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What is a Megatsunami?

Watch Dr Claire Earlie explain the difference between a tsunami and megatsunami.
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Along with earthquakes, volcanoes, hurricanes, and tornadoes, tsunamis are one of the most devastating natural hazards on our planet. Tsunamis are a series of waves with lengths up to hundreds of kilometres, heights as tall as two-story buildings, and speeds of up to 800 kilometres per hour. That’s as fast as a jumbo jet. Tsunamis, or tidal waves, are actually nothing to do with the tide. The waves aren’t created by the weather either, like wind waves or ocean swell waves. They’re created by significant displacement of a body of water. And their impacts aren’t only felt in localised, coastal areas, but they can impact entire oceanic basins, like the entire Pacific Ocean or Atlantic Ocean.
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This is what happened in 2004 in the Indian Ocean. The 2004 Indian Ocean tsunami was one of the most disastrous events we’ve experienced since the 1900s, second only to a series of lethal floods in 1931 that led to the deaths of around 4 million people in China. It was the most disastrous tsunami in recorded history, caused by the third largest earthquake ever recorded. It measured a massive 9.3 in magnitude. The earthquake struck off the coast of Indonesia in an area called a subduction zone, where two tectonic plates come crashing together and one of them is forced underneath the other.
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The impact caused water up to about 15 metres height to burst out across an area of 1600 kilometres, producing a series of powerful waves. These waves rapidly spread across the Indian Ocean hitting the mainland, causing the deaths of around 250,000 people and billions of dollars of damage to 14 countries. In 2004, this tsunami was just one of five tsunamis bigger than magnitude seven that took place. And since 2010, there’s actually been over 300 earthquakes that have triggered tsunami warnings. So when we look at it from this perspective, tsunamis are actually a relatively frequent event on our planet. Tectonic plates are constantly moving, with the endless potential to trigger destructive tsunami waves. In this course, we consider events on geological timescales.
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Which means despite the huge human impact of the 2004 Indian Ocean tsunami, it wasn’t of the scale or magnitude that we looked class as an extreme event. So far, we’ve only looked at tsunamis caused by earthquakes. But there’s another type of tsunami that’s capable of causing even more extreme events, a so-called mega tsunami. Unlike the tsunamis we’ve looked at so far, they’re not caused by earthquakes, but by sudden displacement of material, like land or rocks, into a body of water that triggers massive waves up to tens of metres tall. Only a significant and rare event on a geological timescale is capable of creating a mega tsunami.
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This could be a meteoric impact, the collapse of the flank of an island or landmass into the ocean, or a massive volcanic eruption. Perhaps the most significant mega tsunami occurred over 66 million years ago when a massive asteroid smashed into Earth. This asteroid was 10 to 15 kilometres in diameter and hit the Yucatan Peninsula in Mexico. In terms of energy, the impact of this asteroid was the equivalent of 10 billion atomic bombs and was the cause of the mass extinction of over 75% of plant and animal species on Earth. It was also this impact that was responsible for the extinction of the dinosaurs. It caused the displacement of shallow waters, creating a mega tsunami 100 metres high.
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That’s taller than the Statue of Liberty. Now if the asteroid had hit the Earth in the deep ocean, the volume of water displaced would have caused a wave five kilometres . That’s almost as tall as Mount Kilimanjaro. This event went far beyond the devastation we see in the wake of tsunamis. It was just one of many extreme events our planet has been subjected to and led to catastrophic consequences for early life on the planet.

In this video, Dr Claire Earlie explains the difference between a tsunami and megatsunami.

When looking at the 2004 Indian Ocean tsunami we again pose the question; what is regular activity and what is an extreme event?

There is no doubt that it was one of the deadliest natural disasters in recent history. From the viewpoint of its impact on human life and infrastructure it was devastating.

When we look at what causes tsunamis we see processes that are part and parcel of the dynamic Earth we live on.

Because of that we can confidently predict that we will continue to experience tsunamis of different sizes caused by earthquakes under the ocean.

Tsunamis in 2004

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In that sense the 2004 Indian Ocean tsunami was caused by regular activity and it can be listed alongside a number of others that were of a similar magnitude.

Here’s a list of tsunamis that occurred in 2004:

Date Location and size
5 September 2004 Honshu, Japan (Mw 7.2)
2 November 2004 Vancouver, Canada (Mw 6.7)
28 November 2004 Hokkaido, Japan (Mw 7.0)
23 December 2004 North of MacQuarie Island, Australia (Mw 8.1)
26 December 2004 Sumatra / Indian Ocean (Mw 9.0)

Source: International Tsunami Warning Centre

Chicxulub impact

If we contrast that with the megatsunami we heard about in the video, we see an entirely different and less predictable cause.

The megatsunami at Chicxulub in the Yucatán Peninsula, Mexico was caused by a massive asteroid impact.

Incredibly it created a megatsunami 100 metres high, this is significantly higher than any of the tsunamis that occurred in 2004.

This is incredibly rare and there have been no megatsunamis caused by asteroid impacts in recorded human history.

A truly extreme event.

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This content is taken from Cardiff University online course
Extreme Geological Events
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