[ninetyninenine]

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.

[K0balt]

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.

[chimeracoder]

> 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?

[K0balt]

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.

[jamal-kumar]

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

[ninetyninenine]

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.

[AshleyGrant]

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

[bee_rider]

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.

[jancsika]

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?

[bee_rider]

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.

[golem14]

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?

[yantzr3j]

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.

[k7sune]

Tsunamis are waves the same way a step function is the sum of a series of waves.

[throwaway290]

> 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.