Plenty of earthquakes in New Zealand in 2018 but few of significance

2018-12-28 05:15:54

It was the depth of New Zealand's largest earthquake this year that saved nearby residents from the inevitable clean-up job.

The magnitude 6.2 quake on October 30 began at 3.13pm, deep down on the part of the Pacific Plate being subducted under the Australian Plate below the North Island, about 25 kilometres southwest of Taumarunui.

By the time the energy from the 207km-deep rupture had reached the surface, it had already spread a long way from where it began.

Curiously, as a result, residents living almost on top of the quake felt little or no movement. But those on the east coast of the North Island, on both sides of Cook Strait and as far south as Canterbury experienced the quake as two distinct pulses – the first thump marking the passage of the faster "P", or primary, waves, followed by a spell of shaking as the slower "S", or secondary, waves arrived.

GNS Science seismologist Sam Taylor-Offord said this week the King Country shake was the most "felt" quake of the year, with 15,666 felt reports sent in to GeoNet. Of those, 11,084 people reported either light or moderate shaking, and 979 strong movement.

The second largest quake of the year in the New Zealand region was southwest of the South Island close to the Puysegur Trench, where the Pacific and Australian plates meet. The magnitude 5.8 quake on November 25, centred about 85km west of the Snares Islands and 12km deep, was reported felt by about 50 people on the south coast.

There were also three magnitude 5.2 quakes during the year – 20km west of Waipukurau and 21km deep on February 18, 20km east of Nelson and 73km deep on July 22, and 25km north of Milford Sound and 5km deep on October 6.

Taylor-Offord said it might have seemed like a "relatively quiet year" for earthquakes but there had still been plenty of them. New Zealanders could not afford to be complacent and needed to stay prepared.

"Nearly 20,000 quakes this year shows that we do live in a very seismically active area of the world, but we were fortunate to escape any damaging local earthquakes this year."

However, there were still a couple of days left in 2018 and you could never be sure what might happen next, he said.

In terms of the country's recent quake hotspots, the largest aftershock this year of the November 2016 magnitude 7.8 Kaikōura quake was a magnitude 4.7 shake about 20km west of the town on March 8.

The most significant in the Seddon area was a 4.5-magnitude tremor on January 6 and the largest 2018 aftershock of the Christchurch quake sequence was of magnitude 4.0 on July 15 near New Brighton, with felt reports from 7812 people.

The deepest quake of the year had its focus 547km down about 200km northeast of the Coromandel Peninsula. Nobody felt that magnitude 4.0 event on December 15.

Taylor-Offord said that was very deep for a quake and was down in the mantle layer of the Earth, below the crust, on part of the subducting Pacific Plate.

Next year the public would be even more aware of earthquakes and other hazards than before.

"The advent of our round-the-clock geohazard analysts at the National Geohazards Monitoring Centre means they are locating more earthquakes than were previously picked up by our automated systems."

The slumbering giant for South Islanders remains the Alpine Fault, a more than 650km-long boundary between the Australian and Pacific plates that carves a path along the western edge of the Southern Alps from Marlborough to the entrance to Milford Sound.

A major rupture of the Alpine Fault is likely to generate a quake of about magnitude 8. Research shows these major quakes happen about once every 300 years and the last one was in 1717.

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