Study shows three huge tsunamis have hit east coast in past 1200 years

Thursday, 11 August 2016

A 90-metre-long trench used by GNS scientists on Puatai beach, north of Gisborne, has shown the East Coast has been hit by three huge tsunamis in the past 1200 years.

A new study shows the East Coast was struck by three or four large tsunamis as high as twelve metres over the past 1200 years.

The GNS Science study which involved the digging of a 90metre long, two-metre-deep, trench at Puatai beach, 30km north of Gisborne, revealed shells and wood that could be radiocarbon dated to establish the dates of numerous earthquakes and tsunamis.

East Coast beaches such as Waikanae beach in Gisborne, were hit by huge tsunamis that traveled up to hundreds of metres inland.

The tsunamis produced waves between nine and twelve metres high and would have travelled up to hundreds of metres inland. The last tsunami occurred about 300 years ago.

Scientists found better evidence than they expected over their past two years studying the trench.

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'The study provides the first really concrete evidence of what we have long thought likely, and it shows that hazard preparation in the east coast area is on the right track,' said lead author Nicola Litchfield.

'The tsunamis would have affected the entire East Coast of the North Island. It is hard to estimate from just this one study how big a particular tsunami would have been at various places along the coast. However, we know from past fieldwork and from modelling that 5metre-plus tsunamis can strike anywhere along the East Coast,' Litchfield said.

It has been known for a long time that the area is seismically active and that there is a high risk of tsunamis, but hard evidence about tsunamis has been rare. The study provides solid evidence that the tsunamis occurred.

The investigation showed there were three large earthquakes in the region around 1800, 1200 and 400 years ago. Each quake was around magnitude 7.2 and uplifted coastal land by about 3.5metres.

'The average recurrence interval is 750 years, suggesting the next quake from this source is probably not imminent,' Litchfield said.

The study said there were a number of offshore faults in the region and any one of them could have produced the tsunamis.

'It is also possible they came from faults further afield, such as near South America… Further fieldwork could pinpoint the sources of the tsunamis,' Litchfield said.

The study will be presented to Gisborne District Council and civil defence officials in November.

Gisborne Mayor Meng Foon welcomed the report's findings.

'It's good for us to all be aware of the potential of tsunamis because the East Coast of New Zealand, and more particularly Gisborne are areas of high risk. We've had about 13 recorded minor tsunamis in modern times in the region. These ones in the study though are monstrous. If one of those came we'd be pretty knackered,' he said

'We'll continue educating people about the areas they need to go to and tell them to be aware of tsunamis, both created close to the shoreline and those that have taken 10-12 hours to travel across the Pacific,' Foon said.

Last year, scientists found proof that central New Zealand could be counting down to a highly damaging 'megathrust' earthquake, generated in the southern part of the Hikurangi Trench, offshore Wairarapa, Wellington and Marlborough.

Those researchers, from GNS Science, Geomarine Research in Auckland and the University of Texas, calculated that in the past 1000 years two subduction quakes of at least magnitude 7 or 8 occurred - one between about 880 and 800 years ago and the other between 520 and 470 years ago.

Earlier research suggested a magnitude 8.9 megathrust subduction quake could cause more than 3000 deaths and 7000 injuries in the Wellington region and cost around $13 billion.

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