Earthquake: GNS explains

09:51, Jul 21 2013

This evening's magnitude 6.5 earthquake released energy equivalent to 100 nuclear bombs of the size which devastated Hiroshima, scientists say.

GNS Research seismologist Stephen Bannister said the quake, which was centred in the Cook Strait, 20 kilometres east of Seddon at a depth of 17 kilometres, was one of more than more than 230 earthquakes recorded in the area since Friday, with about 15 rating above magnitude 4.

It was likely linked to a fault in the Cook Strait capable of generating far more severe shaking, GNS Science said.

Dr Kelvin Berryman, GNS Science director of Natural Hazards, said the severe shaking over the past three days was probably linked to one of several faults capable of generating shakes of magnitude 7.0 or more.

The most likely candidate was the Needle fault, although the location of the quakes over the past three days did not quite match up.

He said a larger quake was unlikely but the swarm that kicked off on Friday morning was not following the normal aftershock pattern, in which the tremors became progressively less severe.


This afternoon's 6.5 magnitude shake, the biggest in the latest swarm, meant the shaking was not following the usual pattern, he said.

GNS should have a clearer idea of the probability of another big, or even bigger, quake by tomorrow, he said.

Bannister said a magnitude 6.5 earthquake releases energy equivalent to 100 nuclear bombs of the size which devastated Hiroshima.

"People will be feeling shocks that register above [magnitude] 3, and we have had about 50 of those since Friday.

"It's not a simple main shock followed by aftershocks, it's an evolving sequence," Bannister said.

He said with earthquakes of this size, aftershocks should be anticipated in the coming days.

"We would just encourage people to make sure they have their emergency kits and rations ready for future events. We can't say how many more large ones to expect."

Victoria University geophysics professor Euan Smith said much of Wellington's city centre was reclaimed land which meant it was not as stable as other areas.

"Where rubble has come down in Featherston St, there was no land there before the 1855 earthquake in Wellington.

"Reclaimed land shakes more easily and is not as strong as hard ground. From Lambton Quay and towards the harbour is also reclaimed land, and these areas will shake much more strongly than other areas."

The Hutt Valley was similarly on softer ground.

"If this evening's was the first then I would say we could expect them to decay in the usual way, but given we had the earlier ones we should still be anticipating that there could be quite large earthquakes for the next few days."