Earthquake warning: How New Zealand earthquakes could trigger tsunamis

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Some experts have claimed the series of earthquakes are not concerning considering how often they happen in the area. Speaking to, Professor David Tappin of the British Geological Survey explained how last week’s 5.5 magnitude earthquake off Milford Sound did not raise tsunami concerns because it was not big enough.

“[The earthquake off Milford Sound] at 5.5 was too small to directly generate a tsunami, usually tsunamis need an earthquake greater than 7.0 to trigger a tsunami, but this would be small,” Prof Tappin said.

“Something of 8 would be needed to trigger a significant tsunami and Magnitude 9, such as Japan, 2011 and the Indian Ocean, 2004.

“It is possible however, that an earthquake could trigger a submarine landslide, that if close to the coast, could cause a hazardous tsunami. But it appears from the news I have seen that this did not happen.

Prof Tappin said the country’s greatest tsunami risk exists almost exclusively in specific regions.

“In New Zealand the greatest tsunami hazard is in the east coast, mainly in North Island from earthquake tsunamis, and from submarine landslide tsunamis in South Island,” he said.

“There was tsunami on South Island in November 2016, with the earthquake destroying a lot of Christchurch.

“The Kaikoura earthquake, had magnitude (Mw) of 7.8,and the tsunami was up to 7m high.

“The tsunami could have been partly caused by a submarine landslide.”

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Tsunamis originate under the sea as the seabed is moved, creating a ‘step’ which falls rapidly.

“To cause a tsunami, the earthquake would have to move the seabed vertically.

“As water is incompressible, and earthquakes travel very fast, 3-4km/second, the seabed movement would travel almost instantaneously to the sea surface and create a ‘step’, which would rapidly collapse, causing the tsunami, which would travel outward at great speed, which would be dependent on the water depth.

“The deeper the water, the faster the tsunami. If the earthquake was close to land, the tsunami would strike the adjacent coast, very quickly, within minutes.

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“Fortunately New Zealand has a tsunami warning system, which is triggered by earthquakes.

“So there would be a warning, but as you can see from the above, it might be short. It is likely that people on the coast would have some training on what to do.

“In fact their warning system is mainly for tsunamis that are generated from the east off South America. So for these, there would be a lot of warning.”

But, according to Prof Tappin, the scale of a tsunami “depends on the earthquake size and the landslide size too”.

“New Zealand has tsunamis, but magnitude 9 earthquakes are very rare, not sure if there has ever been one.”

However, the country had protocols in place to ensure tsunamis are monitored and detected in order to act accordingly and timely.

“New Zealand is prepared for tsunamis. in fact they have recently been upgrading their warning system,” Prof Tappin added.

GNS seismologist John Ristau added the recent earthquakes are just “part and parcel” of the country.

He said: “There’s nothing unusual going on in New Zealand that doesn’t happen all the time.

“New Zealand just gets earthquakes all the time and that’s just part and parcel of New Zealand sitting directly over top of the boundary between two huge tectonic plates that are crashing together and moving side-by-side past each other.”

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