The word tsunami (lit. “harbour wave”) is now part of common language with most New Zealanders now having witnessed chilling scenes from the TV coverage of the giant waves following first the Indonesian and now the Japanese earthquake. That is not the same as knowing it could and most certainly has happened here in the not too distant past. What probably saves us from worrying too much is that most of the recent Tsunamis have been mercifully small by the time they reached New Zealand. There is also the fact that comparatively few in the country have witnessed a serious tsunami for themselves. However both the geological record and even some eye witness history since 1840 suggests we probably need to be more vigilant and to take rather more precautions than is currently the practice.
On the Mirimar Peninsular in the Wellington region there is a curious spot in the hills at least 40 metres above sea level where the remains of a number of whales are still evident despite having largely crumbled away. Given the size of whales, for the scientists seeking the explanation, it was a puzzle to work out what gargantuan forces might have carried them there. Although there is also some evidence there had been earthquake uplift in that area the geologists claim this uplift is only 10 meters or so and there is certainly growing consensus that a past huge Tsunami stranded the whales in this unlikely place.
Approximately 500 years ago one or more Tsunami hit the coastal regions of New Zealand with huge waves depositing shells, sand and stones thirty metres above the sea level. The archaeologists now consider this was a significant event devastating Maori settlements of the day.
Perhaps most worrying in terms of potential repeat damage was the earthquake of 1855, an 8.1 – 8.2 earthquake which hit the Wairarapa, with the accompanying tsunami flooding Wellington houses and shops on Lampton Quay, flooding Lyall Bay and Evans Bay and covering the land which is now Wellington airport to a depth of one metre. Some of the waves destroyed sheds at Te Kopi 8 metres above the high tide mark and stranded fish as far North as Otaki. The significant point to note today, is that back in those days there was no high density housing in the vulnerable areas. Today with a much higher proportion of coastal settlement and many houses built on reclaimed land subject to potential liquefaction we would expect much more damage and loss of life for a repeat earthquake of similar size.
Earthquakes are not the only cause. A number of tsunami events, some of them caused by local rock slides into or under the sea, and some caused by volcanic activity have regularly hit parts of New Zealand. Goose Bay south of Kaikoura witnessed a rock slide causing a 10 metre tsunami which if it occurred today would take out State Highway No 1. White Island with its frequent volcanic activity represents a genuine threat to the East Coast of the North Island and more particularly to the city of Tauranga.
The 1868 force 9 Earthquake on the Peru Chile border generated tsunami which if repeated today would have caused widespread devastation in the Pacific. As it was, in New Zealand there was considerable damage on Banks Peninsula and it also affected many ports and destroyed buildings in the Chatham Islands. On August 1868, an earthquake of about magnitude 9.0 offshore from the Peru–Chile border generated a devastating tsunami. The earthquake and tsunami killed thousands of people along the South American coast. Spreading across the Pacific, it became the largest recorded distant tsunami to strike New Zealand, affecting many ports and causing substantial damage on the Chatham Islands and Banks Peninsula. Some indication of the size of the surges was that water rose 7 metres in Lyttleton Harbour and considerable damage was sustained to the wharves and moored ships.
There is no indication that the recent Japanese earthquake and tsunami was particularly unusual in terms of the geologic past, and the fact that the tsunami was generated by a long subduction zone off the coast of Japan should give our civil defence planners pause for thought. New Zealand has a very similar subduction region extending off shore the length of the east side North Island of New Zealand and extending as far as Tonga. Relatively recent earthquake, volcanic activity and indications of past tsunamis suggest that the region is still very much active..
This is only one of many observations that confirm the contention of a number of scientists that based on past record New Zealand is the fifth most likely place in the world for a serious tsunami to strike following closely behind Japan, Indonesia, the West coast of South America and Hawaii.
Although most of the tsunamis appear to have hit New Zealand on the East Coast there is also strong evidence for some Tsunami action on the West Coast of the South Island.
The tsunamis generated on the other side of the Pacific ring of fire give plenty of time for warning in that it takes several hours for a tsunami to cross the Pacific and the time of arrival and likely size can be calculated with plenty of time to spare. However if the source is more local there is no time for the necessary calculations to be done. The recent big tsunami in Japan approached the shore at an estimated 500k per hour. The advice is consequently that if large earthquake is felt, for those in low lying coastal areas the safest course of action is to immediately head for high ground.