Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Big quakes no reason to pack the bags

GeoNet sensors pick up 20,000 shakes a year as part of New Zealand's regular earthquake activity.

The second big earthquake in Christchurch killed 185 people and caused extensive damage. Photo / Brett Phibbs
The second big earthquake in Christchurch killed 185 people and caused extensive damage. Photo / Brett Phibbs

Worried about all the earthquakes? Don't move to Australia just yet.

While it might seem like the shaky isles are getting shakier, scientists aren't at all surprised by the number of big quakes felt up and down the country over the past few years.

"We average around one shallow, on-land, magnitude 7 every decade and a magnitude 6 every year," GNS Science seismologist Caroline Little said.

"But we've had them at irregular intervals, with large earthquakes being relatively infrequent in the 70s and 80s."

Many of the magnitude 6 quakes go relatively unnoticed, striking deeply in sparsely populated areas.

But they could still be catastrophic when shallow enough and near built-up areas.

Ms Little said the February 22, 2011 Christchurch earthquake which left 185 people dead was a case of so many unlucky factors adding up.

Its shallow 5km depth brought the force of 15,000 tonnes of TNT and created ground acceleration about 10 times more powerful than the recent Eketahuna shake, all from a hypocentre 10km east of the city centre.

Quakes between 5.0 and 5.9 are so common that an average 2.5 are recorded each month, and quakes between 4.0 and 4.9 are a daily occurrence.

What are considered "felt" quakes happen so often that between 200 and 300 are recorded each year, and are just the tip of the iceberg when it comes to our regular quake activity.

GeoNet sensors pick up 20,000 a year.

"New Zealanders have been experiencing earthquakes since Maori settlement, so we've learned that they are a part of our life here, and grown resilient to them."

New Zealand experienced more quakes than California, but fewer than Japan.

Our country sits at the boundary of two tectonic plates, the Australian Plate and the Pacific Plate, which are constantly grinding into each other and causing stresses to build up in their brittle upper layers.

Quakes happen when the brittle rock finally breaks and the entire plate interaction zone - running the length of the South Island and up the east coast of the North Island - is potentially a source of moderate-to-large quakes.

And although Canterbury, Wellington and Marlborough had seen more than their share of shakes lately, more earthquakes, although much smaller, are generally recorded in the Mt Tongariro area and Bay of Plenty.

Fiordland, where a 6.2 quake shortly before Christmas generated comparatively few reports, also had a significantly high rate of quakes per square kilometre.

While there was fear of "the big one" landing a direct hit on Wellington, the capital was no greater exposed to quakes than Gisborne, said Professor Euan Smith, of Victoria University's School of Geography, Environment and Earth Sciences.

"The rate at which quakes occur in different parts of the country does vary, but the whole area from the East Cape right through to Wellington and into Marlborough and north Canterbury is not that different in terms of the numbers."

But there were fewer quakes in the north and northwest of the North Island, which was buffered by a zone of "gooey" rock around the Taupo Volcanic Zone, he said.

Professor Smith believed people needed to worry not about where the next big quake was going to strike, but what they could do to protect themselves, their home and family.

In most New Zealand buildings it was safer to stay put during a quake until the shaking stopped.

The Earthquake Commission encouraged people to fix and fasten any large furniture in their homes, secure floor bearers to piles and remove or cap unused brick chimneys.

Hugh Cowan, the EQC's general manager of reinsurance, research and education said: "As we learned in the Canterbury earthquakes, securing a large wardrobe, television or chimney can save lives."

- NZ Herald

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