Question: What causes an earthquake?

Ciarán O'Brien answered on 19 Nov 2014:

The Earth is still mostly a ball of molten rock, and all the land we live on (and the oceans too) is the thin crust that cooled enough to harden like the nasty skin you get on gravy if you let it go cold.

That thin crust is broken up into big chunks called tectonic plates. Here’s a map of the 15 biggest ones, which directions they’re moving, and which countries are sitting on top of them: http://en.wikipedia.org/wiki/List_of_tectonic_plates#mediaviewer/File:Plates_tect2_en.svg

You don’t usually see the edges of these plates, they’re far under the sea, or under mountains (you get mountains when two plates keep moving against each other and the rock piles up)

Those plates float about on the molten rock below, and they bump together and rub against each other as they do so. sometimes they get stuck as they rub against each other, and pressure builds up between them until something gives way, and both plates spring free. That’s what causes an earthquake, two plates getting snagged together and breaking free with a wobble causes shockwaves to ripple out from the point they had been stuck. Everything on top of them shakes about, although the area nearest the plate edge gets it worst. That’s why some countries have more earthquakes than others, they’re right on the edge of a tectonic plate! Because those plates are gaps in the earth’s crust, a lot of magma squeezes through to the surface around a lot of them and that’s why you find most volcanoes on or near the edges of two plates (we call it a fault line).