why do tsunami’s slow down in shallow water?


I also don’t understand how they can be going 300+mph just from a shift. So any explanation would be helpful.

In: Physics

It’s important to remember that a wave isn’t really a singular object that is travelling at any particular speed like we would normally imagine. It is simply the compression of molecules, and the transmission of this compression from molecule to molecule. So, the transmission is happening at hundreds of miles per hour, but the water molecules themselves aren’t really moving much at all. Just getting squeezed momentarily, then squeezing their neighbors, etc…

That being said, as the compression wave moves into shallower waters near the beach the molecules start to compress in a different direction (up) because there simply isn’t enough depth to absorb the compression anymore, so it starts to grow up and out of the normal surface of the water which is a much less efficient movement of the energy, leading to slowing.

It’s not just a simple shift. Even in water 1000ft deep, there are a boggling amount of tons of water above an inch of the floor. Any movement in the ocean flooring causes *all* of the water to be displaced. That is an insane amount of weight all being displaced at once, and as it is a liquid, it’s going to settle, and then displace any water around it. This upwelling sends waves with huge amounts of kinetic energy out in every direction. That’s why they travel so fast. As they reach shallow water, friction and their own energy slows them down.