Explained: The science behind the quake

Multimedia Journalist·NZ Herald·

13 Nov, 2016 06:25 PM7 mins to read

Join Tristram Clayton as he talks to GNS Science seismologist John Ristau on the South Island earthquake.

Scientists investigating New Zealand's big overnight shake say the event may have not been one quake, but two.

GeoNet reports indicated that the combination of these two quakes lasted two minutes, with the most severe shaking at around 50 seconds.

The rupture, which struck at magnitude 7.5 about 15 kilometres north-east of Culverden at 12.02am was widely felt throughout both the North and South Islands.

One of the twin quakes appeared to have been driven by "strike slip" faulting, while the other one was "thrust" faulting - a type of reverse faulting observed in the February 22, 2011, Christchurch Earthquake.

Thrust faulting quakes were out of character for the area the quake - or quakes - struck in.

Explore our interactive: All the quakes above magnitude of 2 in the last 48 hours

In reverse faulting, in a compressed area of the Earth's crust, one rocky block, called the "hanging-wall" block, is pushed up relative to rock, "the footwall block", on the other side.

This is the opposite to normal faulting, where the hanging-wall block moves downward and the crust is being pulled apart rather than compressed, and also different from "strike-slip" faulting, where two blocks slide sideways past each other.

The 7.1 Darfield earthquake in 2010 was a result of strike-slip faulting, 10km below the Canterbury town.

But the 6.3 aftershock that killed 185 people several months later was a thrust-faulting earthquake, occurring just 5km below Christchurch.

Its peak ground acceleration (PGA), the amount of acceleration of movement of the earth at the point recorded, reached 2.2g at the Christchurch epicentre, equivalent to the detonation of 15,000 tonnes of TNT.

Even still, the Culverden event was the largest recorded in New Zealand since the 7.8 Dusky Sound earthquake in 2009.

But, given its location, it was more widely felt and more damaging.

Scientists are still calculating the PGA level at the Culverden epicentre of this morning's quake, but the closest reading, at Ward 25km away, recorded a level of 1.2g.

The PGA in Wellington measured 0.23, similar to that of the 2013 Lake Grassmere and Cook Strait earthquakes.

More than 15,000 people immediately reported feeling it, and, as at 9am, the quake had since generated around 300 aftershocks between Kaikoura and Seddon.

Some of the largest, all striking within 30km of Kaikoura, have measured 6.2, 6.2, 6.0, 5.6 and 5.1.

Professor Tim Stern, of Victoria University's School of Geography, Environment and Earth Sciences, said while New Zealand had been hit with a spate of large quakes in the past decade, this was normal in a historical context.

Since 1843, there had been 17 quakes of over magnitude 7 - an average of one every 10 years.

Having three in the space of a decade - the 2010 Darfield quake, the 2009 Fiordland quake and last night's one - was "unusual, but in the wider context, it's still not that unusual".

GeoNet had received reports of odd light in the sky, but had been unable to investigate it.

"At this point we are only working on the data we know we can collect robustly," GeoNet spokesperson Sarah McBride said.

As for the fault on which the quake or quakes occurred on, it's not clear whether there were one or multiple faults at play.

Scientists believe the quake was formed by a type of reverse faulting called thrust faulting. Image / Supplied

GeoNet duty seismologist Anna Kaiser said the epicentre was possibly just south of the Hope Fault, but it did look like "something else was going on".

GeoNet seismologist Dr John Ristau said: "There are a number of faults in that area and we don't know what it was yet."

Victoria University earthquake scientist Associate Professor John Townend said the quake was unusual in its long duration time.

This would likely mean its ground motions, with the quake's longer, deeper waves, would appear much different to previous big quakes when calculated.

As the length of rupture was also controlled by how fast the quake could ripple along the fault, it was likely the length of the fault was significant.

Also strange was the fact the thrust faulting was out of character with the strike-slip faulting usually seen in the area and along the Hope Fault.

This raised the possibility of a system separate from the Hope Fault.

If a new fault or faults had been revealed, the quake's relatively large depth would have left little evidence of surface rupturing, making it harder for scientists to observe physically.

However, Townend was confident that any new faults could be identified using satellite and GPS technology.

A GPS station at Cape Campbell in Marlborough moved two metres north.

Early reports also indicated that the quakes triggered very large landslides to the north and south of Kaikoura.

One was at least one million cubic metres.

"We have three landslide specialists in helicopters now gathering information about the landslides throughout the affected areas," GeoNet said.

The Hope Fault, which famously caused a magnitude 7 to 7.3 quake in North Canterbury in 1888, crosses the South Island from the Alpine Fault at the Taramakau River to the coast north of Kaikoura.

It's part of the Marlborough Fault System, which accommodates the transfer of displacement along the boundary between the Indo-Australian Plate and Pacific Plate, itself the reason why New Zealand is constantly rocked by earthquakes.

Our shaky isles

The boundary snakes along the North Island's East Coast, through Wairarapa, on past Wellington and over the Cook Strait to Canterbury, and then switches to the South Island's west coast and trails off to the west of Stewart Island.

This is New Zealand's shaky position on the wider Pacific Ring of Fire - an almost continuous belt of volcanoes and earthquakes around the rim of the Pacific Ocean.

It's defined by the 103 million-square kilometre Pacific Plate smashing together with other vast tectonic plates that form parts of the geological jigsaw puzzle that make up Earth's lithosphere - its rigid, outermost shell that combines the crust and the upper mantle.

In New Zealand's case, there's a continual scrum going on between the Pacific Plate and the 47-million square kilometre Australian Plate right along the curving boundary.

How they collide is different in different places.

At the southern end of the South Island, the Australian Plate dives down, or subducts, below the Pacific Plate while in the North Island, the opposite situation occurs with the Pacific Plate being pushed under by the Australian Plate.

In between, through most of the South Island, the two plates grind past each other along the Alpine Fault that runs along the mountainous spine of the island.

Ultimately, this motion creates more than 15,000 quakes large enough to be recorded every year, of which between 100 and 150 are big enough to be felt by people.

As the 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.

New Zealand sits on the boundary of two tectonic plates. Image / Supplied

Scientists often compare this to a bending stick: as it becomes more deformed, it breaks and each of the pieces spring back in a relatively straight but new position from each other.

Different types of earthquakes tended to be characteristic of certain areas.

On the west coast of the South Island, for example, scientists observe many strike-slip earthquakes, while in Wellington, it's more typical to see normal faulting.

But all of these earthquakes are related in the sense that, ultimately, it's the same tectonic processes which is seeing the two major plates colliding.

For New Zealand, the prime earthquake threat remains the Alpine Fault, running along the spine of the South Island, and which is likely to cause a major quake within this generation's lifetime.

It last ruptured in 1717, or 297 years ago, and had a 28 per cent probability of rupturing in the next 50 years - a rate high by global standards.

According to GNS Science, this rupture would produce one of the biggest earthquakes since European settlement of New Zealand, and would have a "major impact" on the lives of many people.