New Zealand’s tsunami risk

Local-source tsunamis since the 1820s

Local-source tsunamis since the 1820s

Tsunami is one of the few Japanese words that have made it into the English language. The etymology of the word is from ‘tsu’ (harbour) and ‘nami’ (waves). Following the Japanese tsunami of 11 March 2011, generated by a massive earthquake of magnitude 9.0, many New Zealanders, especially those who live or work in low-lying coastal areas, will be interested in our tsunami history and level of risk.

Te Ara’s entry on tsunamis was written by Willem de Lange and Eileen McSaveney in 2005, not long after after the 2004 Boxing Day tsunami had devastated many places in the Indian Ocean. Since then, the 29 September 2009 tsunami off Samoa’s south coast, which killed 183, has further raised the profile of the hazard. After the Samoan tsunami Willem de Lange pointed out that while we were now much better prepared from a tsunami generated by a distant earthquake – which takes many hours to get here – we were about as well prepared for a tsunami generated close to our shores as Samoa was in 2009. In other words, there would be no warning.

A major problem is that there are many offshore earthquakes each year but almost all of them pose no tsunami threat, as most are small and do not cause large movements of the sea floor. Offshore quakes cause tsunamis when the sea floor suddenly drops or rises many metres – causing a pulse of energy that ripples outwards in the form of waves (it’s like what happens to the surface of a bucket of water if you drop it onto the ground). There would have to be a judgement call made after a large offshore earthquake as to whether a tsunami warning was issued (or not). This would need to be done in minutes – as the Japanese experience has shown that waves can move at the speed of a jet plane. If warnings were issued and no tsunami eventuated, there would be a danger of people ignoring future warnings. Currently, New Zealand does not have the capability to issue warnings based on science for local- and regional-generated tsunami within minutes.

Increased tidal disturbance around New Zealand after the March 2011 earthquake in Japan

Increased tidal disturbance around New Zealand after the March 2011 earthquake in Japan, recorded by GeoNet (http://www.geonet.org.nz/tsunami/)

Japan is the best-prepared country in the world for a tsunami, with computer systems taking into account 100 pre-computed models of distant-source tsunami, and 100,000 pre-computed models of local-source tsunami. Its system of seismic sensors compares earthquakes in real time with these models, and then generates earthquake and tsunami warnings. In theory, this enables it to produce warnings within three minutes. It is still too early to judge how well this system performed, but early reports suggest that Japan’s earthquake-warning system gave advanced warning of the quake to those in Tokyo.

In New Zealand, local and regional tsunami could be generated from earthquakes in Cook Strait, off Fiordland or off the east coast of the North Island and the Bay of Plenty. They can also be generated by above water and underwater volcanoes, and landslides such as the Ruatōria Giant Avalanche that occurred off the east coast of the North Island 135,000–155,000 years ago. A Kaikoura fisherman recalled a small tsunami caused by an underwater landslide in 1953. The major local and regional risks come from offshore earthquakes and landslides (which can be activated by earthquakes) or occur without warning. When the local-, regional- and distant-source risks are combined, return periods for large tsunami in New Zealand are short – for example in 2001 the return period for 5–10 metre tsunami affecting some part of the Wellington coastline was estimated at once every 84 years. Māori have tsunami in their oral traditions – a wave destroyed coastal settlements in the 1400s in Tasman Bay and deposits left by past tsunami have also been found.

We have a good warning system for distant-source tsunamis, but not for local- and regional-source tsunamis. At the moment, the best that can be said is that if you feel a strong earthquake near the coast get to high ground (35 metres above sea level) immediately. As the 22 February 2011 Christchurch earthquake has seen us scrabbling to inventory our at-risk buildings, it’s perhaps time to review our tsunami warning system for those waves generated closer to our shores. In the meantime, if you feel a quake near the shore, run for the hills.

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