Why do earthquakes cause the ground to shake?

This question was asked by Nikolas from Alexandroupoli, Greece.

Although the surface of our planet seems to be solid, it isn’t. It’s made up of enormous plates that move around on a layer of molten rock, like biscuits floating on top of honey. These are called tectonic plates and there are 8 of them:

Earth's 8 tectonic plates. Image credit: http://www.bbc.co.uk/schools/gcsebitesize/science
Earth’s 8 tectonic plates. Image: BBC Bitesize

These plates move very slowly, shifting between 2cm and 10cm per year. Although they’re constantly shifting, sometimes their edges get stuck against each other and this causes earthquakes.

Because the movement of the plates is so slow, we don’t notice it. But when the jagged edges of the plates get caught on each other (the place where this happens is known as a fault), the energy that usually moves them along begins to build up. When the plates finally manage to slip past each other this energy is released all of a sudden, and that’s what causes earthquakes; the point where the slip occurs is called the epicentre. The energy ripples outwards from the plate edges in all directions and when it reaches the surface it causes the ground to tremble and shake. This energy is known as a seismic wave.

Some faults are only a few metres long and some are many miles long; the longer the fault the bigger the earthquake will be. The earthquake caused by the San Andreas fault in California in 1857 ruptured around 360km of the fault; even with the energy travelling at 3km per second it took almost 2 minutes for that entire stretch to rupture. The ground would have been shaking the entire time.

Scientists measure the force of an earthquake with something called a Richter scale, which is numbered from 1 to 10. A force 2 is 10 times stronger than a force 1, a force 3 quake is 10 times stronger than a force 2 and so on. Most people won’t feel an earthquake unless it’s around force 3 or 4.

Image credit: unknown

 

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