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Story: Active faults

Earthquakes occur on fractures, or faults, within the Earth. Active faults have ruptured repeatedly, and may move again. The most spectacular is the Alpine Fault, where the Southern Alps are being uplifted. The city of Wellington was built, unknowingly, on another active fault. Geologists have now identified these lines of weakness, and there is greater awareness of the hazards of building where the earth may suddenly shift again.

Story by Eileen McSaveney
Main image: The Alpine Fault, South Island

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What are faults?

Faults are cracks in the earth’s crust and can be up to hundreds of kilometres long. When stress builds up in the crust, it can eventually cause the rock to break suddenly along a fault. This movement releases energy, and is felt on the ground surface as an earthquake. Earthquakes, sometimes very violent, may occur many times on the same faults over millions of years.

What is an active fault?

Many of the faults in New Zealand are ancient, and have not moved for thousands of years. Others have moved often, and broken the land surface. These are called active faults, as they are the most likely to move in the future. Several earthquakes on active faults have happened since European settlement.

Where are New Zealand’s active faults?

The country’s longest active faults are:

  • the Wellington–Mōhaka Fault, running from Cook Strait to the Bay of Plenty
  • the Alpine Fault in the South Island.
  • Hope Fault (1888 North Canterbury earthquake)
  • White Creek Fault (1929 Murchison earthquake)
  • Edgecumbe Fault (1987 Edgecumbe earthquake).

Other active faults where earthquakes have been recorded include:

What are the signs of an active fault?

During the 1987 Edgecumbe quake, a 7-kilometre-long crack opened in the Rangitāiki Plains. This is known as the Edgecumbe Fault. It ruptured at the surface, with one side dropping 2 metres. Mountains can be formed from repeated earthquakes and ground movements over thousands of years. The Southern Alps have been lifted up by many earthquakes on the Alpine Fault.

Why is an active fault a hazard?

When a fault ruptures during an earthquake, structures built across it are torn apart, and people can be injured or killed. In the past, the people building New Zealand’s towns and cities did not know about faults. For example, many buildings lie on or near the Wellington Fault, and other centres such as Nelson and Waikanae are built on active faults. Today there are rules preventing new construction near a known fault. The best way to avoid destruction and injury is to make sure that houses and other structures are not built across known active faults.

How to cite this page:

Eileen McSaveney, 'Active faults', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/active-faults (accessed 26 June 2017)

Story by Eileen McSaveney, published 12 Jun 2006