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'Silent' quake gently rocks Wellington

Tuesday 28 May 2013 10:18a.m.

'Silent' quake gently rocks Wellington

The equivalent of a magnitude 7 quake is quietly rocking the capital, though until now you'd have only noticed it if you were a geophysicist.

The huge underground movement called the Kapiti Slip is responsible, and it's not over yet.

"This is what we call a silent earthquake," Geonet scientist Caroline Little said on Firstline this morning. "Instead of this movement happening in seconds, this will take around a year to move."

Normal earthquakes release all their energy when one side of a fault suddenly slips past the other, and can be incredibly destructive – as the people of Christchurch know all too well.

But in a silent earthquake the energy is released slowly, and in some cases can be predicted in advance.

"We started recording with the GPS network in 2002, and since then this is the third Kapiti event we've seen," says Ms Little.

"Both of the previous two have lasted around the year mark. Interestingly it's occurred every five years – 2003, 2008 and then this one in 2013."

Slow-slip quakes are relatively recent discovery, and their effect on conventional quakes is still being researched.

"It does change the stress around the area, and these types of events have been known to interact with conventional earthquakes," says Ms Little.

"We had another event like this around Kapiti in 2008, and that actually caused a swarm of earthquakes – tiny earthquakes – around the Upper Hutt region, around 30 earthquakes around magnitude 2 and 3."

It's not apparent whether they increase or decrease the likelihood of a major quake, like the one in Christchurch.

"It's both, really, because it changes the stress in different areas. It could go both ways," says Ms Little.

The Kapiti Slip is occurring 40km beneath the Earth's surface, and stretches 100km between Kapiti and Marlborough. It's not a phenomenon unique to the area, or even New Zealand.

"It happens in four regions in the North Island of New Zealand, and it also happens at all the other subduction zones around the world like Japan and North America," says Ms Little.

A slow-slip quake happens in the Hawke's Bay and Gisborne every one-to-two years, at only 10 to 15km deep.

GeoNet estimates around 40 percent of the North Island's tectonic plate movement occurs in slow slips.

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