字幕表 動画を再生する 英語字幕をプリント 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. 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