Earthquakes are caused by the movement of the huge tectonic plates that continents sit on. What happens is they come together and one, in this case the pacific plate, is moving below the Japanese or Eurasian plate and it causes that plate to bend. And when it sticks there there’ll be a little too much friction and it’ll stick. Pressure builds up and real suddenly it’s released and flick.
Have a look at this. Now this is the tectonic plate that Japan is sat on. And here’s the pacific plate coming down underneath it. Now normally they want to move together at about nine centimeters a year. But sometimes they get stuck and when they get stuck the energy of them moving together gets translated to strain energy. literally flex and bend in these huge slabs of rock.
Now that energy builds and builds and they’re flexing and bending more and more and when they finally slip that stored energy all that twist and bend in the rock gets released into sudden movement. Like that. As you can see it literally flicks up, moving the ocean floor above it, which moves the ocean, causing a tsunami.
So here we’ve got a bigger, submarine version of what’s going on. And this gives a representation of what’s been happening twenty four kilometers below the ocean surface in the Japan trench. And these things work in what they call a “stick-slip” way. Most of the time they’re stuck but occasionally they slip and that’s what an earthquake is.
So we’ve got it here, this is bent up here. It’s full of pent up energy, waiting to slip. Now when the slip happens that energy gets released into the ocean, causing a series of tsunami waves, moving in both directions across the pacific.