What is the simple explanation for the cause of an earthquake?

Answer 1

The Earth's hard outer shell is called the lithosphere. If too much pressure is applied to the lithosphere it responds by breaking where splits or fractures form through it. This is known as brittle deformation.

As the Earth has formed over time, this hard outer shell of Earth has been broken up into a number of large pieces known as tectonic plates which move around very slowly on the underlying material. Where two or more of these large pieces bump into each other or scrape past each other, these parts of the plates can become locked together. However the rest of the plate is still moving and eventually this causes the parts of the plates that are locked together to be wrenched apart very suddenly. This releases lots of energy and causes the ground to shake. Depending on how long the plates have been locked together and how much movement occurred when they are wrenched apart controls how much energy is released and how violent the shaking that occurs.

Answer 2

Earthquakes are caused by the sudden release of energy in the Earth's crust, usually due to movement along faults. This movement generates seismic waves that create the shaking we feel during an earthquake.