The science of tsunamis and what parts of New Zealand are most at risk
Thursday, 11 March 2021
EXPLAINER: The earthquake struck on a Wednesday in March at 8:32am, 50 kilometres off the coast of Gisborne near the Hikurangi Trough.
Even though it registered magnitude 7.1, it was not widely felt along the coast. The shaking caused no damage.
Thirty minutes later, enormous waves rushed in. There was almost no warning.
The “silent” tsunami hit 115km of coastline on the East Coast, from Mahia Peninsula to Tokomaru Bay.
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A 16-metre wooden bridge near Pouawa was flung almost a kilometre inland. Two men were thrown off their feet and deposited on a nearby road.
Two women and another man were trapped in a cottage as the water hurtled inland. The currents rose to head height. Their home disintegrated as the water receded. Only the kitchen was left.
The size of a tsunami is measured by the maximum height it reaches above sea level, or what scientists call its run-up.
The 1947 Gisborne event was one of the largest tsunamis recorded in New Zealand. Run-up heights of about 10 metres were recorded.
Not only was this a colossal tsunami, it was a strange one.
The waves were caused by what’s called a tsunami earthquake. This type of shake involves a rupture of the Earth’s crust at speeds lower than typically observed during a regular earthquake.
A lot of energy is still released, but it’s released over a longer period of time. Therefore, the intensity of the ground-shaking and the magnitude of the subsequent tsunami do not match up.
This kind of earthquake is a big problem. When people feel the ground shake, they’re much more likely to flee inland. When they don’t, they may well stay put.
The curious case of the 1947 Gisborne earthquake illustrates the uncertain risks we face in New Zealand. Last week’s trifecta of earthquakes rammed that point home.
The country sits on the boundary of the Pacific and Australian plates. Big earthquakes are inevitable. The Hikurangi subduction zone off the East Coast, the Puysegur Trench to the south and the Kermadec Trench to the north can all generate major tremors.
We are also vulnerable to distant tsunamis originating off South America. Landslides, both underwater, and from coastal or lakeside cliffs, can also trigger major waves. Never mind the risk of volcanic eruptions.
Between 1835 and 2013, about 80 tsunamis reached the coastline. At least six of these were more than 5m high, says Dr David Burbidge, tsunami team leader at GNS Science.
This is how tsunamis work, how we seek to protect ourselves, and which parts of the country are most at risk.
The basics
Throw a stone into a lake, and you’ll see the ripples. Now imagine a hunk of rock hundreds of kilometres long suddenly lurching upwards from the sea floor. Imagine how big those ripples would be.
This is a tsunami, a series of waves, caused when water is very suddenly displaced. There are four causes:
A large underwater earthquake. Tsunamis are more likely to be caused by megathrust earthquakes. These occur at subduction zones where two tectonic plates meet, and when one plate is suddenly forced below the other. This causes vertical movement on the ocean floor (the seabed thrusts upwards suddenly), which leads to a huge amount of energy being released, and a lot of water being displaced.
Landslides can and have caused tsunamis in New Zealand. Waves can be generated by slippages both underwater and from cliffs and mountains near a body of water.
The explosion from a volcanic eruption or the sudden collapse of a volcano itself can cause major displacement and tsunami waves.
A meteor suddenly crashing down could cause a tsunami. It had been suggested that the Mahuika crater southwest of Stewart Island was caused by a meteor that may have triggered an enormous tsunami. However, a 2010 scientific paper poured cold water on this claim.
Tsunamis (the Japanese word for harbour waves) are sometimes referred to as tidal waves. The tidal forces of the Earth, sun and the moon have nothing to do with tsunami waves.
What happens when the Earth moves?
An underwater megathrust earthquake occurs typically at a subduction zone, where two plate boundaries meet.
Hundreds of kilometres of the sea floor can be ruptured and tossed upwards by such quakes. Vast amounts of energy are released. Last Friday’s 8.1 magnitude quake was a thrust event.
The 2011 Japanese earthquake released enough energy to shift the axis of the planet and subsequently changed the length of a normal day by 1.8 microseconds. It triggered one of the most destructive tsunamis in history.
