Rebecca Quilliam

Rebecca Quilliam is senior reporter at the APNZ News Service office in Wellington.

Long-running Wellington quake a 'slow-slip' event

The Kapiti slow-slip event's movement from January to May. The most extreme movement, depicted by the darkest colour, is under Otaki and north of Marlborough Sounds. Image / Geonet
The Kapiti slow-slip event's movement from January to May. The most extreme movement, depicted by the darkest colour, is under Otaki and north of Marlborough Sounds. Image / Geonet

The biggest earthquake in 150 years is hitting Wellington at the moment, but none of the locals are diving under desks or sheltering in doorways.

The magnitude 7 quake is centred to the west of the capital and is 40km deep, and has been rolling all year, GeoNet Operations Scientist Caroline Little said.

Despite its strength, even sensitive recording instruments were hardly registering the shake, she said.

"This is because, unlike a normal earthquake, these plate movements happen very slowly in a process known as 'slow-slip events'.

"This Kapiti slow-slip event is affecting an area spanning over 100km from Levin to the Marlborough Sounds," she said.

Conventional earthquakes happen when one side of a fault moves past the other suddenly. A similar process occurs with slow slip, except it takes much longer for the fault to move and release energy.

This is why slow-slip events are often called "silent earthquakes", Ms Little said.

GeoNet's GPS instruments in Wellington and Kapiti show that the Pacific and Australian tectonic plates were slipping past each other more rapidly than usual.

The event has been going on since January and will most likely continue for several months, Ms Little said.

"These movements represent the equivalent of a magnitude 7 earthquake on the plate boundary, at 40km depth."

Slow-slip events were a relatively new discovery but were now recognised as a common occurrence at subduction zone plate boundaries around the world.

Subduction was a process where one plate dived beneath another, Ms Little said.

Recurring slow-slip events were seen in four regions in New Zealand: Kapiti, Manawatu, Hawke's Bay, and Gisborne.

The Kapiti and Manawatu events were deep, larger events that occurred over several months to a year, recurring every five years.

In contrast, the Hawke's Bay and Gisborne events were shallower at less than 10-15km, were shorter (lasting only a few weeks), and occurred every 1-2 years, Ms Little said.

Substantial research efforts were underway by scientists in New Zealand and internationally to better understand why slow-slip events occurred, and how they impacted fault stresses and conventional earthquakes.

Capital riding out silent quake

- APNZ

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