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Research reports reveal Kaikoura tsunami hazard

March 19, 2014
Research reports reveal Kaikoura tsunami hazard

Significant coastal inundation as well as flooding in parts of Kaikoura township are likely if a large South American earthquake sets off a tsunami. This and other findings are included in the latest report on Tsunami and Inundation Modelling for Kaikoura and North Canterbury released today by Environment Canterbury,

At the same time, Environment Canterbury is releasing a report on Kaikoura’s Landslide Tsunami Hazard. This report covers Phase-1 of a study into the risk of tsunamis generated by landslides from the unstable sea floor at the top of the Kaikoura Canyon. It does not include any modelling of new landslide-tsunami events, which will happen in the next two phases of the research due for completion later this year and next year.

Both reports are part of a region-wide tsunami-hazard investigation involving a partnership between Environment Canterbury, NIWA, and scientists from the Universities of Canterbury and Bremen. The Kaikoura District Council and Canterbury Civil Defence Emergency Management Group have also been involved.

“There is a risk to the Kaikoura township and along the coast, especially around the Clarence River Mouth as far as Kekerengu, which will need to be managed,” says Commissioner Donald Couch. “A tsunami generated in a South American earthquake will take several hours to get here, which gives us some warning, but if a tsunami is triggered in the Kaikoura Canyon, there will be only minutes to get to higher ground.”

Mr Couch says the reports will be used as part of civil defence emergency preparedness in Kaikoura. An opportunity to discuss the reports will be given at the Kaikoura District Council Community Preparedness/Tsunami Information road show on March 19 and 20.

“I’m pleased that the findings of these reports are being shared with the local community,” Mr Couch says. “Knowing more about the risk means the community knows what to expect and can therefore be better prepared.”

The distant-source report says the South American-generated tsunami would flood parts of Kaka and Weka Streets in South Bay, and Fyffe and Wakatu Quays in Kaikoura.

The second wave would be the largest, travelling at around a metre a second, the report says. The modelled wave was based on the 1868 Aria (South America) tsunami, the largest inundation historically, under the assumption it arrived at high tide.

The second report, into the stability of the sea floor at the top of the Kaikoura Canyon, concludes that there is potential for large landslides to occur in shallow water, very near the coast to the south of Kaikoura Peninsula.

“This is a unique situation in New Zealand, where a very shallow (as little as 30 metres deep) canyon edge is less than a kilometre from the coast in an area with coastal populations and nationally critical infrastructure,” says Dr Joshu Mountjoy, NIWA marine geologist and project leader. “This research builds on earlier work in the area using state-of-the-art tools that allow us to really see what is happening on the sea floor, and in the sediment and to assess the landslide-tsunami hazard”.

Such landslides are expected to occur during or after an earthquake and could generate a tsunami that would arrive in Kaikoura very quickly. “If a large earthquake is felt in the area, that is the most likely warning sign that a tsunami could eventuate,” Mr Couch says. “If Kaikoura coastline residents feel an earthquake so strong they can’t stand up or one that goes on for more than a minute, then move to higher ground. And do the same if you see the sea suddenly going out or rushing in.”

The first phase of the Kaikoura Canyon study has identified areas of sediment around the canyon rim that could fail during an earthquake, and demonstrates that this is a very active area of sediment movement into the Kaikoura Canyon.

“We have had to re-evaluate previous models for a landslide-generated tsunami here, but have identified several locations where shallow-water sediment bodies at the edge of Kaikoura Canyon pose a potentially hazardous scenario,” says Dr Joshu Mountjoy.

“It’s very important to understand how big these tsunamis could be, and to determine where and when they might occur,” he says.

In February this year, Dr Mountjoy and the research team were working on NIWA’s research vessel Ikatere off the Kaikoura Coast on the second phase of the Kaikoura canyon project, collecting a large amount of geotechnical data. This will be used to determine the size of potential failures and computer simulations will assess the stability of the area during strong earthquakes. This work will underpin further tsunami modelling as part of Phase-3 in 2015.

This local-source tsunami report is in response to the findings of a 2004 report by NIWA for Environment Canterbury, which describes the actual make-up of the sea floor and identifies areas that could start a submarine landslide and tsunami for further investigation.


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