Watch the moment Parliament is shaken by deep 6.2 earthquake, then suspended. / inthehouseNZ
Today's 6.2 quake was likely the effect of contorting deep within one of the two tectonic plates New Zealand straddles.
GeoNet recorded the quake about 25km south-west of Taumarunui – but what mattered more was its 207km depth.
Victoria University's Professor John Townend suspected the earthquake was produced by bending within the Pacific Plate, which is constantly sliding and grinding against the Australian plate.
As the Pacific plate effectively dived beneath the North Island, the further west in the island, the deeper the plate was.
But quakes that struck at the depth today's did still sent their seismic waves propagating to shallower parts of the great slab – which meant the quake would have been felt more strongly in Hawke's Bay and Wellington.
The earthquake's physical orientation - or "focal mechanism" - suggested to Townend that it was reflecting stresses within the Pacific plate induced by a combination of flexure and thermal stresses.
"The cold Pacific slab is heating up as it goes deeper and deeper into the mantle."
The jolt's aftershock potential wasn't immediately clear – but large, deep quakes like today's usually caused fewer aftershocks than shallower ones.
New Zealand's geology could also explain why people in Auckland wouldn't have felt the quake.
The seismic waves it generated would have lost their energy as they propagated northward and hit hot mantle rock deep beneath the North Island.
"It would be like walking through hot toffee – it just soaks up all the energy."
This was also the reason the quake wasn't as felt as strongly even in Taumarunui – as the waves had to propagate directly upward, through the hot mantle.
New Zealand straddles the colliding Australian and Pacific plates. Image / GNS Science
The waves would have also been slowed by crustal rock near the North Island's surface – and then further absorbed by Auckland's volcanic geology.
"But if you were south-east of the quake, and near the plate boundary, it would have been coming at you quite strongly."
Such strong deep quakes happened "intermittently" in New Zealand – although countless more would occur at that depth that were too small to be felt.
GeoNet compared the quake to a another 6.2 event that struck south of Taranaki, at a depth of 241km, on July 3, 2012.
Living on the boundary
The boundary of the Pacific and the Australian plates 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 the centre of the country where today's quake 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.
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.
