14 Nov 2016

The Science Of... Earthquakes

Aerial image of rockfall caused by the earthquake on the Kaikoura coast
From Featured Audio, 5:49 pm on 14 November 2016

At 12.02am on Monday 14 November, a magnitude 7.5 earthquake struck New Zealand. The quake was 15km north-east of Culverden, at a depth of 15km.

The quake lasted several minutes, and was widely felt throughout New Zealand. The areas around Hanmer Springs and Kaikoura reported the worst damage.

A network of earthquake detectors – about 600 located in New Zealand and more around the world – felt the shaking and, within a minute or so, automatically measured the shake, calculated its location, magnitude and depth, and produced a map of ground-shaking intensities.

Over the following hours, a team of GeoNet earthquake experts began to forensically analyse the data from the detectors, seeking to understand in detail how the earthquake unfolded.

Once it was light, another earthquake expert flew over the area to map damage and any visible signs of the fault, while others began recording landslides.

GeoNet duty seismologist Dr Caroline Little said initial aerial observations showed a visible fault rupture running across the landscape. This is a sign of a shallow earthquake, and was also seen in the September 2010 earthquake around Darfield.

Within eight hours of the earthquake, GNS principal scientist Dr Kelvin Berryman told RNZ: “It seems like ‘the earthquake’ was more like two earthquakes, but very closely spaced in time - actually, part of the whole 120 seconds. [It started] off down in the Culverden area, but then jumped quite quickly north to Kaikoura and north along the coast.”

By mid-evening Sara McBride wrote on the GeoNet website that there were further developments in the understanding of the earthquake, and that at least four faults were now suspected of being involved. These are just the visible faults, and the GeoNet team suspect further research will implicate even more hidden faults.

"Rapid field reconnaissance indicates that multiple faults have ruptured:
    Kekerengu Fault at the coast - appears to have had up to 10m of slip
    Newly identified fault at Waipapa Bay
    Hope Fault - seaward segment - minor movement
    Hundalee Fault.

"What we are finding in New Zealand," wrote Dr McBride, "is that quite a few of our larger earthquakes involve jumping from rupture on one plane to another in a complex sequence. We first saw that with the Darfield September 2010 earthquake, where multiple segments ruptured together as a single earthquake. We appear to have seen this again overnight."

Type of earthquake

Earthquakes involve two blocks of land moving relative to one another along a fault line. This movement may be the blocks moving up and down relative to one another (a thrust earthquake), or they can slide sideways or horizontally (strike-slip).

Coastal Road to Kaikoura - rail road cuts through state highway 1

Photo: RNZ / Rebekah Parsons-King

GeoNet scientists have said they think the two earthquakes that occurred at the same time today involved both of these different kinds of fault movement.

Australian-based geologist Dr Mark Quigley, who became well-known during the Canterbury earthquakes when he lived in Christchurch, said that “preliminary data suggest that this is a structurally complicated earthquake, involving a mixture of reverse faults that thrust the northern South Island up over the Pacific Plate, and strike-slip faults, which slid northern crustal blocks sideways and towards the northeast relative to their southern counterparts".

Aftershock sequence

Earthquakes do not occur in isolation – they are followed by a series of increasingly smaller earthquakes, or aftershocks, which can carry on for years.

Coastal Road to Kaikoura - rail road cuts through state highway 1

Photo: RNZ / Rebekah Parsons-King

As the land moves during an earthquake it can either release or create strains in other parts of the fault, and the subsequent release of this loading generates new earthquakes in the surrounding area. 

Dr Berryman said that many of the more than 250 aftershocks experienced today “are north, in the Kaikoura, Ward and Seddon area. We expect those to carry on at a very high level ... with a high likelihood of fives and sixes over the next days".

It took a few hours to get sufficient data to analyse the pattern of aftershocks, but early indications were that they were following “a pretty standard pattern", he said.

A typical series of aftershocks would include earthquakes about one magnitude less than the original quake, and Dr Berryman said the GeoNet team reported there were some earthquakes bigger than magnitude 6 in the first few moments, and subsequent hours, following the magnitude 7.5.

Although aftershocks generally decline in intensity over the following days and weeks, it’s possible to get large damaging aftershocks several months after the original quake. This is what happened in Christchurch - a damaging magnitude 7.1 earthquake in September 2010 was followed by a damaging magnitude 6.3 aftershock in February that devastated central Christchurch, killing 185 people.

