All eyes on Wairarapa earthquakes

By Nathan Crombie nathan.crombie@age.co.nz -
1 comment
Wairarapa faultlines are being closely scrutinised. PHOTO/FILE
Wairarapa faultlines are being closely scrutinised. PHOTO/FILE

SCIENTISTS tracking New Zealand earthquakes have trained their eyes on active faultlines reaching into Wairarapa and "won't blink" for years to come, says GNS Science seismologist Dr Bill Fry.

Similarities had been established between Pongaroa faultline tremors that included a magnitude 5.1 quake on January 4 and seismic activity on the Eketahuna fault including a magnitude 6.2 earthquake on January 20 in 2014, Dr Fry said.

He said the fault at Pongaroa in the far north of Wairarapa had displayed seismic behaviour "genetically similar to the Eketahuna sequence in that it was in the subducting slab" despite being about 60 kilometres apart.

"They look relatively similar from a genetic perspective but they are locationally independent and quite distant. They are not so close that I would call them the same sequence, I wouldn't call them a migrating sequence either. I would say they are two similar sequences," he said.

"We've been looking at this on and off for the last few years. But it's just been the last three or four months that it's been particularly active at Pongaroa. The Eketahuna fault has been busy too and we expect it to have increased activity over coming years as well."

He said the Pongaroa and Eketahuna faultlines were seen as part of the megathrust fault that New Zealand straddles, which separates the Pacific and Australian tectonic plates.

The Pacific plate is subducting under the Australian plate, he said, and the interface is called the megathrust "and that's where we get the very large earthquakes around the Pacific like we saw in Tohoku in Japan in 2011 (magnitude 9) or in Chile in 2010 (magnitude 8.8)".

"Our best information suggests it's the movement of the Pacific plate on this megathrust fault that's actually loading the system, that's providing the stress into the system that drives these other earthquakes."

There are several ways a large quake may trigger other significant tremors, he said, and "depending on the mechanism, the odds are different" of other quakes following in sequence.

"When you get a shallow crust earthquake like the February 22 Christchurch earthquake, you tend to get a lot more aftershocks.

"When you get a deeper earthquake that happens in the slab, you tend to get fewer aftershocks. Of course, you do get exceptions to those rules, but generally that's what we see."

Dr Fry said there was a network of seismometers dotting the country, with stations near an earthquake helping to define the depth and whether the tremor was in the slab or overlying crust, and more distant stations pinpointing the epicentre of the shake.

"What happens when we get one of these earthquakes, especially as large as what hit Eketahuna, this triggers motion across the country and most of our seismometers record that movement."

The greatest known tremor in New Zealand was reckoned to be a magnitude 8.2 quake that struck on the Wairarapa fault on January 23, 1855, he said. The intensity was calculated from extant evidence from the time.

The massive shake was a "crustal earthquake that happened on a strike-slip fault", he said, which some scientists now contend may be linked to the megathrust fault.

"That earthquake was mostly side to side movement and very shallow. Now there's some scientific evidence suggesting it could have happened at the same time that movement happened along the megathrust."

Dr Fry believed the chances were "quite low" of a similar simultaneous tremor involving the same faults.

The Wairarapa fault is relatively shallow, he said, extending downward no more than about 15 kilometres in the midst of the about 40-kilometre deep seismogenic zone, which is the crustal layer where most earthquakes originate.

He said the greater focus on the faults at Pongaroa and Eketahuna - as part of the megathrust rift - will continue for years yet.

"We're constantly monitoring this and actually most of my scientific work now is involved in looking at the megathrust. So for the coming years we won't blink."

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