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  • On June 17, 2017, a bizarre event happened in Greenland.

  • Nuugaatsiaq, a remote fishing village, was devastated by one of the tallest tsunamis

  • in recorded history, a huge, one hundred meter monster of a wave.

  • The wave, about as tall the Statue of Liberty, washed away eleven houses and killed four people.

  • It was so big that its impact registered as a 4.1 magnitude earthquake

  • on nearby seismometers.

  • Now, that confused geologists a bit -- because tsunamis are usually caused by earthquakes

  • out at sea.

  • But when they looked into it further, they found no actual earthquakes had been recorded

  • in the right time frame to cause the wave.

  • Meaning this Greenland tsunami wasn't a conventional tsunami.

  • Instead, scientists determined that this event was something called a megatsunami.

  • So what makes a tsunami mega?

  • It tends to be bigger than an ordinary tsunami, but that's not how you tell the difference.

  • Instead, what distinguishes them from ordinary waves is not their size, but their formation.

  • What you need for a megatsunami is a huge amount of material plunging into a body of

  • water, like an ocean or a lake.

  • It's a little like the splash caused when someone does a cannonball into a swimming pool.

  • But you need more than just your buddies jumping into the ocean.

  • You need like a -- you need a lot a buddies.

  • Or like an asteroid -- or, as was the case for Greenland, a landslide.

  • The Greenland event happened when a huge landslide hit a fjord one kilometer up, displacing a

  • massive area of rock.

  • All that earth tumbled down into the fjord.

  • The resulting wave dissipated quickly, but it was still enough to raise the water level

  • on shorelines about 30 kilometers away.

  • But this isn't the first megatsunami we know of.

  • In recent history, we know of very few besides the Greenland event.

  • On July 9, 1958, a magnitude 7.8

  • earthquake struck Alaska's Lituya Bay.

  • The earthquake caused a rockslide that dumped 82 million metric tonnes of

  • material into the narrow bay.

  • The resulting megatsunami was 524 meters high.

  • That's just recorded history, though.

  • These things are more than big enough to leave their mark for thousands of years.

  • And we've been able to document a few that may have happened long before anyone was around

  • to see it.

  • One of the largest megatsunamis that we know of happened sometime around 73,000

  • years ago, off the coast of West Africa.

  • A huge chunk of the eastern flank of the Fogo

  • volcano fell off at once, striking the surface of the sea.

  • This produced a wave roughly 170 meters tall, almost twice the

  • size of the Greenland wave.

  • But even bigger waves might have struck the Hawai'ian archipelago, more than 100,000 years ago.

  • The clue lies in limestone-bearing gravel

  • on the coastal slopes of the island Lanai.

  • Now, limestone normally forms underwater, but these rocks are found 326 meters above

  • sea level.

  • In a 1984 study, researchers proposed that these rocks were deposited not by simple changes

  • in sea level but by an enormous megatsunami, caused by landslides on the island's steep slopes.

  • To produce this, the wave that struck the island of Lanai would have had to be at least

  • 300 meters high, again twice as large as the Fogo wave.

  • Witnessing that wave would have been like watching a wall of water almost as tall as

  • the Eiffel Tower coming towards you -- out of nowhere.

  • Further back from that, it becomes a little more difficult to directly attribute events

  • to megatsunamis, but we do have a few likely candidates.

  • For example, we know the impact that ended

  • the age of dinosaurs about 66 million years ago splashed down off the coast of the present

  • day Yucatan peninsula -- so it's very likely to have caused a megatsunami.

  • That impact sent shockwaves throughout the entire global ocean system, starting with the nearby

  • Gulf of Mexico.

  • In a 2018 presentation, researchers suggested that the asteroid would have caused a wave

  • 1500 meters high.

  • It's hard to imagine how much water that is, but if you took Mount Kilimanjaro and put

  • it right in the path of that wave, the water would have gotten a quarter of the way up

  • to the summit.

  • Yeah, most of the dinosaurs worldwide were done for anyway -- but the ones in the path

  • of that wave did not last a day.

  • If there's an upside to megatsunamis, it's that they don't last as long as their seismically-created counterparts.

  • Megatsunamis tend to have a stronger local impact.

  • So their impact is limited compared to regular tsunamis, which can still be destructive after

  • traveling thousands of kilometers.

  • But in their local areas, they can be incredibly destructive.

  • Now, here's the bad news: megatsunamis are really rare, but scientists think that

  • climate change might make them more common in our near future.

  • Regions like Greenland and Alaska have permanently frozen soil known as permafrost.

  • Permafrost never thaws -- not even in the summer.

  • Effectively, these places are built on a layer of solid icy ground.

  • But as global temperatures warm, that permafrost does melt.

  • This makes the ground suddenly unstable, which could trigger more landslides.

  • And when landslides happen in coastal areas, well, they could cause more megatsunamis.

  • In theory, early warning systems could help protect the four million or so people who

  • live in the Arctic.

  • Which they're going to need, if more megatsunamis threaten their communities.

  • Thanks for watching this episode of SciShow.

  • Today's the last day of April, and that means it's the last day you can order the Space

  • Pin of the Month -- which is this cute little Vostok 1 satellite.

  • It can be yours, but only if you order it right now.

  • Tomorrow we will have a new pin, and there won't be any more of this one -- but there

  • will be other very good ones.

  • Check it out at the link in the description!

On June 17, 2017, a bizarre event happened in Greenland.

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Megatsunamis: World's Biggest Wave

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    joey joey に公開 2021 年 05 月 19 日
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