GeoNet has predicted a 38 percent chance of a greater than magnitude 7 aftershock happening in the next month, and a 98 percent chance that there will be earthquakes between magnitude 6-6.9.

Large earthquakes in New Zealand

The Culverden-Kaikoura earthquake is one of the largest earthquakes experienced in New Zealand in the past century.

The largest earthquake in recent history was a magnitude 7.8 in Dusky Sound, Fiordland, in July 2009.

A magnitude 7.8 earthquake in Hawke’s Bay in 1931 killed 256 people.

Earthquake detection

More than 600 earthquake detectors are positioned around New Zealand, including 142 main seismograph stations and a further 225 that can be used to help pinpoint positions. A further 200 or so GPS stations detect minute movements of the land, and add another layer of information.

GeoNet has reported that “our GPS stations record how land moved in response to the earthquake. Our station at Cape Campbell in Marlborough moved two metres north".

Dr Little described this as “pretty incredible really ... there was more land movement than was experienced in the 2013 Cook Strait earthquakes".

The GeoNet team is using different computer models to try and understand exactly how the land moved.

Why is New Zealand so prone to earthquakes?

A major tectonic plate boundary runs through New Zealand, and is part of the Pacific Ring of Fire.

The Pacific Ring of Fire is a 40,000km long arc of volcanoes and ocean trenches that runs around the edge of the Pacific Ocean. Most of the world’s earthquakes and volcanic eruptions occur in this area.

The edge of the large Pacific tectonic plate makes up much of the western and northern margins of the Ring of Fire.

Elms Homestead

Photo: RNZ / Rebekah Parsons-King

The boundary of the Pacific Plate and the Australian Plate runs through New Zealand, in a north-east to south-west line. White Island in the Bay of Plenty is near the eastern end, and it continues along the Alpine Fault on the South Island’s West Coast and on south of New Zealand.

This plate boundary is complicated, and different movements happen in different sections. The North Island and the top of the South Island sit above a subduction zone, where the Pacific Plate is pushed down under the Australian Plate.

Along the Alpine Fault the two tectonic plates slide past one another. Further south the motion of the plates changes again, with the Australian Plate being pushed down or subducted beneath the Pacific Plate.

The GNS Science website describes an earthquake like this: "Earthquakes are a response to the motion between plates. As two plates push together at a steady rate, the rocks along the boundary become more and more stressed until eventually something has to give - and an earthquake occurs along a fault somewhere in the plate boundary zone. It's similar to bending a stick. As you bend it with increasing force, the stick becomes more and more deformed until eventually it breaks ('earthquake!') and each of the two pieces of the stick spring back to being more or less straight, but in a new position relative to each other."

Earthquake intensity

GeoNet has reported that the small town of Ward recorded 1.2g peak ground acceleration (PGA). Ward is 25km from the epicentre, and is home to the closest earthquake detector. By comparison, the 22 February 2011 Christchurch earthquake had a peak ground acceleration of 2.2g.

Peak ground acceleration is a measure of how hard the earth shakes at a given geographic point. This ground movement is usually a better indication of likely damage to buildings than magnitude alone, which is the amount of energy released during an earthquake.

PGA is a useful measure when considered alongside the "felt" reports that people give of their experience of the earthquake.

For its "felt" reports GeoNet uses a 12-point ranking system called the New Zealand Modified Mercalli Intensity scale (MMI), to rate the intensity or strength of earthquakes. This describes the effects of an earthquake on people and the environment.

GeoNet said more than 15,000 people went online to fill out felt reports, to describe how they experienced today’s magnitude 7.5 earthquake. Its website reported that “the highest level of impact reported so far has been MMI8 (severe). This level of shaking includes people experiencing difficulty standing, furniture and appliances shifting. Substantial damage to fragile or unsecured objects and a few weak buildings may be damaged".

One of the key variables that affects how people feel an earthquake is the type of ground they are standing on. Soft ground, such as old lake sediments, as well as sand dunes and reclaimed ground, shake much more than solid, rocky ground.

This is especially notable in Wellington, where some of the worst damage has occurred near the waterfront on reclaimed land.

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Photo: RNZ / Simon Morton

More earthquake stories on Our Changing World

Italian seismologist Giulio Selvaggi is calling on scientists to be open and clear in their communication about the risk of natural hazards to make sure people are as well prepared as possible. Dr Selvaggi was convicted, and then acquitted, of manslaughter following a deadly earthquake in the Italian city of L’Aquila.

Seismologist Bill Stephenson explains why soft soils are often severely affected by earthquakes shaking.