Tsunami!

 

A modern jet airliner flies at about 750 kph, which happens to be, more or less, the speed at which a tsunami may travel over the deep ocean where it may rise a mere meter above the surrounding sea. As the wave reaches the shallows, however, things get a lot more violent. Evidence and the cause of past tsunami events on Great Barrier has already been the subject of much study.

The name ‘tsunami’ is Japanese and means ‘wave in the harbour’. Such a wave is created by a variety of disturbances such as earthquake, volcanic activity, underwater landslides or explosions, meteor impacts, even nuclear explosions. They can have a wavelength from crest to crest of over 200km. For instance, a 200km wave travelling at 750kph produces wave periods of about 16 minutes.  The passage of the first wave will almost certainly not be the last, nor necessarily the biggest. Hours may pass before the waves subside. …Get high very quickly indeed!

Various sites existing on the Northland coast have preserved deposits from past tsunami events, but the most convincing site is an area of the east coast of the Barrier. Work here within the last few years by Dr Scott Nichol of the School of Environmental and Marine Science and the departments of Anthropology and Geology has led to the discovery of evidence that our east coast was struck by tsunamis that left a substantial deposit of gravel 14.3m or 50 ft above sea level as far as 200m inland. Analysis of the deposits suggest they may well have occurred when Maori settlers were using the area around 500 years ago, and come roaring ashore from a south-easterly direction.

Tsunami threats to the Barrier could come from a major eruption of one of the Kermadec Ridge volcanoes, and be compounded by the short possible warning time of 1-2 hours before it hits us. The Pacific Tsunami Warning Centre in Hawaii takes an hour or so to interpret and warn of Pacific-wide events that would in most cases give many hours warning to impending victim areas.

Seamounts off the Barrier such as the Rumble III undersea volcano, have become active as recently as 1986, when unusual signals poured into the Naval Listening station above Palmers Beach.

Two recently published books, of an excellent standard, cover all you need to know about the above and much much more -     

‘The New Zealand Coast Te Tai O Aotearoa’ by James Goff, Scott Nichol & Helen Rouse. ISBN 0-86469-438-5 312pp, 2003, Dunmore Press.

‘Deep New Zealand – Blue Water Black Abyss’ by Peter Batson, ISBN 1-877257-09-5,  240pp Canterbury University Press.