Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Science behind the quakes

William Alexander sent us this photo of a massive boulder that fell at Castlepoint yesterday.
William Alexander sent us this photo of a massive boulder that fell at Castlepoint yesterday.

Yesterday's 6.2 quake near Eketahuna was produced by separate seismic mechanisms than last year's Cook Strait quakes, and was about 10 times less powerful than the February 22 Christchurch quakes.

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Scientists are now planning to deploy more seismic monitors in the area and say more aftershocks will follow.

"In the first week of the aftershock sequence, we estimate that there will be about 23 aftershocks of magnitude 4.0 to 4.9, with the final number probably falling between 14 and 33," GeoNet reported.

"We also estimate about two aftershocks of magnitude 5.0 to 5.9 with the final number probably between zero and five."

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As with any aftershock sequence, scientists could cannot rule out the possibility of occurrence of future larger earthquakes.

The aftershock region includes most of the lower North Island.

But the sequence was not likely to last as long as that which followed the rare doublet of severe quakes near the Cook Strait last year, which were triggered by a separate process.

That quake, felt between Auckland and Invercargill and with a depth of 33km, was a product of the inter-plate "high stress zone"that spans the North Island's east coast from Wellington to the East Cape.

GNS Science seismologist Dr Caroline Little said the ground acceleration caused by the now-named Eketahuna Earthquake was also 10 times less powerful than the February 22 Christchurch earthquake.

Because of the geological nature of the area where the quake struck, scientists were not surprised by it.

The quake happened within a part of the Pacific plate called the "subduction slab"- the upper portion of the plate which is brittle and can therefore sustain earthquakes.

Beneath New Zealand, the Pacific Plate is moving to the west-southwest at a rate of about 50mm each year - about the speed a fingernail grows - and constantly grinding against the Australian plate, causing stresses to build up in the brittle upper layers of the plates.

GeoNet scientists locate about 20,000 earthquakes caused by this process each year, of which 250 are felt.

Over the past century, around 35 earthquakes of magnitude 5.5 had occurred within 200km of yesterday's epicentre.

It struck just a few kilometres east of the site of a 7.3 quake in March 1934, and approximately 40 km southwest of two 6.2 and 6.4 earthquakes near Dannevirke in March and May of 1990, respectively.

The US Geological Survey reported yesterday's quake resulted from northeast-southwest oriented "normal faulting".

This form of faulting occurred where Earth's crust was being pulled apart, and the overlying crustal block moved down relative to the block below the fault.

Professor Tim Stern, of Victoria University's Institute of Geophysics and School of Earth Sciences, suspected yesterday's quake was an example of the same seismic processes that caused the 2009 Samoa-Tonga tsunami.

"As the plate bends over, it's under tension at the top and compression deeper down, so you get both extension near the top of the plate and compression beneath," he said.

"You can imagine a beam that has got a free end, and when you bend it, it creates stresses. Or if you bend a Mars bar, you'll get cracking at the top and underneath."

Normal faulting was common in New Zealand quakes.

The process caused the two powerful earthquakes that struck Wairarapa in 1942, causing substantial damage in many towns across the lower North Island.

It was a separate process to "strike-slip"process which triggered the Cook Strait quakes, where each side of the crustal block slides past the other without uplift and down-thrust.

It was also a different process to the most severe quake experienced in recent New Zealand history - the 1855 Wairarapa Earthquake, its magnitude a massive 8.2 - that led scientists to establish the relationship between faults and quakes.

The seismology team at GNS Science is also attempting to understand the earthquakes in the context of recent and ongoing seismicity.

Specifically, efforts are aimed at comparing the current earthquakes with a sequence of events that occurred to the north of Eketahuna in the early 1990s.

Scientists are also considering possible links to the ongoing Kapiti "slow slip event", plate movements to the west of Wellington, 40km below the ground.

The slow slip event, being constantly observed by GNS scientists, was the equivalent of a magnitude 7.0 earthquake, but because of its very slow movement, nothing was felt at the surface.

- NZ Herald

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