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Friday, March 4, 2011

Wellington shook more before Christchurch earthquake

Wellington shook more before Christchurch earthquake

Last updated 05:00 05/03/2011
Wellingtonians anxious that the city seems shakier can think again – the capital trembled more times in the seven days before Christchurch's crippling quake than in the 10 days after.

But the magnitudes of the quakes have increased slightly, with two big shakes in the space of four days leaving residents feeling rattled.

On Tuesday night a 4.5 magnitude quake sent some panicked people fleeing into the streets.

And at 2.19am yesterday people were jarred awake when a 4.7 quake struck, sparking fears that the big one is imminent for Wellington.

Many people took to Twitter to voice their concerns yesterday.

"Another quake in Wellington – 4.7. Second one there in a week. Getting far too close to home," Konrad Peszynski tweeted.

But a seismologist says there is no correlation between these quakes, Christchurch's 6.3 quake, or an impending "big one".

Records from GeoNet show there were more earthquakes in the week before February 22 than there have been since.

Wellington had 14 earthquakes in the week before Christchurch's quake, compared with 10 in the past 10 days.

But there has been a slight increase in magnitude since February 22 – in the week before the range was from 2 to 3.8; since the Christchurch quake, the range has been from 2.2 to 4.7.

GNS Science seismologist John Ristau said it was simply a coincidence that Wellington had felt two large jolts since Christchurch's quake.

"Earthquakes like this for the Wellington region are not unusual. You get a few earthquakes of this size and about 30 to 40 kilometres deep each year. It just so happens that there were two fairly sizeable ones in a few days."

Geographically, Wellington is too far away from Christchurch for its tremors to have any impact on the capital's fault lines, he said.

Fears that Wellington was heading towards "the big one" were also unfounded.

There was no doubt Wellington would be struck by a large earthquake one day, because it sits on several faults, including the Wellington fault where the Australian Tectonic Plate meets the Pacific Plate.

These quakes were normal and would not trigger a massive event, Dr Ristau said.

"Wellington is very prone to earthquakes, and there would be a major earthquake in Wellington, we know that, but these earthquakes, they don't suggest that a larger earthquake is imminent."

There were about 15,000 quakes in New Zealand each year – about 280 a week – and only about 250 of those were actually felt.

There was no way of predicting when a big quake would strike, he said.

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