I'm surprised so many people don't understand what tsunamis are. It's a "wave" created by a sudden shift in the Earth's crust. Imagine, suddenly, water on each of side of that split is now at different heights and has to equalize. It's much closer to just removing a dam that is holding back water equal in height to the new difference between the sea floors.
What you get is not a "wave" but a wall of water.
> What you get is not a "wave" but a wall of water.
Its a wave (or series of waves) with a large wavelength and speed in deep ocean, that becomes a shorter wavelength and very large amplitude by shoaling as it hits shallow water.
Its different from typical wind-driven ocean waves for a lot of reasons; but a big indicator is wavelength -- wind-driven ocean waves have wavelengths up to hundreds of meters, tsunamis have wavelengths (in deep ocean) of hundreds of kilometers.
More like tides than waves, as has been stated elsewhere in the thread, is both technically wrong but substantively (with the caveat that "waves" really means "typical wind-drive waves") correct, in that tides are also manifested through waves, but waves which have wavelengths of thousands of kilometers, and so tsunamis are waves more similar to those making up tides (hence the old colloquial use of "tidal waves", which properly refers to the waves manifesting tides, to refer to tsunamis) than to wind-driven waves.
Not true. As the news reporters here in Japan are repeating every few minutes, there will be many waves and they can get bigger over time. They already have, 20-30cm initial waves had 40-60cm later waves.
Waves can get bigger due to earthquakes not being instantaneous or necessarily a single movement, due to amplification by geography, by reflections, by aftershocks, and many other things. The news is suggesting waves lasted about a day for a previous event in a similar area.
> I'm surprised so many people don't understand what tsunamis are.
“I’m Surprised so many people don’t know what ‘X’ is/are isn’t a very nice thing to say. Your comment could have done without that, the rest of it would have been fine.
I don’t take offence. I’m not the most educated, and I don’t live in or near a tsunami prone area, I know about other natural disasters that are relevant to where I live though, maybe more than the parent poster.
I don't think he's even right. Like what he is saying is in actuality wrong. He's surprised because he's ignorant. I'm all for people saying stuff the way he says it. He believes it's true, then he should stand behind. But then the consequence is that he needs to be accepting of when people call him out for being utterly wrong.
The difference is that people know what 2m (wind driven) waves look like at their cities seawall. A 2m tsunami is a -completely- different phenomenon, because of its length. Depending upon the underwater geography, a 2m tsunami might flood right over their 3m seawall, and wipe out entire parts of the city, sweeping hundreds of people out to sea. A 2m wind wave will get saltwater spray on cars driving by. They are both waves, but they share very little in characteristics other than their fundamental physics. It’s like saying that a slingshot fires a 12mm projectile, and so does a 50 caliber anti material rifle. The fact that they are both projectiles, of the same size, is much, much less informative than other facts about their nature.
Saying that tsunamis are waves is easy to equivocate into tsunamis are waves, like other waves. This is an equivocation that is very misleading and can get people killed.
Insofar as the goal of communication is to communicate meaningful information, it is less accurate to say “tsunamis are waves” than it is to say “tsunamis are nothing like normal waves”, or to say “tsunamis are like a wall of water, not like a wave” or “tsunamis are more like tides than waves”.
So yes, tsunamis are waves, but insisting that tsunamis are waves without qualification that their effective characteristics are fundamentally much different and more dangerous than a regular wave is misleading through omission in a way that could directly put people’s lives in jeopardy.
Being pedantic about definitions and being accurate in conveying meaning are not the same thing, and communicating in good faith normally is about conveying meaning in an accurate manner, not just using words in an accurate manner.
FWIW I also believe that meanings are important, but there is a point where pedantry falls into bad-faith territory.
I appreciate your effort to provide an understandable explanation.
That said, in context the original statement was so extremely misrepresentative of the reality that I felt it left the realm of "inaccurate but effective for communication". I certainly didn't see the objections as pedantic.
A clarification was appropriate because it really did miss the physics, but doubling down on the definition of wave without talking about speed, length, and volume (which is what had confused OP in the first place) was not only suboptimal in teaching useful knowledge to OP, it was also misleading in a way that could (and did, in at least one case of a commenter in this thread) lead to a dangerous misconception about characterising tsunamis.
