Bigger tsunamis predicted

02:01, Sep 30 2013
A Civil Defence tsunami hazard zone notice at the old Stead St wharf, in Invercargill.

Tsunamis could be larger than previously thought in many areas of the country, a report has found, because of uncertainty about the maximum size of earthquakes on nearby plate boundaries.

The areas affected are Southland, Stewart Island, Fiordland, Westland, parts of the Wairarapa coast, East Cape, Bay of Plenty, the Coromandel Peninsula, Great Barrier Island, the north-west part of the Auckland rregion and Northland.

The information is in a review carried out by GNS Science and published today, updating research published in 2005 assessing this country's tsunami hazard.

The review did not find any change in the probability of tsunamis happening.

The maximum possible size of some local and regional tsunamis was bigger than previously understood because there was more uncertainty about the maximum possible size of earthquakes on plate boundaries near the areas involved, Civil Defence Emergency Management director John Hamilton said.

For the most hazardous areas - Northland, Great Barrier, parts of East Cape and Wairarapa - it was possible waves could reach 15 metres above the normal sea level at the shoreline. In contrast, there are parts of the North Island's west coast where the maximum tsunami height is not expected to exceed 5 metres, GNS Science said.

The review considered the biggest tsunami that could be expected in a 2500-year period.

It incorporated research on the 2004 Indian Ocean, 2009 South Pacific, and 2011 Tohoku (Japan) tsunamis. All were produced by earthquakes substantially larger than had been considered likely at those locations, Hamilton said.

"There are now believed to be fewer restrictions on maximum magnitudes of earthquakes that could be created on tectonic plate boundaries. New scientific models take into account this increased uncertainty about the maximum possible size of earthquakes."

It was now also known there was a similar tsunami in Japan in AD 86, indicating the interval between the biggest earthquakes there was more than 1000 years.

The tectonic plates in Japan were converging twice as fast as those around New Zealand, which suggested the interval between the largest earthquakes on our local plate boundaries could be more than 2000 years, Hamilton said.

But New Zealand had only a brief historical record of 200 years, which was not long enough to provide accurate reported information. During that time the country has experienced about 10 tsunamis of 5 metres or more.