I don’t understand the physics as to why an underwater earthquake causes the water on the shore to recede and come back with huge waves.
In: Physics
Tsunamis are just big waves. REALLY big waves. As for how an earthquake can cause a big wave, an earthquake is when a massive piece of a tectonic plate that was deformed under tension suddenly snaps and moves back into shape. An incomprehensibly massive slab of rock suddenly violently shifting underneath is gonna generate some force.
If the sea bed abruptly lifts during an earthquake in deep water, a *huge* volume of water gets moved up…like cubic kilometers (weights on the order of a billion tonnes). And the speed of waves in deep water depends on the depth. So you have this *extremely* fast wave that might be only a few feet high but potentially a kilometer or more wide screaming along the open ocean. If you were in a boat you might not even notice it go by.
By oceans don’t stay deep forever. As the wave approaches the shore it starts to slow down (because the water is getting shallower). But the wave is really wide…the back is still going fast. It starts to pile up on the front, just like a regular wave only *much* larger and faster. This enormous volume of water hits the beach still going stupid fast and now rather tall…but, more importantly, super wide. It just keeps coming and coming and coming.
Watch videos from Fukushima. It doesn’t look like a giant wave, it looks like a very high tide coming in really fast and it just keeps coming. Thats why “tidal wave”.
Edit: typo
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