Tsunamis can be generated by earthquakes, landslides, undersea volcanic eruptions and even crashing meteorites. And despite better warnings, we are more at risk. By Paul Gorman.
When the tsunami came that winter morning in 1868, many Chatham Islanders were asleep.
Accompanied by a rising roar, three waves more than 6m high suddenly surged 6km inland across parts of the northern and eastern coasts of the main island during a 15-minute period from 1am on August 15.
Whare, bridges and roads were smashed and sucked away by the chaotic currents. Some people made it to higher ground when the waves hit, but it is thought at least three families, possibly as many as 20 people, were washed offshore and drowned.
The immense 1868 Arica earthquake, believed now to be of at least magnitude 9 and centred under the Pacific Ocean off the border of Peru and Chile, generated the only tsunami since the arrival of Europeans in New Zealand to cause fatalities. Indigenous oral histories and geological records before then also show damaging tsunami have been frequent visitors to these shores.
Great quakes of more than magnitude 8 in the crustal plate subduction zones off the west coast of South America pose a significant tsunami threat to New Zealand. The Chatham Islands/Rēkohu/Wharekauri and the east and northeast coasts of the North and South islands are in the firing line for these, with waves taking between 12 and 16 hours to cross the Pacific and unleash their energy here.
The most powerful quake ever recorded was the magnitude 9.5 Valdivia or Great Chilean earthquake, which struck on May 22, 1960, and launched a tsunami that created significant surges around the New Zealand coast. The tsunami from the February 27, 2010, Chile quake was also memorable, though on a smaller scale.
More recently, the March 5 East Cape and Kermadec earthquakes and tsunami were a reminder of our vulnerability. Thousands of people in the north and east of the North Island evacuated to higher ground and tsunami waves were seen in several bays, with a 35cm-40cm surge recorded at East Cape.
Some may think the outcome did not justify the efforts to get out of the way. But not all tsunamis are as benign. Even on that scale, they can cause dangerous currents and tidal surges.
Today, we expect immediate access to potentially life-saving advice and warnings of hazards. But in August 1868, the arrival of the tsunami would have sparked fear and bewilderment.
There was no Twitter, no civil defence, no tide gauges or buoys and no Pacific Tsunami Warning Centre. One can only imagine how much of a surprise it was and what people thought had caused it.
A report by GeoNet, a geological hazard monitoring project, based on the master's thesis of GNS Science risk scientist Kristie-Lee Thomas, says the 1868 tsunami smashed whare made of ponga and wood from old shipwrecks and dragged them out to sea. "The land is stripped of trees and shrubs, and only sand, boulders and seaweed are left behind," says the report. "Three families are washed away with their whare; those who survive scramble to higher ground and are left with nothing.
"Nearby, at Waitangi West, a man named Makere is drowned by an incoming wave while trying to save a whaling boat that had been washed along the coast. There and elsewhere on the island, wooden houses, whare, sheds, stores, bridges and roads, some more than 10m above the high-tide mark, are washed away or damaged.
"Dunes are scoured away and beaches are left strewn with seaweed and bits of buildings and what had been inside them."
There was no way of warning mainland New Zealanders. Two hours later, Banks Peninsula residents woke to the sea racing into their homes. In Lyttelton Harbour, boats were ripped from moorings and dashed on to rocks by surging currents. By the end of the day, bridges, jetties and houses around the peninsula had been destroyed or washed away.
"From Mangōnui in the north to Bluff in the south, and in a few places along the West Coast, people watch the sea rising and falling rapidly all day, especially in harbours and narrow bays," the GeoNet report says. "People fishing on rocks in the eastern Bay of Plenty find themselves suddenly up to their necks in water and having to swim ashore."
Strange "tides" and strong currents continued for several days. But it was a fortnight before the schooner Rifleman arrived in Port Chalmers from the Chatham Islands to describe the disaster there.
And it was six weeks until the source of the tsunami was known, with news of the Arica earthquake arriving with the Panama mail steamer in Wellington on October 1.
Thomas (Ngāti Mutunga ō Wharekauri) grew up on the Chatham Islands and moved to the mainland to go to high school and the University of Canterbury.
She says the location of the Arica earthquake, and the arrival of its tsunami at the Chathams about 15 hours later just before high tide, was something of a "worse-case scenario" for a distant-source tsunami hitting New Zealand.
