Big shake was complex and may have been two earthquakes

MORNING REPORT/RADIO NEW ZEALAND

Waves of around 30 centimetres and were detected on the East Cape and Great Barrier. GNS seismologist Bill Fry explains what that means and what the risk of a tsunami is.

Scientists are investigating the possibility Friday's big shake off the East Cape may have been two earthquakes.

The complexity made it hard to estimate how deep the quake was, and to identify whether there was any tsunami risk.

Concerns have also been raised about the time taken to issue a tsunami warning after the quake - reported as a magnitude 7.1, which struck at 4.38am about 125 kilometres from land.

Interactive: Mapping the East Coast earthquake swarm depicting all of the shakes on the morning of September 2.

The Ministry of Civil Defence and Emergency Management (MCDEM) sent out its first national advisory at 5.10am but that was only about the earthquake.

READ MORE:
East Cape earthquake - full coverage
* Panic as 7.1 earthquake strikes East Cape
* East Cape earthquake - what you need to know

A potential tsunami threat notice was not issued until 5.33am, and a request for an emergency announcement to be broadcast was not made until 5.58am.

USGS map showing the severity of the shaking after Friday's big quake(s).
USGS

USGS map showing the severity of the shaking after Friday's big quake(s).

GNS Science said the maximum of the first tsunami wave was measured on an East Cape tide gauge at 4.58am. It was about 24cm above the normal tide level.

GeoNet boss Ken Gledhill said everybody, including overseas agencies such as the US Geological Survey, had trouble working out the depth of the quake.

"We think it's the complexity. We think it might have been two earthquakes on top of each other," Gledhill said.

GeoNet map showing the swarm of quakes on the East Cape and offshore.
GEONET

GeoNet map showing the swarm of quakes on the East Cape and offshore.

"Because of the complexity of the quake, models used by some agencies to make fast tsunami warnings would have been wrong.

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"Everybody got it wrong, even international centres that do it every day."

The size of the quake was "right on the cusp" of whether tsunami action needed to be taken, he said.

"By itself the earthquake didn't pass the criteria for a tsunami warning. That was the issue." It was not apparent until sea level measurements started coming in that something needed to be done.

Had the earthquake been larger it would have been easier to issue a tsunami warning quickly. "As it was we had to wait until we had some confirmation (of a tsunami). It wouldn't have really changed much if we had a different system in place," Gledhill said.

The quake was a bad example for anyone suggesting change was needed to the system used to make tsunami warnings.

Labour's Civil Defence spokeswoman Clare Curran said the delay in sending out a warning was a concern, and could have been more serious if the tsunami had been major.

"That's not a direct criticism of Civil Defence, because they're dealing with the systems that they've got in place and they have to make judgement calls...but it could be the difference between life and death, depending on the circumstances."

Curran said other countries already had the technology for automatic warning systems, but the Government had spent millions trying to create a bespoke system only to "come up with nothing and start from scratch again".

MCDEM director Sarah Stuart-Black said the initial earthquake alert from GNS Science and a magnitude upgrade from 6.3 to 7.1, at 4.41am and 4.53am respectively, both failed to meet the threshold for an tsunami threat.

The Pacific Tsunami Warning Centre did not indicate any risk in their initial assessment, while MCDEM "proactively" sent out a potential tsunami threat notice while it waited for scientists to confirm the risk.

'DON'T WANT TO CRY WOLF'

"There's a real challenge there where it's close to the line between a potential threat, an actual threat or no threat, is whether you end up looking like you're crying wolf if you don't get it right."

Stuart-Black said MCDEM would look at the alerts from the day to determine whether changes were needed, while it was already reviewing its processes for national warnings and public alerts as part of a project.

In a statement, Civil Defence Minister Nikki Kaye said: "After any major event we look at potential lessons learned, and this will happen following today's event."

'PUT GEONET IN CHARGE'

Philip Duncan, the head weather analyst at WeatherWatch, is calling for GeoNet to be given the responsibility for issuing threat warnings.

The time taken to send out a tsunami warning showed Civil Defence, at least from a ministry point of view, was "far too slow in these highly critical moments", Duncan said.

GNS scientist David Burbidge, who analyses tsunami hazards, said a tsunami was a series of waves over a long period of time, 
usually with the biggest wave first. That's what happened on Friday, with the second and third waves being much smaller and no threat.

"It was a really complicated earthquake and took everyone quite a while to get an estimate of how deep it was," he said.

"What happens during an earthquake is that the seafloor goes up and potentially down. If you uplift a mound of water, it doesn't like staying as a mound so it collapses and then moves the ocean towards the shore, in this case, and you get a surge of water towards the shore."

An earthquake the size of Friday's could produce unpredictable and quite large currents, particularly in harbours, Burbidge said.

The challenging day for MCDEM comes as the agency looks for someone to lead and coordinate its policy advice - as part of the National Security Policy team at the Department of the Prime Minister and Cabinet.

Among the many talents required is "a cool, calm approach, particularly in times of stress". The successful candidate will have to be able to get a Top Secret Special security clearance. To do that he or she must have been a New Zealand citizen, or a citizen of UK, Canada, USA or Australia, for at least 10 years. 

The job pays from $116,290 to $145,362.

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