Up to 33 aftershocks likely: GeoNet

RUINED: Paul Dickens beginning the clean-up of his office at his Alfredton home after the earthquake. PHOTO/MARK MITCHELL

Monday'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.

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."

As with any aftershock sequence, scientists could cannot rule out the possibility 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 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.

In 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 about 40km southwest of two 6.2 and 6.4 earthquakes near Dannevirke in March and May of 1990, respectively.

APN News & Media