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.
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.
Tsunamis have been a danger in New Zealand as long as people have lived there. Archaeological studies have shown that during the mid-15th century, many Māori moved their settlements from low-lying coastal sites to hilltops and inland sites. A number of the abandoned coastal settlements show clear evidence of tsunami inundation.
Tsunamis are also recorded in Māori oral tradition. For example, a wave that caused widespread death and damage on the western side of D’Urville Island in Tasman Bay may have been a tsunami:
Unfortunately the community at Moawhitu was eventually wiped out when a massive tidal wave called Tapu-arero-utuutu swept into the harbour and drowned almost everyone, tumbling their bodies into the sand dunes which were piled up by the force of the waves … Traditions do not record whether the tidal wave affected other communities on Rangitoto (D’Urville Island) or the mainland across Te Aumiti (French Pass), but even today kōiwi (human remains) and artefacts are frequently eroded from the dunes at Moawhitu, especially after stormy conditions or exceptionally high tides. 1
On 23 January 1855, a magnitude 8.1–8.2 earthquake, the most powerful to strike New Zealand since European settlement, shook the lower North Island. It generated not one, but several types of tsunami.
During the earthquake, the entire region west of the Wairarapa fault lurched abruptly north-east. Like soup in a bowl that is jostled, the water of Wellington Harbour slopped onto the adjacent land. The next movement of water occurred because the entire Wellington region had tilted – the eastern side of the harbour was now about 80 centimetres higher than the western side. The harbour waters ponderously moved downhill, towards central Wellington. Houses and shops were flooded along Lambton Quay, which at that time was on the shoreline.
The greatest tsunami, however, was generated in Cook Strait. The Remutaka Range rose as much as 6 metres, and part of the floor of the strait was probably uplifted. The tsunami destroyed sheds more than 8 metres above the sea at Te Kopi, on the southern Wairarapa coast. It moved through the strait and up the Kāpiti Coast – stranding fish as far north as Ōtaki – and spread across to the South Island.
The sloop Pandora was anchored in Wellington Harbour at the time of the 1855 earthquake. Her commander, Byron Drury, reported: ‘For eight hours subsequent to the first and great shock, the tide approached and receded from the shore every 20 minutes, rising from eight to ten feet and receding four feet lower than at spring tides. One ship, I heard, was aground at her anchorage four times.’ 2
About 20 minutes after the earthquake, tsunami waves surged into Wellington Harbour through its narrow entrance, then for many hours bounced repeatedly back and forth, reflected off the harbour sides. Water also flooded into Lyall Bay from Cook Strait and Evans Bay from Wellington Harbour, putting the low isthmus between them (now Kilbirnie) under nearly a metre of water.
In 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.
The tsunami reached the Chatham Islands around 1 a.m. on 15 August, about 15 hours after the earthquake. Māori at the village of Tupuangi were woken by water surging into their houses and fled to higher ground. Subsequently two larger waves destroyed the village and the houses of several European settlers. One Māori drowned, carried out to sea while trying to retrieve a boat that had come adrift. The tsunami also damaged buildings at Waitangi.
After the devastating Peru–Chile tsunami of 1868, Ferdinand von Hochstetter, an Austrian geologist who had visited New Zealand in 1859–60, described the effects in Peru, Chile, New Zealand and other Pacific locations. He charted the progress of the tsunami across the Pacific and determined wave speeds and the ocean depth along several paths. Hochstetter’s work was the first detailed scientific analysis of a major tsunami.
Several hours later, on Banks Peninsula in the South Island, a night watchman discovered the ships at Lyttelton’s wharves sitting on the mud bottom – the water had drained from the harbour area. Around 4 a.m., a foaming wall of water surged into the harbour, and the water rose by over 7 metres. Ships’ hawsers snapped, and the ships were dashed against the wharves and each other, causing heavy damage. The sea gradually receded, but more big waves rolled in at intervals of several hours, and water levels rose and fell erratically over several days. In smaller bays around the peninsula, tsunami waves penetrated inland along valleys, damaging homes and carrying away bridges and fences.
At 8.32 a.m. on 26 March 1947, a seemingly minor earthquake jolted the Gisborne area, generating a tsunami that 30 minutes later swamped the coast between Muriwai and Tolaga Bay.
