Seismologist says central North Island earthquake won’t trigger volcanic activity or 'the big one'

An earthquake expert says jolts like the one felt across much of the country yesterday actually happen more often than many would expect but they are too deep to be felt.

GNS Science seismologist John Ristau told TVNZ1's Breakfast this morning that the depth of yesterday's 6.2 magnitude earthquake is significant in terms of why it was felt so widely and why aftershocks aren't a serious concern. 

"We have these deep earthquakes beneath the North Island that happen quite often - we got lots of these types of earthquakes every year - but because of the depth we don't usually feel them," he said.

"The bigger ones, they tend to be widely felt because what happens with deep earthquakes, the seismic waves spread outwards from the epicentre and when they're shallow it doesn't take very long for them to reach the surface.

"But when they're deep, it takes much longer for them to reach the surface and so they spread out from the epicentre so they tend to get felt over a very wide area."

He added the depth even played a role in why some people in Taumaranui, just 25km from the epicentre, didn't feel the quake while others in Bluff, around 1000km away, could.

"The waves, as they spread out up towards the surface, they tend not to travel so much straight up towards the surface so the people directly above it don't really feel it if they feel it at all."

Dr Ristau also said aftershocks shouldn't be an issue.

"Deep earthquakes like this, they don't typically produce in the way of aftershocks and if there are aftershocks they tend to be a lot smaller.

"When they're a similar depth, in this case some 200km, it's highly unlikely anybody would feel them if there are any."

He said those concerned the quake could also trigger volcanic activity or "the big one" need not worry either.

"It's a good reminder that we do live in a very earthquake-prone country and that people should always be prepared because you never know when the next large shell earthquake is going to happen and where it is going to happen."

Civil Defence's Sarah Stuart Black backed-up the calls for preparation.

"I think what yesterday has proven is that it's really important that we all know the right action to take because an emergency can happen at any time in New Zealand.

"Having a plan for you and your family - where you'll meet, what you'll do if an emergency happens when you're at home, at work, the kids are at school and where will you actually meet up afterwards.

"There's also the important message in an earthquake which is about drop, cover and hold, and we saw New Zealanders affected by yesterday's earthquake do that."