Tsunami threat to Bay of Plenty real

Tsunamis may pose more of a threat to the Bay of Plenty than previously thought, according to a report.

A geological study of the region's coastline has found traces of six major tsunami events over the past 4000 years, each at least five metres in height. Eleven smaller tsunamis - less than 3m - have been recorded officially since 1840.

Environment Bay of Plenty and Environment Waikato initiated a joint research project two years ago to assess the tsunami hazard for the eastern seaboard of the Waikato-Bay of Plenty from Colville Channel to East Cape. The councils wanted to understand more about the threat of tsunamis to help them with civil defence planning.

Stage one of the project involved studying sediment cores for signs of early tsunami activity.

Environment Bay of Plenty senior resource planner Stephen Lamb presented the results of the second stage, analysing the general nature of the threat, to the council's strategic policy committee on Tuesday.

He said phase two linked the findings of geological work with more recent historical records. It studied the origins of local tsunamis, their causes and the types likely to have most impact. In the past 160 years, the most substantial tsunamis to have affected the Bay of Plenty and eastern Coromandel areas were generated by "remote" or distant sources. The biggest, in 1868, 1877 and 1960, were triggered by very large earthquakes in the subduction zone along the Chile and southern Peru coastlines of South America - directly opposite and facing New Zealand's eastern seaboard.

Another tsunami in August 1883 was probably generated by an atmospheric pressure wave from the Krakatau eruption in Indonesia.

The Niwa report before the committee suggested the greatest risk to the Bay of Plenty was from tsunamis originating close to shore, such as from an eruption of Mayor (Tuhua) Island or a fault movement in the offshore Taupo Volcanic Zone.

A tsunami of that type could reach the coast in 30 to 60 minutes.

White Island (Whakaari) has previously been discounted for tsunami generation potential due to its deep-water location. Any big waves are expected to disseminate eastward away from the coast.

Mr Lamb said it seemed that local or near-shore events were likely to be more destructive than those generated at a distance.

Tsunamis originating from further afield, such as a landslide in the underwater Hikurangi Trough 250km to 300km away, would take two to three hours to reach the coast. Events of distant origin, say from a South American earthquake, could take 12 hours.

Strategic policy committee chairman Bryan Riesterer said tsunamis were "at the forefront of people's minds" because of the Boxing Day devastation in the Indian Ocean.

The regional council was already addressing the risks to the Bay of Plenty through the three-year research project, he said.

The final stage, to be commissioned soon, would define the most threatened areas of coastline and give constructive options on how to mitigate the risks.