of many people. In between earthquakes, the Alpine Fault is locked. 

When the Alpine Fault next ruptures it is likely to produce an earthquake with a magnitude of around 8. The Darfield earthquake had a magnitude of M7.1. Because of the way earthquake magnitude is measured this means that a magnitude 8 earthquake releases about 30 times more energy than a magnitude 7.

An Alpine Fault earthquake will: 

  • Likely rupture along a larger area of the fault (several hundreds of kilometres)
  • It will last longer (hundreds of seconds rather than tens of seconds)
  • Affect a much larger area than earthquakes such as the Darfield earthquake.
  • The aftershock sequence could involve earthquakes of as much as M7 and continue for many years
  • Violent shaking will cause widespread landslides and weaken rock, making landslides more common for many years
  • Rivers may change course, will carry more sediment and therefore flood more easily
  • Towns and infrastructure such as roads, bridges, power lines and communication services will be disrupted
  • West Coast industries such as tourism, coal and gold mining, forestry and wood processing, fishing, and dairy farming will all be affected by an Alpine Fault earthquake

 

The Alpine Fault

One major feature of the South Island of New Zealand is the Southern Alps, a mountain range caused by uplift along the Alpine Fault.

The Alpine Fault:

  • runs for about 600km up the South Island
  • is one of the world’s major faults
  • marks the boundary of the Pacific and Australian Plates.

How often does the Alpine Fault cause earthquakes?

This fault has suddenly moved or ruptured four times in the past 900 years, each time causing an earthquake of about magnitude 8. Recent research has found that the average time between large earthquakes on the fault is 330 years. But there has been as little as 140 years and as much as 510 years between Alpine fault earthquakes.

Movement on the Alpine Fault

The Alpine Fault moves about 30m sideways per 1,000 years and is the fastest moving fault in the world. Each time it breaks in an earthquake, it has also moved upwards. Over time this has lifted the Southern Alps.

When will the Alpine Fault next rupture? 

There is no way of predicting exactly when an earthquake will happen. There is a chance that the Alpine Fault will rupture in the next 50 to 100 years. This is based on how regularly it has ruptured in the past. The rupture will produce a large earthquake that will have a major impact on the lives of many people. In between earthquakes, the Alpine Fault is locked. 

An Alpine Fault earthquake:

  • The rupture will be several hundred kilometres long
  • The shaking could last between one and two minutes
  • Affect a much larger area than smaller earthquakes such as the Darfield earthquake.
  • The aftershocks will continue for many years
  • Cause widespread landslides and weaken rock, making landslides more common for many years
  • Rivers may change course, will carry more sediment and therefore flood more easily
  • Towns, roads, bridges, power lines and other services will be damaged
  • West Coast industries such as tourism, forestry, fishing, and dairy farming will all be affected by an Alpine Fault earthquake.

 

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How do you think you would be affected by a rupture of the Alpine Fault?

How do you think you would be affected by an Alpine Fault earthquake?



This NASA satellite image of the South Island shows the Alpine Fault as a line along the western edge of the Southern Alps. Image: NASA.

Movement along the Alpine Fault has uplifted the Southern Alps. Image: LEARNZ.

The Southern Alps have been uplifted over 20 kilometres over 12 million years but erosion has kept their height below 4,000 metres. Image: LEARNZ.

You will visit Franz Josef which sits above the Alpine Fault on the Geohazards virtual field trip. Image: LEARNZ.

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