Walls of water

Tsunamis have horrified us for millennia. From early Japanese lore to the 2004 Indian Ocean tsunami, we’ve come a long way in understanding how they work.

 Illustration by Siddharth Mahesh

Illustration by Siddharth Mahesh

The word 'tsunami' conjures up horror like none other. Terrifying images of sea water rising up majestically and crashing into buildings are a staple of many disaster movies. Tsunamis have existed for as long as oceans have. They've wiped out humans and dinosaurs, and always seem to leave a path of destruction in their wake. 

Of all the nations in the world, Japan has been most subjected to tsunamis throughout history. It is said that when village fishermen would go out to fish, their day would pass uneventfully at sea. But when they returned after sunset, they would find their families and homes destroyed by water. This led them to believe that waves rose in shallow waters, and called the perceived phenomenon tsu-nami, a ‘harbour wave’. 

Tsunamis can be generated by several natural events. The most well known of these are earthquakes. Earthquakes in shallow seas affect water in a very specific way. When tectonic plates grate against each other — causing an earthquake — they can sometimes tilt or offset the ocean floor. One plate falls and wedges itself underneath another, causing the latter to rise up. This event happens in a matter of seconds and can displace large quantities of water vertically in the ocean. This steep climb of water will immediately dissipate its energy in the form of waves; in this case, tsunami waves. 

This is precisely what happened during the 2004 Indian Ocean earthquake. A 400km long section of the Sunda Megathrust (a large fault which forms part of the tectonic boundary between the Eurasian and Indo-Australian tectonic plate) ruptured at speeds of up to 2.8km per second, causing the intensely powerful 9.2 magnitude earthquake. The rise in the sea floor created a vertical displacement of over 30 cubic kilometres of water. This rupture occurred because the Indo-Australian plate is a subducting plate, which is slowly lodging itself underneath the overriding Eurasian plate. As the plates push against each other, they exert monumental energy. Occasionally, the fault ruptures, releasing a part of this energy suddenly and violently: as an earthquake.  

But despite displacing such vast quantities of water, tsunami waves are not felt out at sea. Before the 2004 tsunami reached the shore, there were several ships and boats that rode the waves in the ocean without any idea of the impending disaster that was about to strike. The legends of the Japanese fishermen are indeed partly true. A tsunami’s true power is only felt at the coast. Tsunami waves pass relatively calmly in the open sea and aren’t noticed, but rise and unleash their full force in shallow water.

  The large swell of a tsunami only becomes noticeable in shallow waters where it is forced up to great heights.   Veitmueller/Wikimedia Commons  (CC BY-SA 3.0)

The large swell of a tsunami only becomes noticeable in shallow waters where it is forced up to great heights. Veitmueller/Wikimedia Commons (CC BY-SA 3.0)

In the vast ocean, waves stretch out horizontally. But when the wave comes closer to land, the sudden rise in sea level that comes with proximity to the coast, causes the wave to compress. The front part of the wave slows down as it encounters the rising sea floor, but the tail end of the wave is still moving faster than the front. This is what makes the wave grow to terrifying heights. While the speed decreases to less than 100km an hour, its height can continue to grow for several minutes without breaking. 

The rise in height is called the run-up of a tsunami wave. The 2004 Indian Ocean tsunami had a maximum run-up of 50m, as high as a sixteen storey building. And it wasn’t just one wave. Multiple waves from this tsunami event travelled not just around Indonesia, India, and Sri Lanka, but also travelled as far as Yemen, Africa, Canada, Mexico, South America, and even Antarctica. 

Tsunamis almost always come in multiple successive waves. After the first wave, the water recedes back and the subsequent waves are much bigger. In fact, one of the warning signs of an impending tsunami is the receding water from the shoreline. Many tourists in Indonesia (including a 10 year-old girl who had studied tsunamis in school) recognised the drawback of water and managed to save several people around them. However, a tsunami doesn’t always cause the sea to retreat before it strikes. Depending on how the wave propagates, water might just wall up instead of drawing back into the sea; the shallow water bulging before the wave strikes.

 
  If you notice the ocean retreat dramatically from the shoreline, get to high ground; there's a tsunami coming.   Franco Folini/Flickr  (CC BY-SA 2.0)

If you notice the ocean retreat dramatically from the shoreline, get to high ground; there's a tsunami coming. Franco Folini/Flickr (CC BY-SA 2.0)

 

Another well known cause of tsunamis is asteroid impacts. Sixty-six million years ago, an asteroid with a diameter of 15km crashed off the coast of the Yucatan peninsula in Mexico, leaving a crater 180km across and 20km deep. This impact caused the most rapid mass extinction in history. It let off energy more than a billion times that of the Hiroshima bomb, kicked up a cloud of dust that spread worldwide and hung in the air for years, and caused a layer of iridium deposition worldwide. But that wasn’t even the most devastating part. The impact of a piece of rock the size of Mt. Everest into the ocean gave rise to some of the deadliest megatsunamis the world has ever seen. Approaching the shore, these waves would have had run-ups over a whopping 2km high.They washed onto the shores of North America, covering modern-day Mexico, Texas, Florida, and reaching as far as 300km inland. Such megatsunamis today would be capable of killing over 20 million people. 

