The Real Reason New Zealand Gets a lot of Earthquakes

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

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.

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."

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