Four people in the hotel near Tatapouri Point, north of Gisborne, spotted an ominous wave offshore and dashed up a nearby hill. Two successive waves drove through the hotel’s ground floor, filling it to the window sills. Many minutes later the receding water sucked small outbuildings out to sea.
At Turihaua, a 10-metre wave bore down on a cottage. Two men outside were swept inland and dumped on the coast road. Two women and a man were trapped in the kitchen of the cottage, which filled to head height with water. Battered by debris-laden water rushing back to the sea, the cottage crumbled. Only the kitchen was left standing.
At Pouawa Beach, the Pouawa River bridge was carried 800 metres upstream. At Te Mahanga Beach the tsunami shifted a house off its piles, and at Murphy’s Beach six hectares of pumpkins disappeared out to sea.
Less than two months later, on 17 May 1947, another offshore earthquake generated a tsunami that hit the coast between Gisborne and Tolaga Bay. At its maximum, north of Gisborne, this wave was about 6 metres high.
No one died in either of the 1947 tsunamis, but the toll could have been high had they struck during summer holidays, when the beaches are crowded.
Staying at Turihaua Point, Donald Tunicliffe witnessed the wave of March 1947:
‘It sounded like a powerful motorbike that was really getting wound up. … I got to my feet with Novena following and led the way out of the door. … A fascinating but horrifying spectacle met our gaze. Approaching the shore, and us, at breakneck speed and roaring like an express train was a wall of dirty coloured water towering to a good 30 feet [9 metres], boiling and curling as it picked up acres of beach sand on its way to engulf us within seconds.’ 1
The most powerful earthquake of the 20th century was of magnitude 9.5, off the coast of Chile on 22 May 1960. It generated a tsunami that killed several thousand in Chile and across the Pacific, including 61 people in Hawaii and 199 in Japan.
In the late evening and early morning of 23 and 24 May, the first of many tsunami waves began arriving at New Zealand’s east coast. The tsunami caused wild fluctuations in the water level along the coast for several days, damaging boats and harbour facilities.
In the North Island, at Napier, waves that reached 4.5 metres above high-tide level damaged a footbridge over the Ahuriri estuary, wrecked many pleasure boats and swept others out to sea. At Scapa Flow, the waves inundated beach homes and boat houses. At a seaside campground at Te Awanga, north of Hastings, eight people were washed out of their tents, and waves battered cabins.
Further north at Whitianga, the waterfront road and the airport were flooded, and a number of small craft were washed out to sea. During later fluctuations, the sea retreated from the shore, exposing the wreck of HMS Buffalo, which had sunk in 1840. Some people ran out to the wreck to collect relics, but were forced to retreat when the sea returned.
At the port of Lyttelton in the South Island, the tsunami came in at 2.7 metres above the tide level of the time, damaging boats and electrical gear. A hotel and several houses were flooded, and 200 sheep drowned.
Several days later, warnings of another possible tsunami from a major aftershock sparked an evacuation of many areas on the New Zealand coast.
The magnitude 7.8 Kaikōura earthquake sequence on 14 November 2016 ruptured several submarine faults and triggered submarine landslides, causing minor tsunami along the east coast of New Zealand. Because of uplift of long stretches of the coast and a low tide, the effects were minor. The maximum height, 6-7 metres, was at Goose Bay, with 2.5 metre waves at Kaikōura and 1.6 metres at Wellington. The tsunami was only one-metre high at Lyttelton, but at Little Pigeon Bay on Banks Peninsula, a three-metre high wave shoved a holiday house off its foundations.
New Zealand has a tsunami gauge network that consists of water level recorders on open coasts, as well as at Raoul and Chatham Island. They transmit data continuously to GeoNet, and unusual changes in water level may indicate that a tsunami has arrived at the gauge location.
For warnings of tsunamis from distant sources, New Zealand relies on the Pacific Tsunami Warning Center, based in Hawaii. If a large submarine or coastal earthquake greater than magnitude 6.5 occurs, they issue a tsunami information bulletin. They then monitor data from a network of coastal tide stations and deep ocean sensors, and if a tsunami is detected, they issue tsunami watches and warnings for areas that may be affected.