When a quake like this hits, the water above the epicentre is immediately displaced, and shoots upwards. The Earth’s gravity pulls it back down, and the waves of energy released by the quake rush outwards – going back to our earlier example, as if an enormous stone had been tossed in the lake.
It’s best to think of tsunami waves not as a quantity of water moving but as bursts of kinetic energy surging through the ocean.
In deep water, tsunami waves move at high speeds (up to 800kmh or as fast as a jet plane). The section of wave above the water is typically quite small – up to only 1m high.
This is called the wave’s amplitude – the distance from the centre of the wave to the top of a crest or to the bottom of a trough.
Don’t let this fool you. The waves are enormous – they’re just flattened out, with most of their energy (a little like icebergs) beneath the water.
In open waters, the distance between two successive tsunami waves (what’s called the wavelength) can be more than 500km.
When an earthquake strikes on land, the waves of energy released dissipate fairly quickly. We’re not going to feel a quake centred inland in Chile.
But waves can travel through water for a much longer period of time. A tsunami generated off South America can - and has - reached Kiwi shores and done damage.
As the waves reach coastlines, they begin to slow, through a process called shoaling.
You will have seen this happen with regular, wind-generated waves at a beach. As they get closer to shore, their height increases relative to the depth of the water.
It’s the same with tsunamis. Powerful waves that once surged through deep bodies of water are pushed upwards by shallow topography.
As all that energy is still present, the amplitude, or height of the wave, rapidly increases and, as it does, the wavelength drastically decreases.
The waves get very tall and much closer together. For example, the 2004 Indian Ocean tsunami was 30m high.
“When the waves reach land, the only way they can go is upwards. So the wave gets bigger and bigger until they hit the shore,” explains University of Otago earthquake science chairman Mark Stirling.
The Science Learning Hub, a government-led project, also offers an excellent explanation: “It’s a bit like squeezing a toothpaste tube – all the toothpaste is forced upwards.”
The physical make-up of a wave, particularly the bit at the bottom, the trough, also explains why water appears sometimes to be pulled outwards before the tsunami strikes.
Think of it this way. A wave isn’t just water rushing forward, above the surface. There’s also the trough, the lowest point of the wave.
If the trough arrives first, water doesn’t rush in. Instead, it’s sucked out and people may well see what’s called drawback, a classic and terrifying sign of an incoming tsunami crest.
The coastal make-up also has a significant impact on how the tsunami behaves.
Professor Thomas Wilson, of the University of Canterbury’s School of Earth & Environment, points out that some bays can concentrate wave energy, and a longer, shallower sea floor gives tsunamis more time to grow.
This explains why, after the Kaikōura earthquake in 2016, it was Little Pigeon Bay on Banks Peninsula that saw the biggest tsunami surges.
Tsunami waves don’t necessarily look like the surges you see in Hollywood films. They’re more just a big body of water that keeps coming, Stirling says.
How to be ready
Firstly, scientists can model the likelihood of earthquakes hitting a rather broad geographical area, based on mathematical models, but they cannot predict precisely where or when they’ll strike.
This gives us limited time to react to a tsunami wave.
The location of what’s called the ‘’generation event’’ determines how much time people have to be ready.
In the event of a major underwater quake, the ideal process is:
Experts at the National Geohazards Monitoring Centre analyse the data provided by a range of instruments, including seismographs and deep water buoys.
If there’s a tsunami, they’ll immediately flag it with the National Emergency Management Agency (NEMA).
NEMA will issue warnings via a range of channels.
If there’s a danger of flooding, NEMA will send out emergency mobile alerts to every phone they can reach in the affected area.
The problem is in the event of a major offshore quake just off the coast, there may well not be time to do all that.
This kind of threat is defined as a local tsunami – when the waves arrive less than an hour after the quake (or whatever event caused it).
A 2013 tsunami risk assessment by GNS, the most recent major study on this topic, determined these tsunamis now pose the biggest risk – “because earthquakes in some offshore areas, while not more likely, could be bigger than previously thought. This means that some local and regional tsunamis could be larger than previously estimated.”
