Likelihood of tsunami impossible to predict

From the Otago Daily Times By Debbie Porteous on Sat, 3 Oct 2009

Tsunami warnings
> Civil defence organisers are reliant on wave gauges in the Pacific Ocean.

> The closest to New Zealand is near Raoul Island, 1100km northeast of the North Island.

> Information from the Pacific Tsunami Warning Centre in Hawaii is also received.

> Otago's largest tsunami threat is from the Puysegur Fault, southwest of the South Island.

> In the event of an earthquake, the Pacific Tsunami Warning Centre issues an alert informing authorities of the magnitude and depth of the quake.

> Local authorities decide how to react, based on that information.

So many factors determine whether an earthquake will cause a tsunami even scientists cannot predict with any certainty if one will be generated and what impact it will have, a Dunedin civil defence spokesman says.

Dunedin City Council civil defence training officer Glenn Mitchell said if a tsunami was generated by an earthquake at the Puysegur Fault, southwest of the South Island, authorities would have about three hours to warn people to move to higher ground.

The fact the fault has significant tsunami implications for the Otago coast, especially the southern part, was first noted in a 2008 Niwa report commissioned by the Otago Regional Council.

It was previously thought the worst danger came from earthquakes off South America, but for the first time the research showed the Otago coast would be more inundated by a tsunami from the south. Results showed an earthquake of magnitude 8.5 at the Puysegur Fault would generate waves of 2m-4m along coastal areas from the Catlins to Oamaru and could produce waves for eight hours. Such an event was predicted to occur every 600 years. An earthquake at the fault would to have to be at least magnitude 7.8 to generate a tsunami of note. There was no evidence of a tsunami generated from this fault in historical records. Tsunamis generated from South America could still cause less, although still significant, inundation, but were more common, occurring once every 100 years on average.

The coast north of the Otago Peninsula was more likely to hit by a tsunami from South America, which could still be around 3m high, despite travelling so far. For that to happen, the outgoing wave from the earthquake would have to be pointed directly at the Otago coast. Three tsunamis recorded in New Zealand have been generated by Chilean earthquakes. In any future tsunami, the amount of inundation and the speed of the water as it hit land would probably be affected by rising sea levels and erosion of the coast, the report said.

At the Dunedin civil defence headquarters, this new information about the Puysegur Fault had not led to any changes to monitoring or response processes, just raised awareness that the risk existed, Mr Mitchell said. The response to a tsunami warning would remain the same, with information broadcast on local radio networks and distributed to other media outlets as quickly as possible. Civil defence would then organise warning sirens to be sounded in areas likely to be affected as a signal to people to listen to their radios for information. Whether or not that happened depended on several factors, including where the tsunami was generated, which would give an indication of how much time there was to take action.

In the (even rarer) event an earthquake at the Akatore fault, which runs along Otago's coast, triggered a tsunami, there would be virtually no time for any warning.

While council civil defence staff spoke to media during this week's Pacific plate tsunami alert, no media releases were distributed, a situation Mr Mitchell admitted authorities "could have handled better". In this case, the decision was that by 9am the threat had all but disappeared and so no media releases were required, although civil defence continued to monitor the situation, he said.

Updated information had been available on the council website, as it would be during any civil emergency. In the situation of an actual threat, media releases would likely be made available at the same time radio stations were asked to begin playing instructions. "Basically, if you are not hearing any information, then there are no concerns."

Gauging the impact of a tsunami was complicated, involving so many factors, including tidal situation, time of day, the depth and magnitude of the quake, even geological scientists could not make accurate predictions, Mr Mitchell said.

Acting Prime Minister Bill English this week said the Government was "generally happy" with the response of New Zealand Civil Defence to Wednesday's tsunami warning, although there would be a inquiry into why there was some confusion.

A number of vital agencies received confused warnings or no information at all from the Ministry of Civil Defence immediately after the Samoan earthquake and subsequent tsunami, NZPA reported.

It was better if there was no confusion, although Mr English said he was surprised people went to beaches after warnings were issued, meaning staff had to be deployed to get them to go home. The emergency response body has been criticised for not advising local media and Wellington airport what was happening with regard to tsunami warnings on Wednesday.

Civil Defence issued a tsunami warning about an hour after the magnitude 8 quake off Samoa, which hit at 6.48am. It downgraded it to a threat advisory shortly after 11am. Civil Defence Minister John Carter said it was largely up to civil defence controllers in the regions to set up and manage warnings in their areas.

Civil defence systems were continually improving, he said. "It's an imprecise art. It's an act of nature and we do our best, obviously not good enough, and we will be responding to it," Mr Carter said. I think Civil Defence weren't as helpful as they could have been in communicating with the media in the early stages."