Motueka earthquake came from within Earth's crust

Monday, 12 August 2019

A magnitude 5.2 quake on Monday morning registered widely across central New Zealand, with 2100 felt reports on the GeoNet site. (File photo)

The second magnitude 5-plus earthquake in the Nelson-Tasman region in the past 13 months did not occur on any specific fault, GNS Science says.

The epicentre of Monday's 8.44am magnitude 5.2 quake was about 20 kilometres west-southwest of Motueka, close to the eastern slopes of Mt Arthur, and had a focus 102km below the surface.

GNS Science chief executive Dr Ian Simpson at the National Geohazards Monitoring Centre in Lower Hutt. The centre was opened last December.

The quake registered widely with people across central New Zealand, with 2100 GeoNet felt reports from Westport, across Nelson and Marlborough, and on the other side of Cook Strait around Wellington and the Wairarapa.

GNS Science duty seismologist Elizabeth Abbott told Stuff the depth meant the rupture occurred within the boundary zone where the Pacific Plate is being subducted below the Australian Plate.

A GNS Science technician services a seismograph and GPS station at Thirsty Ridge above Jackson Bay in South Westland. The equipment is part of the 600-strong network of instruments in the nationwide GeoNet network.

**READ MORE:

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* Magnitude 5.2 earthquake centred near Nelson shakes lower North Island

* New Zealand earthquake: large aftershocks less likely due to depth, GNS says

* Uncertainty how Friday's quake may affect Cook Strait faults**

'At shallow depths, within the Earth's upper crust, earthquakes occur along faults or break their own small part of the crust.

'However, at greater depths, say more than 30km, the processes that produce earthquakes are different and earthquake do not occur on faults – they occur due to the subducting plate bending or being pulled apart.

'This region is at the southern end of the Hikurangi subduction zone, where the Pacific Plate is diving beneath the Australian plate.'

It was likely, but not known for sure, the quake was within the Pacific Plate.

The region experienced regular, deep quakes but fewer than the central North Island, Abbott said.

An example of a deep North Island quake was the magnitude 6.2 event 207km below Taumarunui on October 30 last year.

The Nelson-Tasman region's last magnitude 5-plus quake was on July 22 last year, when a magnitude 5.2 shake occurred 20km east of Nelson city at a depth of 73km.