On the fault (Image: Sarah Ivey/Newspix/Rex Features)

This week the New Zealand city of Christchurch felt the force of a 6.3-magnitude earthquake. The quake came just five months after an even larger one struck 40 kilometres west of Christchurch, near the town of Darfield. In fact New Zealand experiences around 14,000 tremors each year, although most are too small to be felt. They are a sign of the tectonic processes that are gradually shredding the country.

Why is New Zealand so prone to earthquakes?

Regions that lie close to a boundary between tectonic plates tend to feel more quakes than areas in the middle of a plate. New Zealand may have a total land area of just 27,000 square kilometres, but that area happens to coincide with the margin between the Pacific and Australian plates, leaving parts of the island very seismically active.

Which areas are most vulnerable?

Large areas of both North and South Islands have felt earthquakes with a magnitude greater than 5 within the past 200 years. This is because of New Zealand’s unique tectonic regime: despite its small size, the country feels the impact of three distinct regions of tectonic activity.


The relatively low-density continental crust of the North Island, which sits on the Australian plate, is forcing the dense oceanic crust on the Pacific plate beneath it in a process called subduction. This creates a so-called destructive plate margin that is nibbling away at the Pacific plate. Earthquakes are common where a subducting plate grinds against the underside of an overriding plate.

Something similar is occurring to the south-west of South Island. But here the sliver of continental crust lies on the Pacific plate, and it is the Australian plate that is being destroyed through subduction.

In between, the continental crust on the Pacific and Australian plates slide past one another on South Island, creating a conservative plate margin where crust is neither created nor destroyed. This area is still prone to earthquakes, most notably along the Alpine fault. Further away from these fault zones the ground is generally more quiescent. Christchurch is over 100 kilometres from the Alpine fault.

So what caused the Christchurch quake?

It was caused by a new fault – or, to be more precise, a previously unrecognised fault.

“The fault is likely to have existed previously – and possibly produced earthquakes before – but they have not ruptured recently, in a geological sense,” says John Townend at the Victoria University of Wellington, New Zealand. The unrecognised fault appears to be an offshoot from the Alpine fault. Unfortunately for the residents of Christchurch, that offshoot passes very near South Island’s largest city.

Are more quakes on the fault likely?

Earthquake prediction is an inexact science, despite tantalising evidence that early warning systems may be possible in some cases. But some seismologists are cautiously optimistic.

“An earthquake of this magnitude does a good job of releasing stress,” says Gary Gibson, a seismologist at the University of Melbourne, Australia. Townend agrees: “My interpretation of what we are seeing near Christchurch is temporary, albeit harrowing, activity in what is generally a relatively low-seismicity part of the broad plate boundary.”

What’s the long-term prognosis for New Zealand?

Even if Christchurch dodges major seismic activity in the near future, tectonic forces will continue to act on New Zealand. Hamish Campbell at the research consultancy GNS Science in Lower Hutt, New Zealand, says it’s “very unlikely” that the newly recognised fault will have any serious effect on the country’s geography, but activity on the Alpine fault may well do so.

The rocks on either side of the Alpine fault are grinding past each other quickly – at around 30 millimetres per year. The southern part of South Island has moved at least 480 kilometres relative to the northern part within the past 25 million years. That rate of movement is “colossal”, says Campbell – and not far off the displacement seen on the world-famous San Andreas fault in California, which is itself a conservative plate margin.

Fast forward several million years and New Zealand will continue to twist and turn. The activity that is already shredding the country will ultimately see South Island “split in two along the Alpine boundary”, says Campbell. The town of Kaikoura would be at the northern tip of one island, with Greymouth at the southern tip of the other, he predicts.