"People recall hearing a faint roar, turning into a louder roar and then water surging through their homes. Some people went out to have a look and took action, but a lot of people were caught off guard.
"It ran in quite far; the first wave arrived and woke a few people up – some managed to escape and some didn't. Then the two waves after that completed the destruction and washed a whole lot of things away."
Tsunami blind spot
She says Māori oral histories show people experienced tsunamis in different places around New Zealand hundreds of years before 1868. "In the Chathams, there were Māori oral histories predicting tsunami inundation around that time."
There has always been "a bit of a blind spot" in monitoring tsunamis travelling across the Pacific towards the Chathams. A network of anchored Dart (Deep-ocean Assessment and Reporting of Tsunamis) buoys, which are still being installed, will help, she says.
"There's not much between South America and New Zealand, apart from Rapa Nui/Easter Island and the Chathams."
So, do the Chathams act as an early warning system for the east coast?
"Sort of, although we wouldn't wait until the Chathams get hit – it would be too late by then," says Thomas.
"Mainland New Zealand would be evacuating and have made those decisions, but it would be more a confirmation that it is coming and how big it is."
The February 2010 tsunami and the March 5 events this year have provided important lessons in issuing warnings and about public preparedness, including evacuations. The interconnections between several quakes and tsunamis on March 5 shows how complicated these events can be, she says.
"Even when we get these small events, we want to keep people safe and off the beach and out of the water. Because the water does get angry and there are strong surges, currents and eddies and abnormal tides.
"We have to be prepared for those complex events. That's where the 'long or strong: get gone' messages come in. And all those people who did 'get gone' after that first strong earthquake did amazingly well."
People expect a lot of instant information, Thomas says. "But we need to be responsible for ourselves. We don't sit there on Google at night when our house is on fire and work out what to do.
"If you live, work, play sports, visit marae or places of worship that are in a tsunami-evacuation zone, have a plan for where you would go and what you would take before it happens."
GNS Science tsunami specialists have hundreds of pre-calculated models of tsunamis travelling across the Pacific from various points. "If an earthquake of a certain size occurs in the Pacific, our tsunami modellers will quickly pick the nearest model to get the first estimates. And as the wave actually passes the Dart buoys and is measured, they are able to refine that in terms of arrival time and size.
"After every one of those events, we go back and review the science and calibrate our models and make sure they are doing what they should if we had a similar event again. New Zealand has a really tight whānau of tsunami researchers and experts in this field."
No part of the New Zealand coast is immune from tsunamis, although GNS Science highlights the east coast as being vulnerable to the largest wave heights in events from the nearby Hikurangi subduction zone and distant South America.
Tsunamis can be generated by earthquakes, landslides, undersea volcanic eruptions and even crashing meteorites. A 2013 report calculated that of the 80 tsunamis that had hit New Zealand since 1835, 27 were from distant sources (three hours or more travel time away), 12 from regional sources (one to three hours away), 28 from local sources (less than an hour away) and 13 were from unknown locations.
Our four largest historical tsunamis were generated by the magnitude 8.2 Wairarapa earthquake in January 1855, the magnitude 7.1 quake 50km off the Gisborne coast in March 1947, and the 1868 and 1960 South American quakes.
Eight Dart buoys are already in place around New Zealand, with four more planned this year. All are next to subduction zones, especially the Hikurangi, Tonga, Kermadec and New Hebrides trenches, where they can detect tsunamis that could reach our shores within two hours.
Even with better forecasting enabled by the Dart buoys, GNS Science says it is still important to follow the "long or strong: get gone" advice.
People near the coast should immediately move to higher ground or inland if they feel a strong earthquake that makes it hard to stand up, or earthquake shaking that lasts more than a minute (it doesn't need to be both), or if they see a sudden rise or fall in sea level, or hear loud or unusual noises from the sea.
Thomas says New Zealanders' exposure to tsunamis is growing as coastal development increases, and that worries her. "In historic times, there were fewer things in the way of tsunamis.
But roll forward 150 years from the 1868 event and we now have more people than ever and new infrastructure and developments in those coastal areas, and we're still building in high-risk areas.
"I always wonder how many more 'nice Mother Nature warnings' we have up our sleeve."