Jamie Morton is the NZ Herald's science reporter.

The real reason New Zealand is a land of earthquakes

The Southern Alps were created by two major plates colliding. Photo / Doug Sherring
The Southern Alps were created by two major plates colliding. Photo / Doug Sherring

Were the quakes that struck Wairarapa today and Christchurch last night caused by the weather or planets?

Though they're questions GNS Science seismologists sometimes get asked, no. And no.

But were the two quakes linked?

No again. But yes. Kind of.

Log on to the GeoNet website, check out its earthquake map, and you'll see a dense cluster of dots.

It looks much like someone has splattered paint across a canvas in a rough line, which in this case stretches from the northeast of the North Island down to the west coast at the bottom of the South Island.

The boundary snakes along the North Island's East Coast, through Wairarapa where this morning's twin shakes in Masterton hit, 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.

There's a continual scrum going on between the Pacific Plate and the Australian Plate.
There's a continual scrum going on between the Pacific Plate and the Australian Plate.

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.

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.

GNS Science seismologist Dr John Ristau said different types of earthquakes tended to be characteristic of certain areas.

On the west coast of the South Island, for example, scientists observed many "strike-slip" earthquakes, where the faulting occurred in a side to side motion.

In Wellington, meanwhile, it was typical to see "normal faulting", where chunks of earth rubbed against each other in an up-and-down motion.

"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."

Are Christchurch quakes related to Masterton shakes?

The fact that a magnitude 4.7 quake hit Christchurch just around 12 hours before Masterton residents were shaken this morning by a 5.2 jolt at 7.55am jolt should be put down to co-incidence.

They occurred through different mechanisms on different types of faults hundreds of kilometres away from each other - and effectively weren't large enough, as New Zealand quakes seldom are, to have any influence on each other.

"For instance, the one in Christchurch was very shallow, and that's caused by faults fracturing the crust - while the one in Wairarapa is much deeper, at a depth of around 25km, and actually occurred within the Pacific Plate that's pushing beneath the Australian Plate underneath the North Island."

The 5.2 Masterton earthquake could be linked to a 4.7 quake that struck 40 minutes earlier not far from its epicentre west of the town.

The Christchurch quake, scientists say, was meanwhile part of the same spate that had seen thousands of quakes triggered by the 7.1 earthquake which hit Darfield on September 4, 2010, and included the 6.3, February 22, 2011, quake which cost the lives of 185 people.

The Greendale fault that caused the September 2010 Darfield earthquake was a big fault, but we had no idea it was there.
Dr John Ristau

It was the latter quake's shallow depth - just 5km - its location so close to the city, and its sheer power, erupting with the energy of 15,000 tonnes of TNT, which made the event seem unworthy of simply another aftershock.

Dr Ristau also pointed out that last night and this morning's quakes were a relatively common occurrence, despite the surprise they caused to people.

In the past year, GeoNet seismometers picked up 71 quakes measuring between magnitude 5 and 5.9 and 672 between magnitude 4 and 4.9.

In Canterbury alone over that period, there had been 15 quakes measuring between magnitude 4 and 5, along with three between magnitude 5 and 6, 88 between magnitude 3 and 4 and 1144 under magnitude 3.

In all of those areas where quakes often struck, we could assume there was a dense web of faults lying below ground, of which we're aware of just a tiny fraction.

"I'm not sure what the actual proportion of faults mapped in New Zealand is, but it is a very small percentage of what we think the real number is," Dr Ristau said.

"The Greendale fault that caused the September 2010 Darfield earthquake was a big fault, but we had no idea it was there. There are probably ones like that all over New Zealand."

Alpine Fault

Particularly, scientists are intensely interested in the Alpine Fault, which they probed with a deep drilling project north of Franz Josef Glacier in 2014.

Dr Ristau said earthquakes of magnitude 7.5 or larger struck along the Alpine Fault every 300 years, which was relatively frequent in geological terms.

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.

Kerepehi Fault

Meanwhile, scientists have just revealed new insights into the Kerepehi Fault, which runs for about 80km between Matamata and north into the Firth of Thames.

Research just shared by GNS Science today showed that the fault is more complex than previously thought.

Previously, the fault was thought to consist of up to five loosely connected single strands or segments, but the new findings show it actually consists of a belt of many faults, in a wider zone suggesting a more complicated arrangement of segments.

Scientists had earlier understood the average interval between ruptures of the Kerepehi Fault was between 6000 and 8000 years, based on the better studied segments.

Although there is one period several thousand years ago when three quakes appear to have clustered relatively closely in time.

However, the new research combined the rupture history of all fault segments studied so far, and found the average rupture interval to be about 1000 years.

In other words, the rupture interval on a single segment of the fault was still likely to be several thousand years, but the region may experience a large earthquake every 1000 years.

Previous ruptures have involved up to 2m of vertical displacement of the ground surface per event, which suggests associated earthquakes were between magnitude 6.3 and 7.0 in size.

Geologist Dr Pilar Villamor, who co-led the study, said earthquakes of this size would result in significant damage to weaker buildings, disrupt services, threaten flood defence structures, and cause liquefaction in weak soils where the water table is close to the ground surface.

While the Hauraki Plains is considered a region of only moderate recent earthquake activity - one notable 5.1 quake resulted in 1300 claims to the Earthquake Commission for property damage when it struck near Te Aroha in 1972 - Dr Villamor said further work was needed to estimate the impacts of a major earthquake in the region.

- NZ Herald

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