Earthquake and tsunami risks not an exact science

By Nicki Harper

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UPKEEP: A NOAA (National Oceanic and Atmospheric Administration) DART buoy being secured for maintenance. PHOTO/NOAA Teacher at Sea Programme
UPKEEP: A NOAA (National Oceanic and Atmospheric Administration) DART buoy being secured for maintenance. PHOTO/NOAA Teacher at Sea Programme

The country's east coast is an area vulnerable to earthquakes, and it's unknown if this week's magnitude 7.5 event that was felt throughout the country will trigger another similar or larger earthquake in coming days, says GeoNet and GNS Science seismologist Dr John Ristau.

Hawke's Bay is one of the more seismically active regions in the country because the east coast straddles the boundary of the Australian and Pacific tectonic plates, he said.

"The North Island sits on the Australian plate just off the east coast on the boundary of the Pacific plate which is pushing underneath - a process called subduction.

"The two plates are stuck but every now and then they do a big lurch which creates earthquakes."

When large earthquakes occur on the boundary between the two plates and one subducts underneath the other tsunamis can be caused, such as happened with the 2004 Boxing Day Sumatra earthquake and the 2011 Japanese earthquake and tsunami.

The Kaikoura earthquake was smaller than this and did not displace the seafloor enough to create anything larger than the small 25cm tsunami on the East Cape, he said.

Tsunami risk is measured using seismic and GPS data, and a network of tide gauges, he said, with DART (Deep-ocean Assessment and Reporting of Tsunamis) buoy data also used where possible.

These are used to measure tsunamis in the open ocean, where sea-floor sensors record changes in the water and measure wave height and lengths.

They then send a signal to the buoy that radios to a satellite, which sends the information back to land.

"The only problem with them is that since they are in the open ocean it's difficult to get out and service them, so it's always possible they may not be working."

He said the United States runs a network of about 30 of these buoys around the Pacific Ocean.

Still not possible to predict

This knowledge of how the plates work, however, does not give scientists the means to predict such events, Dr Ristau said.

"We can measure the rate at which stress and strain is building up on the plate boundaries, but there are still two big unknowns.

"One is that we do not know exactly how much stress these faults can hold - we can only make guesses.

"The other is that we do not know where they are at right now."

For example, he said going back to July 2009 when there was a 7.8 magnitude earthquake in Fiordland, scientists could see it put extra stress on the offshore extension of the alpine fault, raising it by three bars.

"We know the alpine fault can hold 30 bars before it goes, but what we don't know is what the original level was."

If it was at 26 it could have moved to 29, but there was no way to find out what that original level was.

"It's a big question."

Relieving pressure?

A common refrain when earthquakes occur is to speculate that such events are easing pressure - in a sense staving off "the big one".

This was a myth, said Dr Ristau.

"The way the magnitude scale works, it is logarithmic not linear.

"When you go up a magnitude unit the amount of energy released, the size of the earthquake, increases 32 times."

As such a M 4 is 32 times more powerful than a M 3, a M 5 is 32 times as powerful as an M 4, and so on.

A M 8 would be 32x32 (1024) times more powerful than a M 6.

"A magnitude 6 would not relieve things to avoid an 8 as it would take 1000 magnitude 6 earthquakes to even equal an 8," he said.

As for the Kaikoura earthquake triggering a similar size or larger earthquake, that was an unknown, only the probabilities could be calculated.

"Our aftershock forecasts based on the revised magnitude of 7.8 that were released on Thursday had the probability of a magnitude 7 or more earthquake of 3 per cent in the following 24 hours, 14 per cent in the following 7 days, and 27 per cent in the following 30 days.

"We consider it extremely unlikely [less than 1 per cent] that the Kaikoura earthquake will trigger a larger earthquake of greater than M 8 in the next 30 days."

- Hawkes Bay Today

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