The underlying cause, during both an earthquake and an extraterrestrial impact, is displacement of large amounts of water in a very short period of time. Imagine slapping your hand into a tub full of water. The water in the tub gets agitated and a large part of it moves towards the edges in small but perceptible waves. Similarly, any large scale event in the oceans that causes heavy displacement of water could trigger a tsunami. Such events include coastal and underwater volcanism, underwater landslides, and the calving of glaciers. Theoretically, even nuclear tests conducted underwater can generate a tsunami. 

The average tsunami is caused by tectonic motions, but large displacement due to impactors or volcanism cause a ‘megatsunami’, typically characterised by run-ups of over 25m. The largest megatsunami ever recorded by humans was in 1958, with a run-up of 520m, it was triggered by a landslide caused by an earthquake in Lituya Bay, Alaska. Luckily, the location is so remote that there were only five casualties. Geologists have also recently found evidence that 73,000 years ago, a megatsunami reaching 170m high engulfed an entire island in the Cape Verde islands west of Africa.

One of the world’s deadliest volcanic eruptions, the Krakatoa eruption, took place in Indonesia in 1883. It raised up billowing clouds of ash over 30km high, ejecting molten rock and magma several kilometres inland. Skies darkened over an area of nearly 500km in radius around the explosion for several days. The sound produced from its explosion was so loud, its shock wave was recorded travelling around the globe four times, and its boom heard a record 5,000km away on the island of Mauritius. 

But these were still not the most destructive effects of the explosion. Krakatoa, which was actually a group of four volcanic peaks, basically blew itself up into the ocean. The explosion expelled tremendous amounts of lava and the volcano's entire body directly into the water. This displaced a mindbogglingly large mass of water, generating multiple waves that were over 45m in height. While the death toll from the eruption and lava flow was one thousand, the resulting tsunamis killed over 35,000 people. 

Considering that most tsunamis occur because of plate tectonics and water from large oceanic bodies, their mechanism of triggering and propagation seems to make them unique to our planet. After all, none of the other planets have large bodies of water or similar plate tectonics, so it’s easy to assume that tsunamis are a very earth-bound disaster. But are they?

Bodies in the far reaches of our solar system, like Pluto and beyond, hold water in the form of ice. But Saturn’s moons: Titan and Enceladus, and Jupiter’s moons: Europa and Ganymede, hold liquid water. In fact, measured by volume of water, Earth is ranked fourth after Ganymede, Titan, and Europa. 

However, oceans on these bodies are not similar to ours. Being so far away from the sun, the temperatures beyond Mars are very low, so these moons all have a frozen crust of ice over large liquid oceans, automatically eliminating the chances of a tsunami arising in these waters. 

We know today that liquid salt water exists on Mars, seeping downwards over slopes during the summers and receding in the winters. Mars was very like our Earth at one point in time, teeming with blue oceans that covered almost a fifth of the planet with water and an active water cycle. Until the solar wind stripped away all the water.

 
  Artist impression of ancient Mars shows oceans which once covered around a fifth the planet's surface .  Ittiz/Wikimedia Commons  (CC BY-SA 3.0)

Artist impression of ancient Mars shows oceans which once covered around a fifth the planet's surface. Ittiz/Wikimedia Commons (CC BY-SA 3.0)

 

A new study, published this year in Nature, claimed that 3.4 billion years ago two pieces of rock from outer space slammed into the Martian ocean, causing two catastrophic megatsunamis. But these were not comparable to tsunamis on Earth. These rose to heights of up to 120m, washing large rocks inland from the ocean and then receding back. 

The evidence of this falling back of water is recorded on Martian rocks as the waves wiped away surface features, especially shorelines. And that’s not all. "Apart from the evidence for these two tsunamis, we can predict there have been many additional, less significant tsunamis on Mars in the past,” explained Natalie Glines, a SETI institute planetary scientist at NASA Ames Research Center. The gravity on Mars is only one third that of Earth's, so it had tsunamis that were nearly three times as high. 

On Earth, a megatsunami of Martian dimensions would have unimaginably disastrous consequences. Our planet is always at risk of being impacted by asteroids, and if something bigger than the asteroid that took out the dinosaurs comes in the Earth’s way, our maps would need to be redrawn. Nearly every piece of land would have water gushing in over the coasts, wiping out millions of people. Glines explained: “A tsunami of equal magnitude on present-day Earth would devastate surrounding coastlines in the most significant natural disaster that modern humanity has yet faced". 

For now, we can breathe easy. So long as we keep our eyes on the skies and prepare for the worst. Unfortunately, we haven’t seen the last of tsunamis, and probably never will. All we can do is get better at warning those in danger and watching both the ground, sea and sky, for any signs of impending doom.

Edited by Tessa Evans and Bryonie Scott