The effects of tsunamis in New Zealand

Technical Abstract

This report summarises the published records of tsunami damage along the New Zealand coastline and comments on the risk and expected effect of future large tsunamis.

A tsunami is a seismic sea wave usually generated by a sudden change in the configuration of the sea floor which is often the result of a moderate-large earthquake. Landslides, volcanoes or atmospheric conditions can also generate tsunamis. Historically tsunamis have caused huge destruction and loss of life in countries bordering the Pacific Ocean, but New Zealand has escaped with relatively little damage. This is partly due to the protection provided by the Australian continent and outlying islands, and also due to dissipation of the energy in the tsunamis on the New Zealand continental shelf.

New Zealand is susceptible to tsunamis from both distant and local sources. The east coast of New Zealand and the Chatham Islands are most vulnerable to tsunamis originating near South America, in particular from Peru or North Chile. These tend to be directed at New Zealand and there is little protection from intermediate landmasses. Historically, three tsunamis generated by very large earthquakes on the Chilean coast have produced wave heights of the order of three metres on the New Zealand coast. Damage has not been extreme, except in the Chatham Islands. However there is a potential for tsunamis originating from this source to be up to 50 percent higher than has previously occurred. Coupled with this is the recent coastline property development and damage could now be substantially greater. The west coast of New Zealand is less vulnerable to tsunamis from distant earthquakes.

Tsunamis originating from local sources have generated larger wave heights in New Zealand than those originating from distant sources, but their influence has only been over a short length of coastline. These past events have not significantly affected major towns or cities, but this possibility cannot be ruled out for the future. The possibility of a major earthquake causing major warping of the sea floor with very large tsunamis causing vast damage cannot be ruled out, but there is no geological evidence for an event of this magnitude in the last several thousand years.