What are tsunamis?
Tsunamis are broad waves in oceans and lakes generated by large disturbances – movements of the sea floor during earthquakes, volcanic eruptions, landslides under or into the ocean, or even meteor impacts. Like the ripples that spread from a stone tossed into a pond, tsunamis consist of a series of waves, but on a giant scale, with wave crests 10 to 500 kilometres apart.
In the deep ocean, tsunamis travel at the speed of a passenger jet (600–900 kilometres per hour), but they are usually less than half a metre high. As they move into shallow water they slow down, the distance between wave crests lessens and the waves increase in height, sometimes to tens of metres.
The size of a tsunami is judged by its maximum height above sea level – called the ‘run-up’. The largest to affect the New Zealand coast have had a run-up height of 10 metres or more. On land, the amount of damage depends partly on the run-up, and partly on the slope of the land. Big tsunamis may inundate low-lying land such as river flats for many kilometres inland.
The first part of a tsunami to reach a shore is often a wave trough, causing the ocean to recede from the shore before the first wave crest hits. Successive wave crests may arrive at intervals of between 10 minutes and an hour. The first wave crest to arrive is often not the largest.
Tsunami waves may come ashore as steep breaking walls of water, or as fast-rising water levels. Unlike normal waves that break and recede in a few seconds, tsunami waves rush in for many minutes, often penetrating far inland along low-lying coasts. Tsunami waves can travel for many kilometres up rivers.
Tsunamis in New Zealand
New Zealand is vulnerable to tsunami damage because of its long coastline, and because 80% of all tsunamis occur in the Pacific Ocean. Local-source tsunamis usually affect limited stretches of coastline, while those from distant sources may affect the entire coast.
Three of the larger historical tsunamis to reach New Zealand (in 1868, 1877 and 1960), with heights of 5 to 10 metres, have resulted from major earthquakes off the western coast of South America. The Aleutian Islands and the Alaskan mainland are also potential sources.
Scientists have proposed that a large area on New Zealand’s continental shelf, south-west of Stewart Island, is a meteor crater. Mahuika crater is about 20 kilometres wide and 150 metres deep, and lies at a depth of 300 metres; the date it was formed is not yet known. The meteor impact would have generated a mega-tsunami more than 50 metres high.
In the late Pliocene, about 2.5 million years ago, an asteroid (known as the Eltanin asteroid) landed in the Bellingshausen Sea, west of the Antarctic Peninsula. A ‘mega-tsunami’ from this ‘Eltanin impact’ may have affected New Zealand.
It is estimated that every century, the country is struck by about a dozen tsunamis of more than a metre in height. Since European settlement, three have exceeded 10 metres, the first occurring around Wellington in 1855. Only one tsunami death has been officially recorded – on the Chatham Islands in 1868. Māori tradition, however, records several tsunamis in pre-European times that caused a large number of deaths.
The widespread devastation caused by the Indian Ocean tsunami in December 2004 and east Japan tsunami of 2011 has led to increased awareness of the hazard around New Zealand’s coast.
New Zealand has frequent shallow earthquakes, and about a third of them occur on submarine faults offshore. From Canterbury to East Cape, and beneath Cook Strait, New Zealand’s continental shelf is fractured by active faults. Movement along these faults caused major tsunamis in 1855 and 1947, and smaller ones in 2016. The West Coast is vulnerable from movement along the Alpine Fault and offshore faults.
Many of the world’s largest tsunamis are caused by subduction earthquakes, which occur when a section of the ocean floor is forced under the edge of a tectonic plate. A major subduction zone, the Hikurangi Trough, lies off the east coast of the North Island and northern South Island. Sediment deposits on the North and South Island coasts, including one from a large tsunami, indicate at least two subduction earthquakes may have occurred on the Hikurangi plate margin in the last 1000 years.
On a local scale, a landslide into Charles Sound during the 2003 Fiordland earthquake created a wave that inundated forest 4 to 5 metres above high tide, and damaged a helipad and wharf. Much larger tsunamis may have been triggered by huge submarine landslides off the edges of New Zealand’s continental shelf, revealed by recent sonar mapping.
Offshore volcanoes, including Mayor Island (Tūhua), Whakaari (White Island) and numerous submarine volcanoes between New Zealand and Tonga, present a tsunami hazard. Some tsunami deposits in the Bay of Plenty appear to be related to the collapse of submarine Healy Volcano. Even inland volcanic eruptions can create atmospheric pressure waves that cause tsunamis – some New Zealand deposits are the same age as a catastrophic eruption of Taupō around 1,800 years ago.