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27/7/2018 — Education, Science and Technology
Nelson quake a puzzle for GNS

Last Sunday's magnitude 5.2 quake near Nelson happened at a depth where the geology, according to a seismologist, “starts to get quite complicated.”

Fairfax Media reported that the seismic monitoring service GeoNet received more than 8,100 felt reports for this afternoon earthquake, centred 20 kilometres north-east of Nelson.

NZResources reported at the time it was fortunate this quake was at a depth of 73 km.

GNS Science duty seismologist Dr John Ristau told Fairfax: “It's not like your typical earthquake rupturing on a fault.”

It was hard to tell what happened in this case. In that area of the upper South Island, the Pacific Plate was pushing down beneath the Australian Plate.

Dr Ristau said earthquakes such as the event on Sunday could happen on the interface between the two plates, but that was unlikely in this case.

“So (it happened) either on the Australian Plate, fairly deep, or on the subducting slab of the Pacific Plate. They can occur for various reasons.”

As the subducting slab went deeper it became hotter, “so the rocks start to change and start to go through transformation to different rock types, and they become more plastic almost.”

If one area of rock started to melt, while the area around it was more solid, then there could be a collapse.

“It starts to get quite complicated down there,” he said.

Fairfax said while being widely felt, Sunday's quake wasn't particularly big. Similar events happened every year or two. Because of its size and depth there were limits to the amount of investigation that could be carried out.

“If it was bigger, we would have more information to go on, and there would be more of an impetus to look into it,” Dr Ristau said.

The data that was available might be fed into models being developed for long-term earthquake forecasting.

Sources: stuff.co.nz/science; nzresources.com

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Location of last Sunday’s earthquake.