Perhaps it wasn’t intentionally pedantic, but the way that it was doubled down on later makes me suspect an argument in bad faith, or at least an epic case of missing the opportunity to usefully inform.
I value this site for the general character of people trying to educate more than just troll, and I think it’s important to try to educate trolls as well to understand a more constructive and respectful way to interact here. Ostensibly, we take off our clown shoes and leave them at the door.
OTOH I may have read multiple comments in similar tone that were not all attributed to one poster , giving me a mistaken impression of the intent. In that case, I owe an apology for perhaps overreacting.
Nitpick: while what you say is generally true, there are several scenarios that can create true dramatic “wall of water” tsunami waves that have leading slopes of 45-90 degrees and heights in the tens of meters.
The most obvious (but relatively rare) are tsunamis amplified by submarine canyons and other coastal bathymetry like the Nazare submarine canyon famous for the biggest waves on the planet (50+ footers are common in season). If an earthquake directs a tsunami at that canyon, the resulting waves will be spectacular and probably drown everything north of the cliffs. Unfortunately we don’t have any historical records about what happened at Nazare after the 1755 Lisbon earthquake so we don’t know just how big those waves can get.
Then there’s landslides like the one that caused the 1958 tsunami in Lituya Bay, Alaska which creates a much more sudden displacement than an earthquake. Based on the surrounding mountainsides the wave created from that landslide might have peaked at ~500 meters without the 100+ mile wavelength you’d see in a normal tsunami wave.
The most common however are tidal bores, which can send a 30+ foot vertical wave down rivers and narrow channels. This phenomenon shows up relatively frequently in earthquake youtube videos near rivers, though the wall is usually only 5-10 ft tall.
Oh yes! It’s absolutely true that underwater geography can steepen the wave front and amplify the height, sometimes by orders of magnitude. The deep water height of a tsunami wave is often a lot different from what you will see at the coastline.
An entertaining anecdote from the pre- smartphone era:
I sailed to the site near Chenega bay where the earthquake wiped out the village in 1958. We got permission from the elders at the Chenega bay village to land at the island, and it was extremely humbling to see the high water mark from the coastline, and to see the wreckage of boats far, far up on mountainsides.
I’m not a big believer in supernatural stuff, and there are plenty of alternative explanations, but it still freaks me out a bit that the photos we took (aside from the digital ones) did not develop any images of the village site. It was white as if it had been overexposed, even in the case of 1/2 frames. On both disposable cameras. Other photos from the same day, taken in other directions, turned out fine. The digital camera fell overboard in 500 fathoms, so we lost those photos the next day.
As for how exactly this could happen in any reasonable version of events, I’ve got nothing. I guess sometimes chance events line up just right to make for a good story.
Interestingly, there are big tidal bores frequently in Turnagain arm, with 1-3m being common. I’ve seen people surfing it with wetsuits in the ice cold water, getting a run of several miles lol.
> Interestingly, there are big tidal bores frequently in Turnagain arm, with 1-3m being common. I’ve seen people surfing it with wetsuits in the ice cold water, getting a run of several miles lol.
That’s going on the bucket list! How dangerous is it? If you biff it and get caught by the wave can you just dive under the wavefront and come up behind it like on a beach?
I don’t know much about the dangers involved, but it isn’t (or wasn’t, AFAIK) a big attraction… so maybe it’s crazy or dangerous? I’m not sure. Or it might be because the water is so cold that hypothermia and death is inevitable in less than an hour? Or because there probably is no realistic chance of rescue if something goes wrong?
At any rate, there was enough people willing to try that I saw it a couple of times, and TBF I think windsurfing isn’t too unusual in the arm now. But there is also trees and stuff in the tidal bores sometimes, so maybe debris is a big problem or the area is too shallow to be safe.
But, if it’s doable, I’m sure it would be memorable!
I think you’re out of touch. A tsunami is a wave both from a pedantic perspective and an intuitive one and most people aren’t deceived into thinking that tsunamis aren’t dangerous at all because it’s a wave. That’s just made up garbage.