Wilson says that if people feel a shake lasting more than 60 seconds, it’s likely the surface of Earth has been ruptured and a significant tsunami is possible. Generally speaking, the earthquake needs to be 7 magnitude or greater to trigger a tsunami.
If the quake is violent and doesn’t last a minute (like the 2011 Christchurch quake), the ocean floor is unlikely to have been ruptured, he says. But such an event could still trigger an underwater landslide.
In this case, the Civil Defence mantra: Remember, Long or Strong, Get Gone very much applies.
In recent years, New Zealand has boosted its tsunami monitoring network, deploying a number of DART (Deep-ocean Assessment and Reporting of Tsunami) buoys to track sea-level changes in the deep ocean, so we can be ready for more distant tsunamis.
These buoys are like the canary in the coalmine – the best tool we have to see just what a tsunami wave is doing and where its energy is focused.
GNS analysts use a mix of the buoy’s real-time data, seismographs, and information from overseas partners like the US Pacific Tsunami Warning Centre, to determine if warnings – via media or text message – need to be issued.
Each local Civil Defence group has also created its own evacuation zone map to outline which areas are most at risk.
Where is at risk?
There’s good news and bad news.
The good news: we know a lot more about where the risks come from.
The bad news: An awful lot of New Zealand faces serious peril from tsunamis. The GNS models below illustrate this well. The models were produced in 2014 by dividing the coastline into 268, 20km-long sections and determining the risk based on a wide range of scenarios.
Below, we show 100-year, 500-year and 1000-year events. (The larger the timeframe, the more significant the tsunami.)
Broadly speaking, the greatest menace appears to be off the East Coast, because of the Hikurangi subduction zone – the massive fault that runs from Marlborough up the east coast of New Zealand. This is where the Pacific and Australian tectonic plates collide.
This fault has the potential to generate earthquakes as high as magnitude 9, some believe.
One scientist has warned that 12-metre-high tsunami waves could follow such a quake.
The water would reach the shore in 10 minutes.
To complicate matters, there’s a whole heap of smaller faults near the subduction zone that can generate serious earthquakes. The 7.8-magnitude Hawke’s Bay earthquake was caused by one such fault.
Northland is also at significant risk because of the continuation of the same plate boundary, which juts northwards through the Kermadec Trench. This is where last week’s largest tsunami originated.
The warning from GNS in 2013 was stark: “For the most hazardous areas – Northland, Great Barrier, parts of East Cape and Wairarapa – it is possible that waves could reach 15m above the normal sea level at the shoreline.”
The south-east coast of the South Island is also at risk from earthquakes generated from the Puysegur Trench, where again the Pacific and Australian plates meet.
The worst-case scenario shows a magnitude-8.7 quake in the Puysegur Trench causing a tsunami surge of up to 7 metres above normal sea level.
An Alpine fault earthquake could also likely trigger major landslips, followed by tsunami in large lakes.
“We’ve had the human impacts of deforestation and overstocking, all of which conspire to produce quite significant landslide hazards around New Zealand,” Stirling says.
The risk for the east coast of the South Island is a little less dramatic. Despite all the seismic activity in Canterbury, the faults off Christchurch’s coast are unlikely to cause a major tsunami.
That said, there is danger from tsunamis generated off the coast of South America, like the event in Chile in 2010.
One last thing
More bad news. Climate change is likely to make tsunamis worse.
“One of the manifestations of climate change is more intense, violent storms, permutations in our weather system, and also rising sea level,’’ Wilson says.
In the next 45 to 90 years, we expect the ocean to be 50cm higher.
“The impact of this isn’t going to be you wake up one day, and you get soggy feet when you walk out the back door. It’s going to be through acute events such as tsunamis, storm surges and coastal inundation.”
The coastal protections we have (for example, sand dunes) are being broken down as sea levels rise.
Hazards, like tsunamis, will only be exacerbated by the rise in sea levels and the creeping damage to our natural protective measures, Wilson says.
This is supported by recent research from the US, which explored how much worse tsunamis could get.
The lead researcher made a rather sobering comment upon its publication.
“Our research shows that sea-level rise can significantly increase the tsunami hazard, which means that smaller tsunamis in the future can have the same adverse impacts as big tsunamis would today.”