Quakes part of evolving sequence - GNS Science

21:30, Jul 21 2013

It was not possible to predict whether the magnitude-6.5 earthquake that hit off the coast of Marlborough early last night was a harbinger of the "big one", GNS Science says.

"We don't have any information whether there will be bigger ones, but it's possible," GNS Science seismologist Stephen Bannister said.

"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."

The quake released energy equivalent to 100 nuclear bombs and was bigger than the devastating February 22 quake, he said.

The 20-second quake, which struck at 5.09pm, was centred 20km east of Seddon at a depth of 17km, which was deeper than the February 22 quake.

The damage would have been similar to that in Christchurch if it had been under a built-up area.


Most of the 230-plus earthquakes since Friday morning centred over the overlapping Australian plate along a horizontal fault in Cook Strait, and were part of "natural movements" between the Australian and Pacific plates, Dr Bannister said. Most of the shakes were 10-20km deep.

"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."

Aftershocks should be expected for several days, Dr Bannister said. But it wasn't possible to predict whether a bigger one was likely.

In November 2005, Wellington and Marlborough experienced a swarm of earthquakes over about 10 days, ranging between magnitudes 4 and 5, he said.

They were located around 15km from Seddon, with some at a depth of just 5km.

Many people have been concerned about the risk of a tsunami after the earthquakes, but GeoNet says they have not been big enough. An offshore earthquake needed to be at least magnitude-7.5 for a tsunami to be considered possible.

Victoria University geophysics professor Euan Smith said he expected 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."

The Marlborough Express