Tsunami threat heightens

The Bay of Plenty is more at risk of being hit by a tsunami than first thought, with the coastline between Mauao to Maketu having been identified as one of the region's most vulnerable spots.
A geological study found the greatest threat to the Bay was from tsunamis originating close to shore, such as an eruption of Mayor Island, a fault movement in the offshore Taupo Volcanic Zone or a landslide in the underwater Hikurangi Trough which was 250km to 300km away.
Six major tsunamis have hit the Bay over the past 4000 years, each producing waves of at least five metres.
Stephen Lamb, Environment BOP senior resource planner, said the study's results showed the region had no automatic exemption from the impact of tsunamis.
"They have occurred in the past and are likely to occur again," he said.
A preliminary report prepared by the National Institute of Water and Atmospheric Research has been given to Environment Bay of Plenty and has named five areas along the region's coastline from Coromandel to East Cape that are more exposed than others to a tsunami.
But the report said more geophysical information and modelling was required before the relative vulnerability of the areas could be determined.
The other areas named were:
Open coast from Otama Beach to Port Charles and out to Great Mercury Island.
Mercury Bay, especially Whitianga.
Open coast between Matata and Torere.
Papatea and Whangaparaoa Bay near Cape Runaway.
The report said those areas were more vulnerable than others because incoming tsunami waves could "pick out and excite the resonance" of a harbour or bay, causing the wave to grow in height and persist longer in these areas compared with other parts of the region's coastline.
Environment BOP and Environment Waikato began researching the region's tsunami hazard just over two years ago.
They wanted to know more about the potential threat to help them with civil defence planning.
The report, presented to Environment BOP's strategic policy committee, detailed the second stage of the project's findings which linked geological discoveries with historical records of tsunami events.
The aim was to pinpoint where future tsunami threats could come from.
The final stage of the project will clearly define the areas most at risk and present options for managing the tsunami risk.
Apart from the six major tsunamis, four smaller ones between 1m and 3m high occurred in August 1868, May 1877, August 1883 and May 1960.
The report mentioned 11 smaller tsunamis since 1840, the most recent of which occurred in June 2001 and July 1998, with wave heights of less than half a metre.
The largest tsunamis to hit the Bay and eastern Coromandel coasts in the past 160 years were generated by remote or distant sources and took longer than three hours to travel to New Zealand's shores.
The 1868, 1877 and 1960 tsunamis were caused by large earthquakes in the subduction zone along the Chile and southern Peru coastlines - directly opposite the east coast of the North Island.
An atmospheric pressure wave from the Krakatoa eruption in Indonesia probably caused the 1883 tsunami, generating a 1.8m-high wave that hit the Bay and Coromandel coasts.
The NIWA report said there had not been any tsunamis caused by "local sources" since 1840 but this was not unexpected as major earthquakes and volcanic eruptions rarely occurred.
Mr Lamb said a review currently being carried out of tsunami return periods would help assess the level of future risk.