You’re like coming up to me and saying hurricane is not wind because it’s dangerous to think of a hurricane as only wind.
Dude. Nobody is thinking hurricanes are just chill just because hurricanes are wind. This is a fucking non-issue.
I think what you’re trying to say is that the wave length of a tsunami is much longer than the amplitude even though the amplitude is still epically high. But don’t try to conflate this with a safety issue of people dying because somebody called it a “wave” that’s just garbage.
So, I’m sorry that I evidently didn’t manage to convey my point effectively. The problem is that wave is accompanied by a measurement that deceptively buries the lede.
Everyone knows hurricanes are wind. So they look for the wind speed to understand the threat. And it’s effective at characterizing the threat. A 100mph wind is going to be similarly destructive as any other 100mph wind. It works and is semantically and linguistically accurate.
Everyone knows a tsunami is a wave, and it is a strong intuition to believe that a wave is defined by its height. , and the height of the tsunami is actually one of the most widely reported metrics. But intuition about the effects of a tsunami by wave height is dangerously wrong. A tsunami is not at all similar to the vast, vast majority of waves in character and effect. Its speed and length at way, way out of band, and are seldom reported.
My understanding is the most deadly/destructive parts of hurricanes are usually:
1. the storm surge, the potential wall of water brought by the continuous winds and waves near the shore, followed by 2. the flooding from heavy rains, then 3. followed by the wind.
So your example might also be hitting the same issue you're trying to avoid.
Note, the worst storm surge is from the eye towards the side where the winds are blowing in the direction of the shore. That's only part of the area with the peak winds.
Good points. Where I am at it’s mostly the wind because I am a well drained higher elevation, so I’m sure that coloured my perception. But you are right, the storm surge and flooding also do a great deal of damage.
Idk. I don't live anywhere where tsunamis are an issue but seeing measurements like a 1m wave does make me wonder about waves I see at the beach that are that high regularly. I find myself going "oh not so bad then" only to read about thousands of people being evacuated and major damage
Obligatory https://xkcd.com/1053/
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This doesn't make sense to me intuitively. It must be a wave.
Imagine you have a fault line. There is a left side and a right side to the fault line. If the left side lowers with a shift then that shift MUST be localized to the area around the fault. Because if it wasn't then that means there's an elevation change across the board for everything to the left of the fault. You see how that doesn't make sense? So if the entire country of japan was on the left side of the fault then the entire country of japan shifts in elevation which is unrealistic.
So that means, if what you say is semi-true then the shift in elevation is localized to the area left along the fault but the elevation further left remains the same. It's like a slight dip or bump along the fault line. It must be like this because the alternative is just unrealistic. This MUST be what happens when tectonic plates "shift". You won't see the ENTIRE plate shifting in elevation.
With naive logic, one would think that the water simply fills the localized gap but given how deep the ocean is relative to the actual shift way down in the abyss I'm betting if you were on a boat on top of the fault you wouldn't notice anything. But the movement does create a slight imperceptible "filling" that you don't notice. This is a "wave" but it's invisible.
The wave will translate leftward if the movement of the "shift" was sort of in that direction, but you don't see it. BUT as the sea floor gets nearer and nearer to the surface of the ocean the energy of the wave gets squuezed into less and less ocean water mass (i'm remembering how tsunamis work now) and THEN it becomes visible. Right? Just imagine a sideways cross section. As the tiny wave travels from big ocean with huge depth to coastline with no depth the energy of the wave gets concentrated into a thinner and thinner layer of water.
My intuition just sort of converged with my obscure memory of how tsunamis work so I'm pretty sure this is what's going on.
So it is indeed a "wave" that is acting on wave like phenomena beyond simply "filling a gap". In fact say there's an elevation lowering on the left side of the fault by 1 meter. The resulting wave on the coast line hundreds of miles away will be a wave that extends upward by MORE then 1 meter above sea level which is the opposite of water "filling up a gap." That's totally a wave.
Additionally water from tsunamis always recede. This wouldn't happen if the "wall of water" was simply equalizing. If that's the case the water would never recede.
