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What Happens Underwater During a Tidal Wave?

VO: Noah Baum WRITTEN BY: Caitlin Johnson
Tsunamis and tidal waves are often huge natural disasters, causing widespread damage and destruction. When these disasters strike they're headline news all over the world, as tragic stories unfold. But, we only see what happens on land. So, what about under the water? In this video, we show what happens to underwater ocean habitats during a tsunami or tidal wave. How do sea animals cope against the powerful waves? And how does the natural world recover from such an event?
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What Happens Underwater During a Tidal Wave?


Earth’s oceans stretch out as far as the eye can see, blue and endless in every direction. But, as mysterious as those infinite horizons are, at 0.3 billion cubic miles in volume, there’s far more going on beneath the water’s surface. It’s even speculated that we know less about what lurks in the furthest depths of our own oceans than we do about the furthest reaches of outer space… But, one thing we do know, is that Earth’s seas have the potential to cause complete devastation.

It’s all well and good staying away from the ocean and all of its unknowns, but sometimes the planet forcibly brings the water to us – via tidal waves, storm surges and tsunamis. And, when that happens, it’s not only people on the surface who are in danger. Some of the deadliest natural disasters in history have been tsunamis, with the 2004 Indian Ocean Earthquake and Tsunami killing almost a quarter of a million people in South East Asia. While their effects on mankind are clear enough, the effects on the areas they originate from are often hard to see.

Technically, all regular waves could be considered “tidal waves”, since they’re caused by the motion of the tides and, ultimately, by the gravitational pull of the moon. Tidal waves are really just a typical wave, only bigger – often thanks to increased winds. On the other hand, tsunamis and megatsunamis are the most dangerous. Tsunamis aren’t caused by the natural pull of the moon, but are triggered by seismic activity deep underwater – earthquakes and volcanic eruptions. The earth’s surface is made up of tectonic plates, large slabs of crust which move about because of convection currents inside the planet’s mantle. When these plates come into contact with each other, the force of a collision or a sudden release of pressure causes an earthquake.

On land these are already dangerous enough, with the deadliest ‘quake on record – in China in 1556 – killing an estimated 830,000 people. But, when they happen at sea they can be doubly devastating. If the earthquake is close enough to land, there’s often widespread destruction even before the tsunami hits – and afterwards, the aftershocks can lead to more danger. However, the distance a tsunami has to travel before it hits land isn’t what determines how dangerous it is; the danger actually comes from how deep the epicentre is in the water. The deeper the water, the deadlier the wave.

People often find themselves trapped at sea during tsunamis. Sailors have described the sensation as like running aground on rocks, even though they’re out in open water with no rocks to be seen. Scuba divers and surfers are also at risk, although some have beaten the odds to survive partly because they were already in the water (and a considerable distance out) when the wave gathered pace. For those on land from the outset, the best bet is always to get to high ground as soon as possible.

The same rules are true for lots of sea creatures. Large creatures that prefer deep water will be able to survive a tsunami with little to no effort, and whales and dolphins are even capable of sensing changes in pressure when a tsunami draws close. Since one of the primary threats from these disasters comes from being thrown into objects at great force by the water, in the sea (where there aren’t any objects) the animals are at much lower risk. Given that the ocean is also their natural habitat, the human threat of drowning doesn’t exist either.

It’s along the immediate coastlines that things are just as dangerous for animals and humans alike, however. Out in deep water, tsunamis travel very quickly but don’t rise up; they only begin to gain significant height when they get closer to the shore, sucking up all the water on the beaches as they approach. This receding of the water is the closest thing we have to a five-minute tsunami warning; so, if you’re ever on a beach and witness this happening, you definitely need to run.

But, this process can have devastating consequences for certain sea creatures who live in the shallows and can’t escape quickly enough, like crustaceans, certain turtles, coral reefs and other ocean flora. Sudden exposure to open air can be particularly damaging to reefs, especially when the almighty force of the tsunami comes crashing down all around them. Lots of smaller fish can also suffer if they can’t get away quickly enough, eventually finding themselves stranded in wrecked buildings and roads once the initial wave has passed.

That’s not to say that animals and environments found further out at sea are definitely safe… Because, tsunamis are capable of physically ripping apart the sea floor, causing a kind of underwater landfall, killing reefs and destroying habitats. However, this destruction doesn’t necessarily cause long-term problems. A study carried out by Kyoto University on the long-term effects of the 2011 Tōhoku Earthquake and Tsunami – which was the most powerful earthquake to ever hit Japan – proved that coral reefs are actually incredibly resilient, and able to recover at a surprisingly steady rate. Similarly, while there were some problems with the tsunami bringing invasive species from other parts of the sea, fish populations soon returned to expected levels.

That said, coral reefs are only so good at repairing themselves when left alone by humans, and pollution is another major threat that a tsunami poses to the ocean. While the giant waves bring millions of gallons of water onto land, they also take significant parts of the land back out to the sea with them – when they eventually return. In fact, the same 2011 tsunami in Japan led to four times as much ocean pollution being recorded. There was a massive influx in waste plastic and other non-biodegradable objects in the surrounding waters, but also across the globe as up to 100,000 pollutant items landed on the coast of North America in the four years after the tsunami – reportedly as a direct result of the disaster itself.

Given that human-triggered ocean pollution is one of the main ecological concerns of this generation, the long-term environmental effects of a tsunami are constantly monitored. However, it’s also true that the seas would be polluted by such a wave regardless of human influence, because of naturally-occurring sedimentation – when excess soil and dirt from the land is dragged back into the water. With or without humans, this would always happen whenever a big enough wave strikes, and it can have an impact on some species evolution – although it isn’t likely to permanently damage marine ecosystems.

Evidence exists of all of these things – sedimentation, displacement of wildlife, and damage to the sea floor – dating back to prehistoric tsunamis. The Eltanin Impact, for example, happened off the coast of South America approximately 2.15 million years ago, and was caused by a 4km asteroid crashing into the ocean. Fossil records show that marine life was widely displaced by this extreme event, which saw a wave that may have been as tall as 660 feet.

Tsunamis aren’t the only waves we need to be concerned about, however – nor are they the only ones that can affect sea life. While many fish have adapted to survive storms which cause increased wave activity, with some even knowing to flee the area if storm surges get too high, underwater waves pose a different threat entirely. Also known as “internal waves”, they were extensively researched in a 2015 study on the Luzon Strait in the South China Sea. According to research, internal waves can be so powerful that they affect the celestial motion of the moon itself, causing it to sometimes move further away from Earth.

While they work in a similar way as waves on the surface, they’re much vaster, and most of them are the size of skyscrapers. In fact, the average height of these underwater events is the same as the most extreme surface tsunamis ever known. The largest surface tsunami on record is the 1958 mega-tsunami in Lituya Bay, Alaska – which was 1,700 feet tall! All internal waves are to this scale, however. And, while they’re typically slow-moving, they’re a relatively common phenomenon – churning warm water with cold, and irretrievably changing underwater landscapes.

So, while deadly surface waves pose the biggest (and most obvious) threat to us, the mysteries of the deep remain as bountiful as always. But the underwater world is, in general, better-adapted to deal with these disasters than humans are; after all, internal waves are just a fact of life for deep sea creatures, rather than a frightening revelation.

What happens underwater during a tidal wave? Well, besides the coastal creatures in shallow waters, the majority of sea life can endure even the most extreme situations… It’s those of us living on land that face the gravest danger.
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