Scientists explain complex nature of country, ‘oblique-reverse’ faults and what they mean by ‘liquefaction’ of the ground • New Zealand earthquake: get the latest updates

How did New Zealand's earthquake happen? Was it two quakes in one?

A 7.5-magnitude earthquake has hit the South Island of New Zealand, causing two deaths and mass evacuations following a tsunami warning.

What caused the earthquake?

The exact cause of the quake (or quakes) on Sunday night in New Zealand is not yet completely clear.

But the US Geological Survey says it was a result of an “oblique-reverse” fault.

That occurs when two plates are moving towards one another, and after enough tension is built up, one slips up over the other, releasing the tension.

Most NZ’s South Island is on a plate called the Pacific plate. But the northern part of South Island (and all of North Island) sit on the Australian plate.

The boundary between the two is exactly where the earthquake occurred.

At the location of the quake, the Pacific plate is moving roughly west at about 40mm a year relative to the Australian plate.

In very general terms, that movement is causing the two plates to collide, creating tension which is released in earthquakes.

Was it one quake or two?

According to the US Geological Survey, the size, depth and direction of the fault points towards a kind of fault called a “subduction zone”, which can cause very large quakes.

Subduction occurs when one plate is pushed down under another plate.

But according to the US Geological Survey, scientists didn’t think that there was any subduction happening where this earthquake occurred.

They say the plate boundaries have such a complex structure that, with the strain that builds up being applied to many different structures, there may have been more than one fault that caused the earthquake sequence.

Dr Behzad Fatahi from the University of Technology Sydney says: “The complexity of fault lines and tectonic activities is due to the fact that tectonic plates in some locations such as east of the North Island move toward each (creating compressive stresses), and in some other regions such as South Island slide past each other (generating shear stresses), both creating shallow earthquakes.”

James Goff from the University of New South Wales says: “Yet again, New Zealand’s earthquakes are proving to be anything but easy to understand.”

Scientists from New Zealand’s GNS Science have said they’re not sure yet which faults were involved.

What is this ‘liquefaction’ people are talking about?

There have been reports of widespread liquefaction of soil, particularly on reclaimed land around the Wellington waterfront and the Picton foreshore.

Soil liquefaction can happen when soil is shaken up, and the tension it can normally hold – which is often enough to hold up roads and buildings – is reduced to nearly zero. It usually happens with soil that is saturated with water and then shaken.

During earthquakes, the shaking can not only loosen the soil but also cause water to rise up from deeper down, making liquefaction more likely.

The effect can look like quicksand, with objects falling into soil that was previously able to support their weight.

Fatahi says: “It is foreseen that due to the main earthquake and follow up aftershocks, as a result of liquefaction of soil, significant damage to transport infrastructure mainly roads, and lifelines including water and sewage, oil and gas distribution pipelines, may happen. Indeed, although liquefaction of loose materials may happen well below the ground surface, it can be manifested as the ground surface subsidence.”