Tsunami Watch bulletins from the Pacific Tsunami Warning Center are received by New Zealand’s National Emergency Management Agency, which convenes a panel of tsunami experts to determine the hazard. They then notify regional authorities, the police and New Zealand Defence Force, and local emergency organisations. The public are alerted via radio and television broadcasts, internet apps, social media, websites, phone text alerts and electronic highway notice boards. Some areas also use sirens.
Tsunamis from major earthquakes off South America may take 12 hours or more to reach the New Zealand mainland, giving authorities time to prepare and if necessary evacuate coastal settlements.
If you are on the coast during an earthquake that is strong or lasts longer than a minute, hear unusual noises from the sea or see water rising or receding from the shore, move immediately to the nearest high ground or as far inland as possible. If you do not have time to move to higher ground or inland, go to an upper storey of a sturdy building. Stay away from coastal rivers and streams, and never go to the coast to look for a tsunami.
Tsunamis produced by local sources such as offshore earthquakes may arrive at the coast in just a few minutes. This is not enough time for New Zealand’s earthquake monitoring system, GeoNet, to locate the earthquake, determine if it could produce a tsunami, and notify the National Emergency Management Agency. It is important for the public to recognise natural warning signs, and act quickly.
Following major tsunamis in the Indian Ocean and Japan, New Zealand regional councils [and local authorities] developed tsunami zone maps and evacuation plans for population centres. Some areas have introduced signs showing evacuation routes, and safety zones marked by blue painted lines on streets and footpaths.
Museum of New Zealand Te Papa Tongarewa is on Wellington Harbour, which has been hit by tsunamis in the past and is at risk from major earthquakes and tsunamis. The building was designed so that no art or artefacts are displayed or stored on the ground floor. In an earthquake that might result in a local tsunami, museum staff will move visitors to the upper floors.
On 23 May 1960, a tsunami from a magnitude 9.5 earthquake in Chile caused damage in New Zealand coastal areas. Two days later, just before noon on 25 May, a radio message from Hawaii said that a tsunami from a major aftershock of the Chile earthquake might hit New Zealand within about an hour and a half. It sparked a major tsunami evacuation. Warned by nationwide radio broadcasts (‘A wave is reported approaching at 400 miles an hour’) and telephone messages, port facilities along the east coast were cleared, schools in low-lying coastal areas were closed and the children taken to safe areas. 1
Almost the entire populations of Whitianga, Mercury Bay, Waihī Beach, Whakatāne, Ōhope, Ōpōtiki and Kaikōura headed for high ground. It was the largest evacuation in New Zealand’s history. The warnings, however, had an unintended side effect – many people went to the coast to watch the tsunami arrive. It proved to be very minor, and hard to separate from the fluctuations continuing from the main tsunami. Had it been larger, however, many onlookers could have drowned, as tsunami waves travel much faster than people can run.
Between 2005 and 2016, Civil Defence warnings were issued for a number of tsunamis – from earthquakes in southern Fiordland and the Pacific Ocean near Samoa in 2009, from Chile in 2010, from the Japanese Tohoku tsunami in 2011 and more Chile earthquakes in 2014 and 2015. None caused any substantial damage in New Zealand.
The magnitude 7.8 Kaikōura earthquake sequence on 14 November 2016 triggered tsunami along the east coast of New Zealand. In the early hours of the morning, hundreds of people were evacuated from their homes to higher ground in Kaikōura and from low-lying suburbs of cities such as Wellington and Christchurch. However, conflicting reports meant that warning sirens in some areas, such as Wellington, did not sound until after the waves had arrived. There was also confusion among the public over how to respond to the sirens.
‘We revelled in it, whooping and hollering as we followed the ebb’. Rossi King-Turner describes her memory of the 1960 tsunami.
The tsunami in May 1960, caused by a Chile earthquake, is the one most often recalled by New Zealanders. Despite the potential of tsunamis to create tragic destruction, the 1960 event was not a major disaster in New Zealand. No lives were lost or even threatened. So when we invited stories about natural disasters, the 1960 tsunami accounts tended to be more amusing than terrifying.
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McFadgen, Bruce. Hostile Shores - Catastrophic Events in Prehistoric New Zealand and Their Impact on Maori Coastal Communities. Auckland: Auckland University Press, 2007.
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