Seddon shake bigger than Feb 22

The magnitude-6.5 earthquake released energy equivalent to 100 nuclear bombs and was bigger than the devastating February 22 quake, GNS Science says.

The 20-second quake, which struck at 5.09pm, was centred 20km east of Seddon at a depth of 17km.

GNS Science seismologist Stephen Bannister said it was deeper than the February 22 quake and located 50km away from Wellington but was larger in size.

It released energy equivalent to 100 nuclear bombs of the size that devastated Hiroshima, he said.

"If it had been under the city, we would have been looking at equal damage or close to what happened in Christchurch."

By comparison, the February 22 quake was shallower and closer to that city's centre - 5km deep and just 10km southeast of Christchurch.

Most of the 230-plus earthquakes since Friday centred over the overlapping Australian plate along a horizontal fault in the Cook Strait and were part of "natural movements" between the overlapping Australian and Pacific plates, Bannister said.

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

With earthquakes of this size, aftershocks should be anticipated for a number of days, Bannister said.

But it wasn't possible to predict whether this was a harbinger of the "big one".

"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. The Hutt Valley was also on softer ground.

"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."

Smith anticipated that the quakes were an "ongoing sequence", with yesterday's 6.5 quake likely to have been the main event.

"That quake will have its own aftershocks independent of the earlier shakes.

Everyone should be prepared for these to continue for quite some time."

Fairfax Media