Deep quake ‘quite complicated’

Sunday’s magnitude 5.2 quake near Nelson happened at a depth where the geology ‘‘starts to get quite complicated’’, a seismologist says.

GeoNet received more than 8100 felt reports for the 3.14pm earthquake, which was centred 20 kilometres east of Nelson at a depth of 73km.

‘‘It’s not like your typical earthquake rupturing on a fault,’’ GNS Science duty seismologist Dr John Ristau said.

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.

Earthquakes such as the event on Sunday could happen on the interface between the two plates but that was unlikely in this case, Ristau said. ‘‘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 got 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.’’

Despite being widely felt, Sunday’s jolt wasn’t particularly big. Similar events happened every year or two.

Because of the 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.’’