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Aftershocks not big enough for tsunami – Civil Defence

Aftershocks from last year’s Canterbury earthquake are not likely to cause a devastating tsunami, scientists say.

Following larger aftershocks such as Friday morning’s magnitude 4.9 jolt, there were concerns from residents of a possible tsunami. However Civil Defence operations manager David Coeztee said the quakes are not large enough to generate a devastating tsunami.

“Generally, we start thinking tsunami from about magnitude 7 upwards. At least 6.5 upwards,” he said. “Of course the earthquake must be offshore and relatively shallow – less than 50km deep normally.

“It is very unlikely we will get an earthquake of that magnitude as part of the aftershocks sequence off the coast of Christchurch.”

Mr Coetzee said GNS Science had done modelling on the September 4 and February 22 earthquakes, should they have happened offshore.

He said the September 4 magnitude 7.1 quake would have created a wave no larger than 2m, while the February 22 magnitude 6.3 would have gone no further than half a metre above shore.

“[GNS Science] tell us that it is very unlikely that any of the aftershocks around Christchurch would be big enough to cause a tsunami, because they wouldn’t cause displacement of the sea floor at those magnitudes that we see now.”

A bigger risk of tsunami for Christchurch comes from quakes striking further abroad, in particular South America, Mr Coetzee said.

Fortunately residents would have 13 or 14 hours warning for such events.
“But our public advice remains, if you are near the coast and you feel a strong earthquake where it is hard to stand up or it goes on for more than a minute, then go to higher ground.

“If it is in an area as flat as Christchurch is, we say go at least one and a half kilometres inland.

“Don’t wait for the warning. We just can’t warn fast enough in some cases.”

– By Paul Harper,


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