Any expert who says otherwise, let me know.
edit: Actually why the fuck am I using my intuition to explain it? Just cite a source:
https://www.noaa.gov/explainers/science-behind-tsunamis
tsunamis are 100% waves as explained in the link. Anyone who says otherwise clearly doesn't know what they are talking about, that includes the person I'm responding to. End of story.
Yes, they are waves, but they are often very long waves. A typical 1m wave might be 20m long. A tsunami wave might be a kilometer long or longer. That is why people say they are like a tide. The wave arrives, then does not recede for several minutes. So, while a 4m wind driven wave might break over a seawall and even wash a car off the road, a 4m tsunami washes ships over that same seawall and floods the city.
It’s a wave, but it is often not at all like a regular ocean wave. I’ve been at sea when a 3m tsunami passed, we barely felt it. If it had been a 3m wind wave in that otherwise calm sea, it would have knocked dinner off the table.
> I’ve been at sea when a 3m tsunami passed, we barely felt it.
How far out at sea were you? And how did you know at the time?
We were about 50 miles offshore, off the continental shelf (in very deep water) we got the information of the wave from our regular meteorological diligence, since it was my job to get our satellite weather and any notices to mariners on a 6 hour rotation.
I saw the wave on radar first, since it lifted ships that were below our horizon up to where they could be seen again for a few sweeps. But it just felt like A gentle lifting. I didn’t even feel the subsidence of the wave. Interestingly, ships 20 miles away from us but near the edge of the shelf reported isolated severe and chaotic waves.
https://www.youtube.com/watch?v=wAFYVpX45xs
Here's a video of what it looks like from the 2011 event, from the POV of the coast guard approaching it. Waves don't typically look like a sheet has been flapped across one front of the entire horizon of what is visible on the ocean
Yeah that's a wave bro. Notice how the ocean rises above it's own typical sea level? That's not water "filling in a gap" the way tides do it as sea level changes.
That's a huge ass wave as it's a pulse traveling on top of the ocean, above sea level.
That's what it is like out at sea. There's a reason tsunamis are referred to as "tidal waves." For example, watch this video of a tsunami hitting a port today: https://www.youtube.com/watch?v=M1B1J6sgFxk
Yeah, they are waves I think. Just, really incredibly big waves with lots of mass behind them. I think people want to say “not a wave” to emphasize the fact that they are much bigger than the waves that the local environment is used to, so they can be really surprising.
Maybe the easiest way is explain it by volume of water coming at you. A 'normal' wave comes at you for maybe 2-5 seconds, then recedes. A tsunami wave might come at you for what, a few minutes? So moves more than 20x-50x the water than an equivalent 'normal' wave, which has no other way to go?
Since we're intuiting, I'm just imagining something like quickly adding a "D.C. offset" of some given height to the crests and troughs you'd measure by sampling ocean waves.
In fact, I'm not sure I should have quotes around that. Isn't your interlocutor saying a tsunami is literally a direct current of water flowing toward the shore?
I guess it is like a step function, or at least a step function on one side and a really long decay on the other. Is a step function a wave? I’m not sure, my signal processing class was too early in the morning. Maybe it depends on who you ask, mathematicians vs engineers. I’ll go along with the ones that might make a taser or something.
Sure it's a wave, but tides, swells and waves all oscillate just on different frequencies and amplitudes. When they all align you get rogue waves and to the casual observer of a tsunami, a wall of water coming your way.
Tsunamis are waves the same way a step function is the sum of a series of waves.
> If the left side lowers with a shift then that shift MUST be localized to the area around the fault. Because if it wasn't then that means there's an elevation change across the board for everything to the left of the fault. You see how that doesn't make sense
Yo heard of fluid dynamics? Good luck localizing this;) maybe you can build a wall or something real quick
Obviously it is all technically waves. Even if EVERYTHING to the left lowered we would be talking about waves caused by it. But it don't need to be all lowered because waves propagate. And point is these particular waves, tsunami are not the waves you think about because you saw some on the beach. It's an ocean rising for a while. Watch some vids to get a vibe for it.